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ANSI/ASHRAE Standard 62.1-20101 includes three alternative procedures for determining minimum outdoor airflow (OA) rates: the ventilation rate procedure (VRP), the indoor air quality procedure

(IAQP), and the natural ventilation procedure (NVP).

The VRP prescribes minimum zone-level OA rates and procedures to find system-level OA intake rates. The IAQP allows compliance based on contaminant concentrations and perceived air qual-ity, which constitute IAQ performance criteria. The NVP prescribes minimum outdoor air opening sizes to ensure lo-cal natural (passive) ventilation; in most cases, naturally ventilated zones must use mixed-mode ventilation, so either the VRP or the IAQP must also be ap-plied to determine required mechanical ventilation whenever passive ventilation is undesirable or ineffective.

Previous articles2,3 have described compliance with the VRP require-ments. Because most engineers use the VRP, most building codes base minimum outdoor airflow require-ments on it, and several energy la-beling programs and building rating systems require compliance with the VRP. This article, however, describes compliance with the IAQP require-ments (see sidebar IAQP Require-ments); although not widely used (primarily because it requires signifi-cant judgment on the part of the de-signer), this valid compliance alter-

native deserves a detailed discussion and consideration in some cases.

Summary of the IAQPThe following steps must be complet-

ed to find the minimum required breath-ing zone OA rate for each zone using the IAQP:

1. Identify all contaminants-of-con-cern and mixtures-of-concern (see side-bar Mixtures-of-Concern) for the zone.* For simplicity, this article refers to all indoor contaminants and mixtures and mixture constituents as zone pollut-ants.

2. Identify both indoor and outdoor sources for each zone pollutant.

3. Determine the emission rate for each zone pollutant from each of its identified sources.

4. Specify a concentration limit and exposure time for each individual con-taminant, citing an appropriate cog-

About the AuthorDennis Stanke is staff applications engineer for Trane Commercial Systems, Ingersoll Rand, in La Crosse, Wis. He is chair of SSPC 189.1.

By Dennis Stanke, Fellow ASHRAE

*Although Section 6.3.1 seems to require identification of either contaminants-of-concern or mixtures-of-concern, designers are likely to interpret this as a requirement to identify all contaminants-of-concern and all mixtures-of-concern; it just makes sense. The Standard 62.1 committee should be asked to interpret this requirement.

Minimum Outdoor Airf low Using the IAQ Procedure

This article was published in ASHRAE Journal, June 2012. Copyright 2012 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

June 2012 ASHRAE Jou rna l 27

nizant authority (see sidebar Cognizant Authority). Specify concentration limits and exposure times for constituents of mixtures based on threshold limit values (TLV), for instance, or reference exposure limits (REL) as shown later.

5. Specify the design limit for perceived IAQ in terms of the minimum percentage of occupants or visitors expressing satisfaction with the IAQ in the completed zone.

6. Using mass balance calculations, determine the mini-mum breathing zone OA rate necessary to meet the concentra-tion established in Item 4 for each individual contaminant and for each mixture.

7. Find the perceived-IAQ OA rate required to meet the de-sign limit specified in Item 5 by either:

a. Conducting a subjective evaluation in the completed zone at different OA rates; repeat until the OA rate results in perceived IAQ, meeting the criteria specified in Item 5, or

b. Establishing that the current zone design meets the cri-teria for a substantially similar zone wherein a subjective evaluation has been conducted and the minimum perceived IAQ specified has been achieved; the breathing zone OA rate in the new zone must be no less than that determined for the similar zone.

8. Find the minimum breathing zone OA rate, i.e., find the largest OA rate among those determined by mass balance for individual contaminants and for mixture, and by subjective evaluation of perceived IAQ.

9. Finally, find system-level outdoor air intake flow based on the breathing zone OA rate found for each zone, using the appropriate system equations provided in Appendix D for single-zone systems or the calculation procedures specified by the VRP for 100% outdoor-air and multiple-zone systems.

The standard allows designers to apply the IAQP in some zones within a system, and the VRP in other zones, and it re-quires designers to document design assumptions and calcula-tions, but here we focus on the nine steps listed.

Identify Zone Pollutants: Contaminants and Mixtures-of-Concern

The ASHRAE Handbook5 classifies air contaminants as ei-ther particulate matter or gases. Designers must consider both when identifying potential contaminants-of-concern. Design-ers must also consider mixtures-of-concern to comply with the IAQP.

Particulate matter comprises both solid and liquid particles, including bioaerosols.

