How to Use This Reference Guide
This reference guide is designed to elaborate upon and work in conjunction with the rating system. Written by expert users of LEED, it serves as a roadmap, describing the steps for meeting and documenting credit requirements and offering advice on best practices.
Within each section, information is organized to flow from general guidance to more specific tips and finally to supporting references and other information. Sections have been designed with a parallel structure to support way finding and minimize repetition.
Each credit category begins with an overview that discusses sustainability and market factors specific to the category. For each prerequisite and credit, readers will then find the following sections:
- Intent & Requirements outlines the rating system requirements for achieving the prerequisite or credit. They were approved through the rating system development process and can also be found on the USGBC website.
- Behind the Intent connects credit achievement with larger sustainability issues and provides information on how the credit requirements meet the intent stated in the rating system.
- Step-by-Step Guidance suggests the implementation and documentation steps that can be used by most projects, as well as generally applicable tips and examples.
- Further Explanation provides guidance for lengthy calculations or for special project situations, such as tips for nonstandard project types or different credit approaches. It includes a Campus section and, sometimes, an International Tips section.
- Required Documentation lists the items that must be submitted for certification review.
- Related Credit Tips identifies other credits that may affect a project team's decisions and strategies for the credit in question; the relationships between credits may imply synergies or trade-offs.
- Changes from LEED 2009 is a quick reference of changes from the previous version of LEED.
- Referenced Standards lists the technical standards related to the credit and offers weblinks to find them.
- Exemplary Performance identifies the threshold that must be met to earn an exemplary performance point, if available.
- Definitions gives the meaning of terms used in the credit.
More about the Further Explanation section
Further Explanation contains varied subsections depending on the credit; two of the common subsections are elaborated upon here.
Campus refers to the Campus Program for Projects on a Shared Site, which certifies multiple projects located on one site and under the control of a single entity. In the ID+C context, an example would be multiple nonconsecutive floors within a single office building with similar fit-out. Only project teams using the Campus Program need to follow the guidance in the Campus section; the guidance is not applicable to projects that are in a campus setting but not pursuing certification using the Campus Program.
There are two approaches to certifying multiple projects under the Campus Program:
- Group Approach allows allows project spaces that are substantially similar and are in a single location to certify as one project that shares a single certification.
- Campus Approach allows project spaces that share a single location and site attributes to achieve separate LEED certification for each project space or group on the master site.
For each approach, the reference guide gives any credit-specific information and notes two possible scenarios:
- Group Approach
- "All project spaces in the group may be documented as one." The project spaces may meet the credit requirements as a single group by, for example, pooling resources or purchasing, and then submitting a single set of documentation.
- "Submit separate documentation for each project space." Each project space in the group project must meet the credit requirements individually for the project to earn the credit.
- Campus Approach
- "Eligible." This credit may be documented once at the level of the master site, and then individual projects within the master site boundary earn the credit without submitting additional documentation.
- "Ineligible. Each LEED project may pursue the credit individually." Each project space within the campus boundary may earn the credit but each project space must document compliance separately.
Projects Outside the U.S.
The International Tips section offers advice on determining equivalency to U.S. standards or using non-U.S. standards referenced in the rating system. It is meant to complement, not replace, the other sections of the credit. Helpful advice for projects outside the U.S. may also appear in the Step-by-Step Guidance section of each credit. When no tips are needed or available, the International Tips heading does not appear.
Units of measurement are given in both Inch-Pound (IP) and International System of Units (SI). IP refers to the system of measurements based on the inch, pound, and gallon, historically derived from the English system and commonly used in the U.S. SI is the modern metric system used in most other parts of the world and defined by the General Conference on Weights and Measures.
Where "local equivalent" is specified, it means an alternative to a LEED referenced standard that is specific to a project's locality. This standard must be widely used and accepted by industry experts and when applied, must meet the credit's intent leading to similar or better outcomes.
Where "USGBC-approved local equivalent" is specified, it means a local standard deemed equivalent to the listed standard by the U.S. Green Building Council through its process for establishing non-U.S. equivalencies in LEED.
Taking an Integrative Approach to Design and Construction
The realization of benefits associated with LEED starts with a transformation of the design process itself. Success in LEED and green building design is best accomplished through an integrative design process that prioritizes cost-effectiveness over both the short and long terms and engages all project team members in discovering beneficial interrelationships and synergies between systems and components. By integrating technical and living systems, the team can achieve high levels of building performance, human performance, and environmental benefits.
Conventionally, the design and construction disciplines work separately, and their solutions to design and construction challenges are fragmented. These "solutions" often create unintended consequences—some positive, but mostly negative. The corollary is that when areas of practice are integrated, it becomes possible to significantly improve building performance and achieve synergies that yield economic, environmental, and human health benefits.
In the conventional design process, each discipline's practitioner is expected to design the subassemblies and system components under his or her control for the most benefit and the least cost. In an integrative process, an entire team—client, designers, builders, and operators—identifies overlapping relationships, services, and redundancies among systems so that interdependencies and benefits (which otherwise would have gone unnoticed) can be exploited, thereby increasing performance and reducing costs.
To work this way requires that project teams, whose members represent various disciplines, come together so that the knowledge, analyses, and ideas from each discipline can inform and link with the systems and components of all other disciplines. In this way, LEED credits become aspects of a whole rather than separate components, and the entire design and construction team can identify the interrelationships and linked benefits across multiple LEED credits.
The coordination of building and site systems—and how they relate to the tenant space— should be addressed early, preferably before schematic design. The Integrative Process credit formally introduces this way of working into LEED so that the team members' expertise in building and site systems can inform the performance, efficiency, and effectiveness of every system. The strategies in the Integrative Process credit are recommended for all LEED projects because they encourage integration during early design stages, when it will be the most effective. The credit introduces an integrative process by focusing on engaging energy- and water-related research and analysis to inform early design decisions through high levels of collaboration among all project team members.
Approaching certification using an integrative process gives the project team the greatest chance of success. The process includes three phases:
- Discovery. The most important phase of the integrative process, discovery can be thought of as an extensive expansion of what is conventionally called predesign. A project is unlikely to meets its environmental goals cost-effectively without this discrete phase. Discovery work should take place before schematic design begins.
- Design and construction (implementation). This phase begins with what is conventionally called schematic design. It resembles conventional practice but integrates all the work and collective understanding of system interactions reached during the discovery phase.
- Occupancy, operations, and performance feedback. This third stage focuses on preparing to measure performance and creating feedback mechanisms. Assessing performance against targets is critical for informing building operations and identifying the need for any corrective action.
Achieving economic and environmental performance requires that every issue and all team members (clients, designers, engineers, constructors, operators) be brought into the project at the earliest point, before anything is yet designed. The structure to manage this flow of people, information, and analysis is as follows:
- All project team members, representing all design and construction disciplines, gather information and data relevant to the project.
- Team members analyze their information.
- Team members participate in workshops to compare notes and identify opportunities for synergy.
This process of research, analysis, and workshops is done in an iterative cycle that refines the design solutions. In the best scenario, the research and workshops continue until the project systems are optimized, all reasonable synergies are identified, and the related strategies associated with all LEED credits are documented and implemented.
Devising a LEED Work Plan
It is recommended that LEED applicants follow a series of steps to certification.
Step 1. Initiate discovery phase
Begin initial research and analysis (see Integrative Process Credit). When sufficient information has been gathered, hold a goal-setting workshop to discuss findings.
Step 2. Select LEED rating system
The LEED system comprises 21 adaptations designed to accommodate the needs of a variety of market sectors (see Rating System Selection Guidance). For many credits, Further Explanation highlights rating system and project type variations to help teams develop a successful approach.
Step 3. Check minimum program requirements
All projects seeking certification are required to comply with the minimum program requirements (MPRs) for the applicable rating system, found in this reference guide and on the USGBC website.
Step 4. Establish project goals
Prioritize strategies for certification that align with the project's context and the values of the project team, owner, or organization. Once these values are articulated, project teams will be able to select appropriate strategies and associated LEED credits to meet the goals.
The recommended method for establishing project goals is to convene a goal-setting workshop (see Integrative Process Credit) for the project team members and the owner. Understanding the owner's goals, budget, schedule, functional programmatic requirements, scope, quality, performance targets, and occupants' expectations will promote creative problem solving and encourage fruitful interaction.
To capture the most opportunities, the workshop should occur before any design work and include wide representation from the design and construction disciplines.
Step 5. Define LEED project scope
Review the project's program and initial findings from the goal-setting workshop to identify the project scope. Special considerations include building or site amenities or shared facilities that may be used by project occupants.
Next, map the LEED project boundary along departmental or ownership lines. If the project boundary is not obvious because of scope of work, partial renovations, or other issues, see the minimum program requirements. Share the final project boundary decision with the entire team, since this definition affects numerous prerequisites and credits.
Finally, investigate any special certification programs that may apply based on the project's scope, such as the Volume Program or the Campus Program. If the project owner is planning multiple similar projects in different locations, Volume may be a useful program to streamline certification. If the project includes multiple project spaces in a single location, Campus may be appropriate.
Step 6. Develop LEED scorecard
Use the project goals to identify the credits and options that should be attempted by the team. The Behind the Intent sections offer insight into what each credit is intended to achieve and may help teams align goals with credits that bring value to the owner, environment, and community of the project.
This process should focus the team on those credits with the highest value for the project over the long term. Once the high-priority credits have been selected, identify related credits that reinforce the priority strategies and provide synergistic benefits.
Finally, establish the target LEED certification level (Certified, Silver, Gold, or Platinum) and identify additional credits needed to achieve it. Make sure that all prerequisites can be met and include a buffer of several points above the minimum in case of changes during design and construction.
Step 7. Continue discovery phase
Project team members should perform additional research and analysis as the project progresses, refining the analysis, testing alternatives, comparing notes, generating ideas in small meetings, and evaluating costs. Examples of research and analysis for energy- and water-related systems are outlined in the Integrative Process credit.
The project team should reassemble occasionally to discuss overlapping benefits and opportunities (e.g., how best to use the waste products from one system to benefit other systems). This approach encourages the discovery of new opportunities, raises new questions, and facilitates testing across disciplines.
Step 8. Continue iterative process
The above pattern of research and analysis followed by team workshops should continue until the solutions satisfy the project team and owner.
Step 9. Assign roles and responsibilities
Select one team member to take primary responsibility for leading the group through the LEED application and documentation process. This leadership role may change from the design to the construction phase, but both the design and the construction leaders should be involved throughout the process to ensure consistency, clarity, and an integrative approach.
Cross-disciplinary team ownership of LEED credit compliance can help foster integrative design while ensuring consistent documentation across credits. On a credit-by-credit basis, assign primary and supporting roles to appropriate team members for credit achievement and documentation. Clarify responsibilities for ensuring that design decisions are accurately represented in drawings and specifications and that construction details match design documentation.
Establish regular meeting dates and develop clear communication channels to streamline the process and resolve issues quickly.
Step 10. Develop consistent documentation
Consistent documentation is critical to achieving LEED certification.
Data accumulated throughout the construction process, such as construction materials quantities, should be gathered and assessed at regular intervals to allow the team to track ongoing progress toward credit achievement and ensure that information is not misplaced or omitted. Maintaining Consistency in the Application, below, and the credit category overviews discuss the numeric values and meaning of terms that affect achievement of multiple credits within a credit category.
Step 11. Perform quality assurance review and submit for certification
A quality assurance review is an essential part of the work program. A thorough quality control check can improve clarity and consistency of the project's LEED documentation, thereby avoiding errors that require time and expense to correct later in the certification process. The submission should be thoroughly proofread and checked for completeness. In particular, numeric values that appear throughout the submission (e.g., site area) must be consistent across credits.
Maintaining Consistency in the Application
Certain concepts and numeric values recur across multiple credits and credit categories and must be treated consistently throughout the submission.
Spaces that earn LEED certification should be completed by the time they have submitted their final application for LEED certification. Complete means that no further work is needed and the project is ready for occupancy. For ID+C projects, spaces are considered incomplete if they do not include the furnishings, fixtures, and equipment intended for regular operations of the space. No more than 40% of the certifying gross floor area of a LEED project may consist of incomplete space. Additionally, projects that include incomplete spaces must use Appendix 2 Default Occupancy Counts to establish occupant counts for incomplete spaces.
For incomplete spaces in ID+C projects, the project team must provide supplemental documentation.
- Submit a letter of commitment, signed by the owner, indicating that the remaining incomplete spaces will satisfy the requirements of each prerequisite and credit achieved by this project if and when completed by the owner. This letter may cover the commitment in general terms and need not address each prerequisite or credit individually.