Gaseous contaminants comprise organic and inorganic gas-es. Particles and gases may be contaminants-of-concern indi-vidually, constituents of mixtures-of-concern, or both.

Mixtures-of-concern can be divided into two categories: source-related mixtures and impact-related mixtures. Source mixtures comprise a wide range of contaminants originating from a known source or process (e.g., diesel fumes or tobacco smoke); these mixtures can usually be kept out of the zone by, for example, proper intake placement, prohibiting indoor smoking or making proper use of local exhaust, so they are not considered

6.3 Indoor Air Quality (IAQ) Procedure. Breathing zone outdoor airflow (Vbz) and/or system outdoor air intake flow (Vot) shall be determined in accordance with Sections 6.3.1 through 6.3.5.

6.3.1 Contaminant Sources. Contaminants or mixtures of concern for purposes of the design shall be identi-fied. For each contaminant or mixture of concern, indoor sources (occupants and materials) and outdoor sources shall be identified, and the emission rate for each con-taminant of concern from each source shall be deter-mined.

6.3.2 Contaminant Concentration. For each contami-nant of concern, a concentration limit and its correspond-ing exposure period and an appropriate reference to a cognizant authority shall be specified.

6.3.3 Perceived Indoor Air Quality. The design level of indoor air acceptability shall be specified in terms of the percentage of building occupants and/or visitors expressing satisfaction with perceived IAQ.

6.3.4 Design Approach. Zone and system outdoor airflow rates shall be the larger of those determined in accordance with Section 6.3.4.1 and either 6.3.4.2 or 6.3.4.3, based on emission rates, concentration limits, and other relevant design parameters (e.g., air cleaning efficiencies and supply airflow rates).

6.3.4.1 Mass Balance Analysis. Using a steady-state or dynamic mass-balance analysis, determine the mini-mum outdoor airflow rates required to achieve the con-centration limits specified in Section 6.3.2 for each con-taminant or mixture of concern within each zone served by the system.

6.3.4.2 Subjective Evaluation. Using a subjective oc-cupant evaluation conducted in the completed building, determine the minimum outdoor airflow rates required to achieve the level of acceptability specified in Section 6.3.3 within each zone served by the system.

6.3.4.3 Similar Zone. The minimum outdoor airflow rates shall be no less than those found in accordance with Section 6.3.4.2 for a substantially similar zone (i.e., in a zone with identical contaminants of concern, concentra-tion limits, air cleaning efficiency, and specified level of acceptability; and with similar contaminant sources and emission rates).

6.3.5 Combined IAQ Procedure and Ventilation Rate Procedure. The IAQ Procedure in conjunction with the Ventilation Rate Procedure may be applied to a zone or system. In this case, the Ventilation Rate Procedure shall be used to determine the required zone minimum outdoor airflow, and the IAQ Procedure shall be used to determine the additional outdoor air or air cleaning necessary to achieve the concentration limits of the con-taminants of concern.

IAQP Requirements

28 A SHRA E Jou rna l ash rae .o rg J u n e 2 0 1 2

here. Impact mixtures comprise individual contaminants known to impact the same human organ (e.g., those which impact the respiratory system, the eyes or the nervous system). Like individ-ual contaminants, mixture constituents must be dealt with using source control, air cleaning, and/or dilution ventilation.

This article lumps all contaminants-of-concern and mix-tures-of-concern as zone pollutants. Zone pollutants gener-ated indoors, as well as those from outdoor sources, must be identified. Some lists of potential indoor and outdoor contami-nants1,6 exist, but a complete list of zone pollutants has not been published. A complete list(s) would include particles, gases, and constituents of both source and impact mixtures, perhaps similar to Table 1. (Note: The remainder of this article ignores source mixtures; such mixtures can be most easily and successfully treated by not allowing the source mixture into the building by, for instance, prohibiting smoking or locating outdoor air intakes a substantial distance from idling trucks or buses.) Without such a list, designers must use a variety of resources to identify zone pollutants.

Its clear the most important step in the IAQPestablishing zone pollutants, including both contaminants- and mixtures-of-concernis not easy and requires considerable designer judgment.

Sources of ContaminantsHaving established a list of zone pollutants, the sources of

each must be identified. Possible indoor sources include peo-ple, processes, plants, furnishings, wall coverings, cleaning products, paints, adhesives, ceiling tiles, plywood, carpet and so on. Possible outdoor sources include the outdoor air (see sidebar What About Outdoor Air Quality?), motor vehicles, industrial operations, dumpsters, cooling towers, cooking ex-haust, plants, and so on.