- For incomplete spaces intended to be finished by tenants (i.e., parties other than the owner), submit a set of nonbinding tenant design and construction guidelines, with a brief explanation of the project circumstances.
For prerequisites with established baselines (e.g., WE Prerequisite Indoor Water Use, EA Prerequisite Minimum Energy Performance) and the credits dependent on the calculations in the prerequisites, the proposed design must be equivalent to the baseline for the incomplete spaces. Project teams that wish to claim environmental performance or benefit beyond the baseline for incomplete spaces should provide a binding tenant sales and lease agreement for the incomplete space. This must be signed by the future tenant and include terms related to how the technical credit requirements will be carried out by the tenant. If the tenant and the owner are the same entity, a signed letter if commitment is sufficient. An unsigned or sample lease agreement is not acceptable.
Several credits require the assessment of a piece of land to determine whether it has been previously developed, defined as follows:
previously developed altered by paving, construction, and/or land use that would typically have required regulatory permitting to have been initiated (alterations may exist now or in the past). Land that is not previously developed and landscapes altered by current or historical clearing or filling, agricultural or forestry use, or preserved natural area use are considered undeveloped land. The date of previous development permit issuance constitutes the date of previous development, but permit issuance in itself does not constitute previous development.
Tricky lands to assess include those with few buildings present. If the land previously had buildings, then it is considered previously developed even if those buildings have since been torn down. Another frequently confusing situation is parkland. Pay careful attention to the type of parkland. Improved parks with manicured landscaping and constructed features like playgrounds (e.g., a city park) are considered previously developed. Land that has only been cleared or graded, with no additional improvements, is not considered previously developed. Land maintained in a natural state (e.g., a forest preserve) is not considered previously developed, even if minor features like walking paths are present.
Density can be calculated separately for residential and nonresidential elements or as a single value. The following definitions apply:
density a ratio of building coverage on a given parcel of land to the size of that parcel. Density can be measured using floor area ratio (FAR); dwelling units per acre (DU/acre) or dwelling units per hectare (DU/hectare); square feet of building area per acre of buildable land; or square meters of building area per hectare of buildable land. It does not include structured parking.
buildable land the portion of the site where construction can occur, including land voluntarily set aside and not constructed on. When used in density calculations, buildable land excludes public rights-of-way and land excluded from development by codified law.
Land voluntarily set aside and not built on, such as open space, is considered buildable because it was available for construction but set aside voluntarily. For example, 5 acres (2 hectares) of park space required by local government code would be considered nonbuildable, but if a developer voluntarily sets aside an additional 3 acres (1.2 hectares) for more park space, those 3 acres (1.2 hectares) must be categorized as buildable land.
After determining buildable land, calculate residential or nonresidential density or a combined density. To calculate residential density, divide the number of dwelling units by the amount of residential land. To calculate nonresidential density, use floor area ratio (FAR):
floor-area ratio (FAR) the density of nonresidential land use, exclusive of structured parking, measured as the total nonresidential building floor area divided by the total buildable land area available for nonresidential buildings.
For example, on a site with 10,000 square feet (930 square meters) of buildable nonresidential land area, a building of 10,000 square feet (930 square meters) of floor area would have a FAR of 1.0. On the same site, a building of 5,000 square feet (465 square meters) would have a FAR of 0.5; a building of 15,000 square feet (1395 square meters) would have a FAR of 1.5; and a building of 20,000 square feet (1860 square meters) would have a FAR of 2.0.
To calculate the combined density for residential and nonresidential areas, use FAR.
Many kinds of people use a typical LEED project space, and the mix varies by project type. Occupants are sometimes referred to in a general sense; for example, "promote occupants' comfort, well-being, and productivity by improving indoor air quality." In other instances, occupants must be counted for calculations. Definitions of occupant types are general guidelines that may be modified or superseded in a particular credit when appropriate (such changes are noted in each credit's reference guide section). Most credits group users into two categories, regular building occupants and visitors.
Regular Building Occupants
Regular building occupants are habitual users of a building. All of the following are considered regular building occupants.
Employees include part-time and full-time employees, and totals are calculated using full-time equivalency (FTE).
A typical project can count FTE employees by adding full-time employees and part-time employees, adjusted for their hours of work.
Equation 1. FTE employees = Full-time employees + (Σ daily part-time employee hours) / 8
Staff is synonymous with employees for the purpose of LEED calculations.
Volunteers who regularly use a building are synonymous with employees for the purpose of LEED calculations.
Residents of a project are considered regular building occupants. This includes residents of a dormitory. If actual resident count is not known, use a default equal to the number of bedrooms in the dwelling unit plus one, multiplied by the number of such dwelling units.
Primary and secondary school students are typically regular building occupants (see the exception in LT Credit Bicycle Facilities).
Hotel guests are typically considered regular building occupants, with some credit-specific exceptions. Calculate the number of overnight hotel guests based on the number and size of units in the project. Assume 1.5 occupants per guest room and multiply the resulting total by 60% (average hotel occupancy). Alternatively, the number of hotel guest occupants may be derived from actual or historical occupancy.
Inpatients are medical, surgical, maternity, specialty, and intensive-care unit patients whose length of stay exceeds 23 hours. Peak inpatients are the highest number of inpatients at a given point in a typical 24-hour period.
Visitors (also "transients") intermittently use a LEED building. All of the following are considered visitors:
Retail customers are considered visitors. In Water Efficiency credits, retail customers are considered separately from other kinds of visitors and should not be included in the total average daily visitors.
Outpatients visit a hospital, clinic, or associated health care facility for diagnosis or treatment that lasts 23 hours or less Peak outpatients are the highest number of outpatients at a given point in a typical 24-hour period.
Volunteers who periodically use a building (e.g., once per week) are considered visitors.
Higher-education students are considered visitors to most buildings, except when they are residents of a dorm, in which case they are residents.
In calculations, occupant types are typically counted in two ways:
Daily averages take into account all the occupants of a given type for a typical 24-hour day of operation.
Peak totals are measured at the moment in a typical 24-hour period when the highest number of a given occupant type is present.
Whenever possible, use actual or predicted occupancies. If occupancy cannot be accurately predicted, one of the following resources to estimate occupancy:
- Default occupant density from ASHRAE 62.1-2010, Table 6-1
- Default occupant density from CEN Standard EN 15251, Table B.2
- Appendix 2 Default Occupancy Counts
- Results from applicable studies.
If numbers vary seasonally, use occupancy numbers that are a representative daily average over the entire operating season of the building.
If occupancy patterns are atypical (shift overlap, significant seasonal variation), explain such patterns when submitting documentation for certification. Table 1 lists prerequisites and credits that require specific occupancy counts for calculations.
Table 1. Occupancy types for calculations, by project type variation
Table 2. Credit attributes
Minimum Program Requirements
The Minimum Program Requirements (MPRs) are the minimum characteristics or conditions that make a project appropriate to pursue LEED certification. These requirements are foundational to all LEED projects and define the types of buildings, spaces, and neighborhoods that the LEED rating system is designed to evaluate. View the Minimum Program Requirements
Rating System Selection Guidance
Projects are required to use the rating system that is most appropriate. However, when the decision is not clear, it is the responsibility of the project team to make a reasonable decision in selecting a rating system before registering their project. This guidance helps project teams select a LEED rating system. View the Rating System Selection Guidance
Credit Category Overviews
The Location and Transportation (LT) category rewards thoughtful project team decisions about the location of their tenant space, with credits that encourage compact development, alternative transportation, and connection with amenities, such as restaurants and parks. The LT category is an outgrowth of the Sustainable Sites (SS) category, which formerly covered location-related topics. Whereas the SS category now specifically addresses on-site ecosystem services, the LT category considers the existing features of the surrounding community and how this infrastructure affects occupants’ behavior and environmental performance.
Well-located tenant spaces are those that take advantage of existing infrastructure—public transit, street networks, pedestrian paths, bicycle networks, and services and amenities. By recognizing existing patterns of development and land density, project teams can reduce strain on the environment from the social and ecological costs that accompany sprawling development patterns. In addition, the compact communities promoted by the LT credits encourage robust and realistic alternatives to private automobile use, such as walking, biking, vehicle shares, and public transit. These incremental steps can have significant benefits: a 2009 Urban Land Institute study concluded that improvements in land-use patterns and investments in public transportation infrastructure alone could reduce greenhouse gas emissions from transportation in the U.S. by 9% to 15% by 20501; globally, the transportation sector is responsible for about one-quarter of energy-related greenhouse gas emissions2.
If integrated into the surrounding community, a well-located tenant space can also offer distinct advantages to the owner and users of the space. For owners, locating the tenant space in a vibrant, livable community makes it a destination for residents, employees, customers, and visitors. For occupants, walkable and bikable locations can enhance health by encouraging daily physical activity, and proximity to services and amenities can increase happiness and productivity.
Design strategies that complement and build on the project location are also rewarded in the LT section. For example, by limiting parking, a project can encourage building users to take alternative transportation. By providing bicycle storage, a project can support users seeking transportation options.
Walking and bicycling distances are measurements of how far a pedestrian and bicyclist would travel from a point of origin to a destination, such as the nearest bus stop. This distance, also known as shortest path analysis, replaces the simple straight-line radius used in LEED 2009 and better reflects pedestrians’ and bicyclists’ access to amenities, taking into account safety, convenience, and obstructions to movement. This in turn better predicts the use of these amenities.
Walking distances must be measured along infrastructure that is safe and comfortable for pedestrian: sidewalks, all-weather-surface footpaths, crosswalks, or equivalent pedestrian facilities.
Bicycling distances must be measured along infrastructure that is safe and comfortable for bicyclists: on-street bicycle lanes, off-street bicycle paths or trails, and streets with low target vehicle speed. Project teams may use bicycling distance instead of walking distance to measure the proximity of bicycle storage to a bicycle network in LT Credit Bicycle Facilities. When calculating the walking or bicycling distance, sum the continuous segments of the walking or bicycling route to determine the distance from origin to destination. A straight-line radius from the origin that does not follow pedestrian and bicyclist infrastructure will not be accepted.
Refer to specific credits to select the appropriate origin and destination points. In all cases, the origin must be accessible to all building users, and the walking or bicycling distance must not exceed the distance specified in the credit requirements.
1U.S. Environmental Protection Agency, Smart Growth and Climate Change, epa.gov/dced/climatechange.htm (accessed September 11, 2012).
2International Council on Clean Transportation, Passenger Vehicles, theicct.org/passenger-vehicles (accessed March 22, 2013).
The Water Efficiency (WE) section addresses water holistically, looking at indoor use, outdoor use, specialized uses, and metering. The section is based on an “efficiency first” approach to water conservation. As a result, each prerequisite looks at water efficiency and reductions in potable water use alone. Then, the WE credits additionally recognize the use of nonpotable and alternative sources of water.
The conservation and creative reuse of water are important because only 3% of Earth’s water is fresh water, and of that, slightly over two-thirds is trapped in glaciers1. Typically, most of a building’s water cycles through the building and then flows off-site as wastewater. In developed nations, potable water often comes from a public water supply system far from the building site, and wastewater leaving the site must be piped to a processing plant, after which it is discharged into a distant water body. This pass-through system reduces streamflow in rivers and depletes fresh water aquifers, causing water tables to drop and wells to go dry. In 60% of European cities with more than 100,000 people, groundwater is being used faster than it can be replenished2.
In addition, the energy required to treat water for drinking, transport it to and from a building, and treat it for disposal represents a significant amount of energy use not captured by a building’s utility meter. Research in California shows that roughly 19% of all energy used in this U.S. state is consumed by water treatment and pumping3.
In the U.S., buildings account for 13.6% of potable water use4, the third-largest category, behind thermoelectric power and irrigation. Designers and builders can construct green buildings that use significantly less water than conventional construction by incorporating native landscapes that eliminate the need for irrigation, installing water-efficient fixtures, and reusing wastewater for nonpotable water needs. The Green Building Market Impact Report 2009 found that LEED projects were responsible for saving an aggregate 1.2 trillion gallons (4.54 trillion liters) of water5. LEED’s WE credits encourage project teams to take advantage of every opportunity to significantly reduce total water use.
The focus of the interiors version of the WE category is indoor water for fixtures, appliances, and processes. Several kinds of documentation span these components, depending on the project’s specific water-saving strategies.
Floor plans. Floorplans are used to document the location of fixtures, appliances, and process water equipment (e.g., cooling towers, evaporative condensers), as well as submeters.
Fixture cutsheets. Projects must document their fixtures (and appliances as applicable) using fixture cutsheets or manufacturers’ literature. This documentation is used in the Indoor Water Use Reduction prerequisite and credit.