Since there are so many potential zone pollutants, accu-rately identifying the major sources for each can be very time consuming for the designer.

Source StrengthWith a list of sources for each contaminant-of-concern and

impact-mixture constituent, source strengths must next be de-termined. All sources must be evaluated for each contaminant to determine the total emission rate. For instance, if formalde-hyde has been identified as a contaminant-of-concern and/or a mixture constituent, its emission rate from all potential sources must be determined. Emission rates for various contaminants from various products have been established, but for many con-taminants and for many sources, source strengths must be de-termined using appropriate field or materials-laboratory testing. Contaminant source strength for people-related zone pollutants can be significant, and it depends on factors such as age, gender, diet and activity level.

Determining source strengths for each contaminant and mixture constituent can be expensive and time consuming as well as risky, but mass balance analysis depends on emission rates, so they must be found or estimated.

Establish Target ConcentrationsFor each contaminant- and mixture-of-concern, designers

must cite an acceptable concentration and exposure-time limit established by a cognizant authority. Different au-thorities often list different acceptable limits and exposure times for the same contaminantthe designer must judge which authority to rely upon. Cognizant authorities have not established acceptable limits for mixtures, so designers tend to ignore this requirement. But, cognizant authorities have established threshold limit values (TLVs) for many impact-mixture constituents and ACGIH has established a target limit of 1.0 for the sum of the ratios of constituent

A mixture-of-concern may be either a complex mix-ture of contaminants from a specific source (such as die-sel exhaust or environmental tobacco smoke), or it may be a group of individual contaminants from one or more sources, where each contaminant is known to impact the same human organ or system (such as the respiratory system, the eyes, the skin, and so on). Here, we refer to the first type as source-mixtures and the second type as impact-mixtures. Standard 62.1-2010 doesnt cur-rently make this distinction, although it probably should in the future.

Mixtures-of-Concern

Standard 62.1-2010 defines cognizant authority as an agency or organization that has the expertise and jurisdiction to establish and regulate concentration limits for airborne contaminants; or an agency or organization that is recognized as authoritative and has the scope and expertise to establish guidelines, limit values, or con-centrations levels for airborne contaminants.

Cognizant Authority

Outdoor air introduced into a building can dilute and remove indoor-source zone pollutants, but it can also add contaminants and mixtures from outdoor sources. Some outdoor pollutants (such as ozone, particulate mat-ter, and odors) can be reduced by air cleaning before they enter the building, but some (carbon monoxide, for instance) cannot. In any case, contaminants from outdoors must be considered either individually, as a mixture con-stituent, or both. But, remember, most indoor-source zone pollutants are not present in the outdoor air, so even dirty outdoor air can dilute and remove indoor-source pollutants. Some outdoor airflow rate is always needed, regardless of the procedure used to find it.

What About OA Quality?

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Pollutant Category

Type Examples Example Sources Impact

Particulate Matter

Solid

Dust (Less than 100 mm) Wind, Volcanoes, Grinding, De-

molition, Burning Fuels, Unpaved Roads, etc.

Asthma, Nose and Throat Irritation, Lung Damage

BioaerosolsLiving Things, Including Viruses,

Bacteria, Fungi, Mites, Plants and Insects

Allergies, Asthma, Various Diseases, Including Tuberculosis, Legionnaires

Disease, and Influenza

Liquid

Mist Cooling TowersEvaporation May Increase

Concentration of Bioaerosols

Fog CondensationEvaporation May Increase

Concentration of Bioaerosols and Very Small Particles

Gaseous Contaminants

Inorganic

Ammonia Cleaning Products Respiratory Irritant

OzoneElectrostatic Appliances, Printers,

Copiers, Outdoor AirReduced Lung Function,

Asthma, Eye Irritation

Carbon MonoxideIncomplete Combustion,

Outdoor AirRespiratory System Damage

Radon Soil Lung Cancer

Organic

FormaldehydeBuilding Materials, Adhesives,

InsulationEye, Nose, and Throat Irritation, Asthma, Respiratory Symptoms

BenzeneBurning, Automobile Exhaust,

Some Glues, Paints, and Detergents

Cancer

Other Volatile Organic Compounds (VOCs)

People, Processes, Cleaning Products

Odor, Wide Range of Physical Symptoms

Mixtures

Source Mixtures

Smoke Burning Organic Material See Particulate Matter

Tobacco Smoke Burning Tobacco Cancer; See Particulate Matter

Diesel Exhaust Diesel Fuel Combustion Cancer; See Particulate Matter, Inorganic and Organic Gases

Smog Products of Combustion See Ozone and Particulate Matter

Impact Mixtures*

Example Constituents: Acetalde-hyde, Acrolein, Ammonia, etc.