Occupancy calculations. The Indoor Water Use Reduction prerequisite and credit require projections based on occupants’ usage. The Location and Transportation credit category also uses project occupancy calculations. Review the occupancy section in Getting Started to understand how occupants are classified and counted. Also see WE Prerequisite Indoor Water Use Reduction for additional guidance specific to the WE section.
1 U.S. Environmental Protection Agency, Water Trivia Facts, water.epa.gov/learn/kids/drinkingwater/water_trivia_facts.cfm (accessed September 12, 2012).
2 Statistics: Graphs & Maps, UN Water, http://www.unwater.org/statistics_use.html (accessed March 8, 2013).
4 USGBC, Green Building Facts, https://www.usgbc.org/press/benefits-of-green-building.
5 Green Outlook 2011, Green Trends Driving Growth (McGraw-Hill Construction, 2010), aiacc.org/wp-content/uploads/2011/06/greenoutlook2011.pdf (accessed on September 12, 2012).
The Energy and Atmosphere (EA) category approaches energy from a holistic perspective, addressing energy use reduction, energy-efficient design strategies, and renewable energy sources.
The current worldwide mix of energy resources is weighted heavily toward oil, coal, and natural gas1. In addition to emitting greenhouse gases, these resources are nonrenewable: their quantities are limited or they cannot be replaced as fast as they are consumed2. Though estimates regarding the remaining quantity of these resources vary, it is clear that the current reliance on nonrenewable energy sources is not sustainable and involves increasingly destructive extraction processes, uncertain supplies, escalating market prices, and national security vulnerability. Accounting for approximately 40% of the total energy used today3, buildings are significant contributors to these problems.
Energy efficiency in a green building starts with a focus on design that reduces overall energy needs, such as building orientation and glazing selection, and the choice of climate-appropriate building materials. Strategies such as passive heating and cooling, natural ventilation, and high-efficiency HVAC systems partnered with smart controls further reduce a building’s energy use. Interiors Design and Construction projects can encourage these methods by choosing to locate in buildings that have been designed with efficiency in mind and then continuing the process through actions such as installing efficient lighting and appliances. The generation of renewables on the project site or purchase of green power allows portions of the remaining energy consumption to be met with non–fossil fuel energy, helping to balance the demand on traditional sources.
The commissioning process is critical to ensuring high-performing buildings. Early involvement of a commissioning authority helps prevent long-term maintenance issues and wasted energy by verifying that the design meets the owner’s project requirements and functions as intended. In an operationally effective and efficient building, the staff understands what systems are installed and how they function.
The American Physical Society has found that if current and emerging cost-effective energy efficiency measures are employed in new buildings and in existing buildings as their heating, cooling, lighting, and other equipment is replaced, the growth in energy demand from the building sector could fall from a projected 30% increase to zero between now and 20304. The EA section supports the goal of reduced energy demand through credits related to reducing usage, designing for efficiency, and supplementing the energy supply with renewables.
sup>4 Energy Future: Think Efficiency (American Physical Society, September 2008), aps.org/energyefficiencyreport/report/energy-bldgs.pdf (accessed September 13, 2012).
The Materials and Resources (MR) credit category focuses on minimizing the embodied energy and impacts associated with the extraction, processing, transport, maintenance, and disposal of building materials. The requirements are designed to support a life-cycle approach that improves performance and promotes resource efficiency. Each requirement identifies a specific action that fits into the larger context of a life-cycle approach to embodied impact reduction.
The Waste Hierarchy
Construction and demolition waste constitutes about 40% of the total solid waste stream in the United States1 and about 25% of the total waste stream in the European Union2. In its solid waste management hierarchy, the U.S. Environmental Protection Agency (EPA) ranks source reduction, reuse, recycling, and waste to energy as the four preferred strategies for reducing waste. The MR section directly addresses each of these recommended strategies.
Source reduction appears at the top of the hierarchy because it avoids environmental harms throughout a material’s life cycle, from supply chain and use to recycling and waste disposal. Source reduction encourages the use of innovative construction strategies, such as prefabrication and designing to dimensional construction materials, thereby minimizing material cutoffs and inefficiencies.
Building and material reuse is the next most effective strategy because reusing existing materials avoids the environmental burden of the manufacturing process. Replacing existing materials with new ones would entail production and transportation of new materials, and it would take many years to offset the associated greenhouse gases through increased efficiency of the building. LEED has consistently rewarded the reuse of materials. LEED v4 now offers more flexibility and rewards all material reuse achieved by a project—both in situ, as part of a building reuse strategy, and from off site, as part of a salvaging strategy.
Recycling is the most common way to divert waste from landfills. In conventional practice, most waste is landfilled—an increasingly unsustainable solution. In urban areas landfill space is reaching capacity, requiring the conversion of more land elsewhere and raising the transportation costs of waste. Innovations in recycling technology improve sorting and processing to supply raw material to secondary markets, keeping those materials in the production stream longer.
Because secondary markets do not exist for every material, the next most beneficial use of waste materials is conversion to energy. Many countries are lessening the burden on landfills through a waste-to-energy solution. In countries such as Sweden and Saudi Arabia, waste-to-energy facilities are far more common than landfills. When strict air quality control measures are enforced, waste-to-energy can be a viable alternative to extracting fossil fuels to produce energy.
In aggregate, LEED projects are responsible for diverting more than 80 million tons (72.6 million tonnes) of waste from landfills, and this volume is expected to grow to 540 million tons (489.9 million tonnes) by 20303. From 2000 to 2011, LEED projects in Seattle diverted an average of 90% of their construction waste from the landfill, resulting in 175,000 tons (158,757.3 tonnes) of waste diverted4. If all newly constructed buildings achieved the 90% diversion rate demonstrated by Seattle’s 102 LEED projects, the result would be staggering. Construction debris is no longer waste, it is a resource.
Life-Cycle Assessment in LEED
Through credits in the MR category, LEED has instigated market transformation of building products by creating a cycle of consumer demand and industry delivery of environmentally preferable products. LEED project teams have created demand for increasingly sustainable products; in turn, suppliers, designers, and manufacturers are responding. From responsibly harvested wood to recycled content to biobased materials, the increased supply of sustainable materials has been measurable over the history of LEED. Several MR credits reward use of products that perform well on specific criteria. It is difficult, however, to compare two products that have different sustainable attributes—for example, cabinets made of wheat husks sourced from all over the country and bound together in resin versus solid wood cabinets made from local timber. Life-cycle assessment (LCA) provides a more comprehensive picture of materials and products, enabling project teams to make more informed decisions that will have greater overall benefit for the environmental, human health, and communities, while encouraging manufacturers to improve their products through innovation.
LCA is a “compilation and evaluation of the inputs and outputs and the potential environmental impacts of a product system throughout its life cycle.5” The entire life cycle of a product (or building) is examined, the processes and constituents identified, and their environmental effects assessed—both upstream, from the point of manufacture or raw materials extraction, and downstream, including transportation, use, maintenance, and end of life. This approach is sometimes called “cradle to grave.” Going even further, “cradle to cradle” emphasizes recycling and reuse at the end of life rather than disposal.
Life-cycle approaches to materials assessment began in the 1960s with carbon accounting models. Since then, LCA standards and practices have been developed and refined. In Europe and a few other parts of the world, manufacturers, regulators, specifiers, and consumers in many fields have been using life-cycle information to improve their product selections and product environmental profiles. Until relatively recently, however, the data and tools that support LCA were lacking in the U.S. Now a growing number of manufacturers are ready to document and publicly disclose the environmental profiles of their products, and programs that assist this effort and help users understand the results are available.
LEED aims to accelerate the use of LCA tools and LCA-based decision making, thereby spurring market transformation and improving the quality of databases. Recognizing the limitations of the life-cycle approach for addressing human health and the ecosystem consequences of raw material extraction, LEED uses alternative, complementary approaches to LCA in the credits that address those topics.
Required Products and Materials
The scope of the MR credit category includes the building or portions of the building that are being constructed or renovated. Portions of an existing building that are not part of the construction contract are excluded from MR documentation unless otherwise noted. For guidance on the treatment of additions, see the minimum program requirements.
Qualifying Products and Exclusions
The MR section for interior design and construction includes permanently installed building products as well as furniture. “Permanently installed building products” is defined by LEED as the products and materials that create the building or are attached to it. Examples include structure and enclosure elements, installed finishes, framing, interior walls, cabinets and casework, doors, and roofs. Most of these materials fall into Construction Specifications Institute (CSI) 2012 MasterFormat Divisions 3-10, 31, and 32. Some products addressed by MR credits fall outside these divisions. Furniture items within the project’s scope of work are also required to be included in credit calculations.
In past versions of LEED, all mechanical, electrical, and plumbing (MEP) equipment, categorized as CSI MasterFormat divisions 11, 21-28, and other specialty divisions, was excluded from MR credits. In this version of LEED, some specific products that are part of these systems but are “passive” (meaning not part of the active portions of the system) may be included in credit calculations. This allows flexibility for the optional assessment of piping, pipe insulation, ducts, duct insulation, conduit, plumbing fixtures, faucets, showerheads, and lamp housings. If optional products or materials are included in cost-based credit calculations, such as Option 2 under each Building Product Disclosure and Optimization credits, they must be included consistently across all cost-based credit calculations. Additionally, if optional products and materials are included in product based calculations, such as Option 1 under each Building Product Disclosure and Optimization credits, they are not required to be included in cost-based credit calculations. However, unlike furniture, if some of these products are included in credit calculations, not all products of that type must be included.
Special equipment, such as elevators, escalators, process equipment, and fire suppression systems, are excluded from the credit calculations. Also exclude products purchased for temporary use on the project, like formwork for concrete.
Defining a Product
Several credits in this category calculate achievement on the basis of number of products instead of product cost. For these credits, a “product” or a “permanently installed building product” is defined by its function in the project. A product includes the physical components and services needed to serve the intended function. If there are similar products within a specification, each contributes as a separate product. Here are a few scenarios.
Products that arrive at the project site ready for installation:
- Metal studs, wallboard, and concrete masonry units are all separate products.
- For wallboard, the gypsum, binder, and backing are all required for the product to function, so each ingredient does not count as a separate product.
- Concrete is a single product as it typically arrives at the site as a mixed product ready to pour.
Products that arrive as an ingredient or component used in a site-assembled product:
- Lumber for custom millwork.
- Concrete mix components (admixture, aggregate, and cement) may be considered separate products if they arrive to site as separate products.
Similar products from the same manufacturer with distinct formulations versus similar products from the same manufacturer with aesthetic variations or reconfigurations:
- Paints of different gloss levels are separate products because each paint type is specified to serve a different function, such as water resistance. Different colors of the same paint are not separate products because they serve the same function.
- Carpets of different pile heights are separate products because they are used for different kinds of foot traffic. The same carpet in a different color is not a separate product.
- Desk chairs and side chairs in the same product line are different products because they serve different functions. Two side chairs differing only in aesthetic aspects, such as the presence of arms, are not different products.
Determining Product Cost
Product and materials cost includes all expenses to deliver the material to the project site. Materials cost should include all taxes and delivery costs incurred by the contractor but exclude any cost for labor and equipment after the material is delivered to the site.
The Building Design and Construction (BD+C) rating systems use a default materials cost calculation. This approach is not applicable to Interior Design and Construction (ID+C) rating systems.
Location Valuation Factor
Several credits in the MR section include a location valuation factor, which adds value to locally produced products and materials. The intent is to incentivize the purchase of products that support the local economy. Products and materials that are extracted, manufactured, and purchased within 100 miles (160 kilometers) of the project are valued at 200% of their cost (i.e., the valuation factor is 2).
For a product to qualify for the location valuation factor, it must meet two conditions: all extraction, manufacture, and purchase (including distribution) of the product and its materials must occur within that radius (Figure 1), and the product (or portion of an assembled product) must meet at least one of the sustainable criteria (e.g., FSC certification, recycled content) specified in the credit. Products and materials that do not meet the location radius but do meet at least one of the sustainability criteria are valued 100% of their cost (i.e., the valuation factor is 1).
The distance is measured as the crow flies, not by actual travel distance. The point of purchase is considered the location of the purchase transaction. For online or other transactions that do not occur in person, the point of purchase is considered the location of product distribution.
Figure 1. Example material radius
For this project in western Pennsylvania, the concrete does not meet the location valuation factor requirements: even though the concrete was mixed and purchased within the 100- mile radius, its components—the silica, gravel, Portland cement, and lime—were extracted or made farther away.
For the location valuation factor of salvaged and reused materials, see MR Credit Building Product Disclosure and Optimization—Sourcing of Raw Materials, Further Explanation, Material Reuse Considerations.