Multiple Sources Respiratory System Impact

Example Constituents: Arsenic, Benzene, Carbon Disulfide, etc.

Nervous System Impact

Example Constituents: Benzene, Butadiene, Carbon Disulfide, etc.

Reproductive System Impact

*Example impact-mixture categories based on Office of Environmental Health Hazard Assessment (OEHHA) table (see www.oehha.org/air/allrels.html).

Table 1. Zone pollutants.

concentrations to TLVs (see sidebar Impact Mixture Ad-ditivity).

Since different concentration limits can be selected from different cognizant authorities, designer judgment once again must be used.

Perceived IAQA minimum target level for perceived IAQ must be speci-

fied for each zone. For instance, the designer might specify that at least 70% of visitors to a zone must express satisfac-tion with the air quality. Setting this target level high means more filtration or higher OA rates for the zone while setting it low may lead to odor complaints. Determining the minimum

perceived-IAQ target places another judgment burden on the designer.

Mass Balance CalculationsGiven the emission rates and source strengths found above,

and accounting for the efficiency of air cleaning devices, the standard requires mass-balance calculations to determine the minimum breathing zone OA rate needed to achieve the target concentration limit for each contaminant, and the target sum-of-constituent-ratios (1.0) for each impact mixture (see sidebar Im-pact Mixture Calculations). For individual contaminants these calculations can be time consuming but concentration limits are clear. For impact mixtures, constituent concentrations must be

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found repeatedly by adjusting OA rate, for instance, or filtra-tion efficiency, until the sum-of-constituent-ratios is less than or equal to 1.0. The mass-balance calculations result in a mini-mum breathing zone OA rate for each zone contaminant- and impact-mixture-of-concern. Each zone requires the highest of these rates to ensure acceptable objective IAQ.

Most designers using the IAQP dont identify impact mix-turesthey simply focus on the concentration limits for spe-cific contaminants, and assume they need only introduce the OA needed for the worst-case contaminant. They dont make the additivity adjustments, which are the basis for the rates and calculations in the ventilation rate procedure. By ignoring the additive nature of impact-mixtures, designers purporting to use the IAQP often find inappropriately low OA rates. Many studies in offices show the effective OA rate must be between 15 cfm to 20 cfm (425 L to 566 L) per person for at least 80% visitor satisfaction. If an IAQP design (without air cleaning) for an office zone results in an effective OA rate significantly less than 15 cfm (425 L) per person, the additive quality of impact-mixtures has probably been overlooked.

According to Informative Appendix B, contaminants that act on (impact) the same organ systems should be analyzed and sufficient OA rate or air cleaning should be provided so that the sum of concentration ratios for the impact mixture does not exceed 1.0.

When many chemicals are present in the air, as they al-most always are in indoor air, then some way of addressing potential additive effects is warranted. The ACGIH guid-ance on the subject instructs that when two or more sub-stances acting on the same organ system are present, their combined effect, rather than that of either individually, should be given primary consideration. Information on af-fected organs is readily available on the websites of the

Impact Mixture Additivity cited references for OEHHA and ATSDR. If no contradic-tory information is available, the effects of the different substances should be considered as additive. A formula is given wherein the ratios of the concentrations of each sub-stance with the same health-related endpoint to the thresh-old limit value for each substance are added. If the sum of all these ratios exceeds unity, then it is considered that the concentration value has been exceeded.

C

T

C

T

C

Tn

n

1

1

2

2

1 0+ ++ .

whereCi = the airborne concentration of the substance.Ti = the threshold limit value of that substance.

According to the ACGIH, when two or more substances acting on the same organ system are present, their combined effect, rather than that of either individually, should be given priority. One way to do this is to: 1) find the steady-state concentration of each individual constitu-ent, given an OA rate, generation rate, air cleaning ef-ficiency, and so on, 2) find the ratio of each constituent concentration to its TLV, and 3) add all the ratios together. If the ratio sum exceeds 1.0, the concentration of one (or more) of the constituents must be lowered by increasing the OA rate, decreasing the generation rate, and/or in-creasing air cleaning efficiency for that constituent. Re-peat Steps 1 through 3 until the sum of the concentration ratios is less than 1.0. This results in an acceptable OA rate for the impact-mixture being considered. Of course,

Impact-Mixture Calculations these calculations must be repeated for each identified impact-mixture.