Determining Material Contributions of an Assembly
Many sustainability criteria in the MR category apply to the entire product, as is the case for product certifications and programs. However, some criteria apply to only a portion of the product. The portion of the product that contributes to the credit could be either a percentage of a homogeneous material or the percentage of qualifying components that are mechanically or permanently fastened together. In either case, the contributing value is based on weight. Examples of homogeneous materials include composite flooring, ceiling tiles, and rubber wall base. Examples of assemblies (parts mechanically or permanently fastened together) include office chairs, demountable partition walls, premade window assemblies, and doors.
Calculate the value that contributes toward credit compliance as the percentage, by weight, of the material or component that meets the criteria, multiplied by the total product cost (Figure 2, Table 1).
product value ($) = Total product cost ($) x (%) product component by weight x (%) meeting sustainable criteria
Figure 2. Sustainably produced components of $500 office chair
Table 1. Example calculation for $500 office chair
1U.S. Environmental Protection Agency, epa.gov/osw/conserve/rrr/imr/cdm/pubs/cd-meas.pdf.
2European Commission Service Contract on Management of Construction and Demolition Waste, Final Report, http://www.eu-smr.eu/cdw/docs/BIO_Construction%20and %20Demolition%20Waste_Final%20report_09022011.pdf (accessed April 9, 2013).
3USGBC, Green Building Facts, USGBC, usgbc.org/ShowFile.aspx?DocumentID=18693 (accessed September 13, 2012).
4City of Seattle, LEED Projects Analysis, seattle.gov/dpd/greenbuilding/docs/dpdp022009.pdf (accessed March 26, 2013).
5ISO 14040 International Standard, Environmental management, Life cycle assessment, principles and framework (Geneva, Switzerland: International Organization for Standardization, 2006).
The Indoor Environmental Quality (EQ) category rewards decisions made by project teams about indoor air quality and thermal, visual, and acoustic comfort. Green buildings with good indoor environmental quality protect the health and comfort of building occupants. High-quality indoor environments also enhance productivity, decrease absenteeism, improve the building’s value, and reduce liability for building designers and owners1. This category addresses the myriad design strategies and environmental factors—air quality, lighting quality, acoustic design, control over one’s surroundings—that influence the way people learn, work, and live.
The relationship between the indoor environment and the health and comfort of building occupants is complex and still not fully understood. Local customs and expectations, occupants’ activities, and the building’s site, design, and construction are just a few of the variables that make it difficult to quantify and measure the direct effect of a building on its occupants2. Therefore, the EQ section balances the need for prescriptive measures with more performance-oriented credit requirements. For example, source control is addressed first, in a prerequisite, and a later credit then specifies an indoor air quality assessment to measure the actual outcome of those strategies.
The EQ category combines traditional approaches, such as ventilation and thermal control, with emerging design strategies, including a holistic, emissions-based approach (Low- Emitting Materials credit), source control and monitoring for user-determined contaminants (Enhanced Indoor Air Quality Strategies credit), requirements for lighting quality (Interior Lighting credit), and advanced lighting metrics (Daylight credit). A new credit covering acoustics is now available.
Floor Area Calculations and Floor Plans
For many of the credits in the EQ category, compliance is based on the percentage of floor area that meets the credit requirements. In general, floor areas and space categorization should be consistent across EQ credits. Any excluded spaces or discrepancies in floor area values should be explained and highlighted in the documentation. See Space Categorization, below, for additional information on which floor area should be included in which credits.
The EQ category focuses on the interaction between the occupants of the building and the indoor spaces in which they spend their time. For this reason, it is important to identify which spaces are used by the occupants, including any visitors (transients), and what activities they perform in each space. Depending on the space categorization, the credit requirements may or may not apply (Table 1).
Occupied versus unoccupied space
All spaces in a building must be categorized as either occupied or unoccupied. Occupied spaces are enclosed areas intended for human activities. Unoccupied spaces are places intended primarily for other purposes; they are occupied only occasionally and for short periods of time—in other words, they are inactive areas.
Examples of spaces that are typically unoccupied include the following:
- Mechanical and electrical rooms
- Egress stairway or dedicated emergency exit corridor
- Closets in a residence (but a walk-in closet is occupied)
- Data center floor area, including a raised floor area
- Inactive storage area in a warehouse or distribution center
For areas with equipment retrieval, the space is unoccupied only if the retrieval is occasional.
Regularly versus nonregularly occupied spaces
Occupied spaces are further classified as regularly occupied or nonregularly occupied, based on the duration of the occupancy. Regularly occupied spaces are enclosed areas where people normally spend time, defined as more than one hour of continuous occupancy per person per day, on average; the occupants may be seated or standing as they work, study, or perform other activities. For spaces that are not used daily, the classification should be based on the time a typical occupant spends in the space when it is in use. For example, a computer workstation may be largely vacant throughout the month, but when it is occupied, a worker spends one to five hours there. It would then be considered regularly occupied because that length of time is sufficient to affect the person’s well-being, and he or she would have an expectation of thermal comfort and control over the environment.
Occupied spaces that do not meet the definition of regularly occupied are nonregularly occupied; these are areas that people pass through or areas used an average of less than one hour per person per day.
Examples of regularly occupied spaces include the following:
|Airplane hangar||Hospital operating room||Private office|
|Auditorium||Hospital patient room||Reception desk|
|Auto service bay||Hospital recovery area||Residential bedroom|
|Bank teller station||Hospital staff room||Residential dining room|
|Conference room||Hospital surgical suite||Residential kitchen|
|Correctional facility cell or day room||Hospital waiting room||Residential living room|
|Data center network operations center||Hospital diagnostic and treatment area||Residential office, den, workroom|
|Data center security operations center||Hospital laboratory||Retail merchandise area and associated circulation|
|Dorm room||Hospital nursing station||Retail sales transaction area|
|Exhibition hall||Hospital solarium||School classroom|
|Facilities staff office||Hospital waiting room||School media center|
|Facilities staff workstation||Hotel front desk||School student activity room|
|Food service facility dining area||Hotel guest room||School study hall|
|Food service facility kitchen area||Hotel housekeeping area||Shipping and receiving office|
|Gymnasium||Hotel lobby||Study carrel|
|Hospital autopsy and morgue||Information desk||Warehouse materials-handling area|
|Hospital critical-care area||Meeting room|
|Hospital dialysis and infusion area||Natatorium|
|Hospital exam room||Open-office workstation|
Examples of nonregularly occupied spaces include the following:
Fire station apparatus bay
Hospital linen area
Hospital medical record area
Hospital patient room bathroom
Hospital short-term charting space
Hospital prep and cleanup area in surgical suite
Lobby (except hotel lobby)*
Residential laundry area
Residential walk-in closet
Retail fitting area
Retail stock room
*Hotel lobbies are considered regularly occupied because people often congregate, work on laptops, and spend more time there than they do in an office building lobby.