For example, assume that a project designer identifies a particular zone with a respiratory system impact mix-ture comprising two constituents: acetaldehyde (8-hr Ref-erence Exposure Level of 300 g/m3) and acrolein (8-hr REL of 0.7 g/m3). (Note: This example uses inhalation RELs published by OEHHA, an agency of the California Environ-mental Protection Agency.) If a mass balance analysis at a given breathing zone OA rate shows that the concentra-tion of acetaldehyde rises to 200 g/m3 and acrolein rises to 0.3 g/m3, the sum-of-ratios described above results in 200/300 + 0.3/0.7 = 0.67 + 0.43 = 1.10. Since this sum exceeds 1.0, increasing breathing zone OA rate slightly lowers the concentration of each constituent, so that the ratio is equal to or less than 1.0.

Subjective EvaluationSubjective minimum OA rates must also be found to en-

sure acceptable perceived IAQ in terms of occupant or visitor satisfaction. This requires a subjective evaluation of air quality using either of two approaches:

a. After completion of design, construction and instal-lation, but prior to full operational occupancy, a subjective evaluation must be conducted in each IAQP zone. While the standard does not require a specific subjective evaluation pro-cedure, informative Appendix B describes one such proce-dure. If the initial breathing zone OA rate based on the mass balance calculations results in less than the target satisfaction percentage level specified, action must be taken (e.g., increase the OA rate, reduce emissions by replacing some building ma-terials or furnishings, or increase air cleaning for odors) and the subjective evaluation must be repeated until the specified level of acceptability is achieved.

b. Alternatively, designers may identify a substantially similar zone in a previously constructed building that was successfully evaluated using the subjective evaluation ap-

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proach described above. The OA rate used in the similar zone establishes the minimum OA rate needed to satisfy the subjec-tive evaluation requirement. Of course, identifying a similar zone can be difficult, especially for zones in one-of-a-kind buildings. A substantially similar zone has similar pollutant sources and emission rates, similar or identical contaminants, source-mixtures and impact-mixtures, identical concentration limits, identical air-cleaning efficiency for each pollutant, and an identical specified level of perceived acceptability.

SummaryThe IAQP is a valid procedure that

has apparently been successfully ap-plied to design ventilation systems in some buildings, especially where the same building design with substantial-ly similar zones is repeated in multiple locations. However, for one-of-a-kind buildings, the IAQP requires designers to make many difficult judgments. In so doing, the required minimum OA rates found using the IAQP are likely to vary considerably from one designer to the next, and may be too low in many cases due to incomplete evaluation of the ad-ditive nature of impact-mixtures.

For these reasons, many designers are uncomfortable with this approach. Furthermore, most mechanical codes require a prescriptive procedure based on the VRP, and only allow the IAQP as a code variance.

Perhaps more designers will use a future version of the IAQP, when more is known about mixtures, sources, air cleaning and subjective evaluation and when designers can make fewer judgments with more con-fidence. But for now, most designers con-tinue to use the prescriptive VRP to reduce design time, cost, and risk.

References1. ANSI/ASHRAE Standard 62.1-2010,

Ventilation for Acceptable Indoor Air Quality.

2. Stanke, D. 2005. Standard 62-2001 Ad-dendum 62n: single-path multiple-zone sys-tem design. ASHRAE Journal 47(1):28 35.

3. Stanke, D. 2005. Standard 62.1-2004: designing dual-path, multiple-zone systems ASHRAE Journal 47(5):2028.

4. 2009 ASHRAE HandbookFundamen-tals, Chapter 11.

5. 2009 ASHRAE HandbookFundamen-tals, Chapter 11, Tables 11 and 12.

6. ASHRAE. 2010. 62.1 Users Manual: ANSI/ASHRAE Standard 62.1-2010.

Minimum Outdoor Airflow RatesThe minimum zone OA rate determined using the mass bal-

ance calculations must be compared to the rate determined using subjective evaluation. The higher of these two values es-tablishes the minimum breathing zone OA rate for each IAQP zone. Based on these minimum zone OA rates, system-level outdoor air intake flow can be found using the equations in Appendix D of Standard 62.1-2010 (for single-zone systems) or using the equations in the VRP (for multiple-zone systems).

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