Occupied space subcategories
Occupied spaces, or portions of an occupied space, are further categorized as individual or shared multioccupant, based on the number of occupants and their activities. An individual occupant space is an area where someone performs distinct tasks. A shared multioccupant space is a place of congregation or a place where people pursue overlapping or collaborative tasks. Occupied spaces that are not regularly occupied or not used for distinct or collaborative tasks are neither individual occupant nor shared multioccupant spaces.
Examples of individual occupant spaces include the following:
Bank teller station
Correctional facility cell or day room
Data center staff workstation
Hospital nursing station
Hospital patient room
Hotel guest room
Military barracks with personal workspaces
Examples of shared multioccupant spaces include the following:
Active warehouse and storage
Auto service bay
Correctional facility cell or day room
Data center network operations center
Data center security operations center
Facilities staff office
Food service facility dining area
Food service facility kitchen area
Hospital autopsy and morgue
Hospital critical-care area
Hospital dialysis and infusion area
Hospital exam room
Hospital operating room
Hospital surgical suite
Hospital waiting room
Hospital diagnostic and treatment area
Hotel front desk
Hotel housekeeping area
Retail merchandise area and associated circulation
Retail sales transaction area
School media center
School student activity room
School study hall
Shipping and receiving office
Warehouse materials-handling area
Occupied spaces can also be classified as densely or nondensely occupied, based on the concentration of occupants in the space. A densely occupied space has a design occupant density of 25 people or more per 1,000 square feet (93 square meters), or 40 square feet (3.7 square meters) or less per person. Occupied spaces with a lower density are nondensely occupied.
Table 1 outlines the relationship between the EQ credits and the space categorization terms. If the credit is listed, the space must meet the requirements of the credit.
Table 1. Space types in EQ credits
Table 2 outlines the relationship between the EQ credits and the space categorization terms specific to each rating system (see Definitions). Unless otherwise stated, if the credit is listed, the space must meet the requirements of the credit.
Table 2. Rating system–specific space classifications
*Hotel guest rooms are excluded from the credit requirements.
The following credits are not affected by space classifications:
- Environmental Tobacco Smoke Control
- Enhanced Indoor Air Quality Strategies, Option 1 A
- Enhanced Indoor Air Quality Strategies, Option 1 B
- Enhanced Indoor Air Quality Strategies, Option 2 A
- Enhanced Indoor Air Quality Strategies, Option 2 D (no specific spaces; applicable spaces are determined by the project team)
- Low-Emitting Materials
- Construction Indoor Air Quality Management Plan
- Indoor Air Quality Assessment, Option 1, Flush-Out (the floor area from all spaces must be included in calculation for total air volume; the flush-out must be demonstrated at the system level.)
- Interior Lighting, Option 2, strategy B
- Interior Lighting, Option 2, strategy C
- Interior Lighting, Option 2, strategy F
- Acoustic Performance (Healthcare)
Pay extra attention to how the following types of spaces are classified in specific credits.
- Minimum Indoor Air Quality Performance and Environmental Tobacco Smoke have specific requirements and considerations for residential projects.
- See the Project Type Variations sections in Thermal Comfort and Interior Lighting for guidance on providing appropriate controllability in residential buildings.
- Exceptions to Daylight and Quality Views are permitted. See the Project Type Variations sections in Daylight and Quality Views.
- See the Project Type Variations section in Thermal Comfort for guidance on dealing with high levels of physical activity.
- Exceptions to Quality Views are permitted. See the Project Type Variations section in Quality Views.
- For Thermal Comfort and Interior Lighting, Option 1, Lighting Control, most of the areas in a transportation terminal can be considered shared multioccupant. Most areas in transportation terminals are also regularly occupied.
Dormitories and Military Barracks
- These spaces fall in-between a work space and residence.
- Dorm rooms or military barracks with personal workspaces are considered individual occupant spaces. Military barracks without personal workspaces are considered shared multioccupant.
- For Thermal Comfort and Interior Lighting, Option 1, Lighting Control, most of the active warehouse and storage areas are considered multioccupant.
- Most areas in industrial facilities are also regularly occupied.
Sustainable design strategies and measures are constantly evolving and improving. New technologies are continually introduced to the marketplace, and up-to-date scientific research influences building design strategies. The purpose of this LEED category is to recognize projects for innovative building features and sustainable building practices and strategies.
Occasionally, a strategy results in building performance that greatly exceeds what is required in an existing LEED credit. Other strategies may not be addressed by any LEED prerequisite or credit but warrant consideration for their sustainability benefits. In addition, LEED is most effectively implemented as part of a cohesive team, and this category addresses the role of a LEED Accredited Professional in facilitating that process.
Because some environmental issues are particular to a locale, volunteers from USGBC chapters and the LEED International Roundtable have identified distinct environmental priorities within their areas and the credits that address those issues. These Regional Priority credits encourage project teams to focus on their local environmental priorities.
USGBC established a process that identified six RP credits for every location and every rating system within chapter or country boundaries. Participants were asked to determine which environmental issues were most salient in their chapter area or country. The issues could be naturally occurring (e.g., water shortages) or man-made (e.g., polluted watersheds) and could reflect environmental concerns (e.g., water shortages) or environmental assets (e.g., abundant sunlight). The areas, or zones, were defined by a combination of priority issues—for example, an urban area with an impaired watershed versus an urban area with an intact watershed.
The participants then prioritized credits to address the important issues of given locations. Because each LEED project type (e.g., a data center) may be associated with different environmental impacts, each rating system has its own RP credits.
The ultimate goal of RP credits is to enhance the ability of LEED project teams to address critical environmental issues across the country and around the world.
1 U.S. Environmental Protection Agency, Health Buildings Healthy People: A Vision for the 21st Century, epa.gov/iaq/pubs/hbhp.html (October 2001) (accessed July 25, 2013).
2 Mitchell, Clifford S., Junfeng Zhang, Torben Sigsgaard, Matti Jantunen, Palu J. Lioy, Robert Samson, and Meryl H. Karol, Current State of the Science: Health Effects and Indoor Environmental Quality, Environmental Health Perspectives 115(6) (June 2007).