D. JEFF BURTON, MS, PE, FAIHA (former CIH and CSP, VS), is an industrial hygiene engineer with broad experience in ventilation used for emission and exposure control. He is chair of the ANSI/ASSP Z9.10 subcommittee responsible for the Fundamentals Governing the Design and Operation of Dilution Ventilation Systems in Industrial Occupancies standard. He is also an adjunct faculty member at the Rocky Mountain Center for Occupational and Environmental Health at the University of Utah in Salt Lake City.
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Dilution Ventilation
A Requirement for Every Indoor Occupancy

Dilution ventilation is the addition of fresh, clean air into potentially contaminated air to reduce the concentration of airborne contaminants—for example, vapors, gases, viral aerosols, particles, or smoke—to acceptable exposure levels chosen by the OEHS professional. These levels could be the OSHA Action Level or a certain percentage of a relevant occupational exposure limit, but they could also refer to levels “below the odor threshold,” “non-detectable levels,” and so forth.
The old mantra “dilution is the solution to pollution” is still valid.
This article provides an overview of the basics of dilution ventilation and introduces examples of techniques and relationships that can enhance our usage of dilution ventilation.
CONTROLS Although we recognize that emission source control, where possible, is important for cost-effectively controlling emissions and exposures, the old mantra “dilution is the solution to pollution” is still valid, and dilution ventilation is always required as a backup and enhancement for source controls.
Dilution ventilation is often used as the primary exposure control where source control is inadequate, not entirely reliable, impractical, or too expensive, and when standards require it. It is most cost effective when air pollutants are relatively lower-toxicity vapors, gases, or fine particles; when emissions occur uniformly in time, are widely dispersed, and do not occur close to occupants’ breathing zones; and where moderate climatic conditions prevail and the dilution air is much less contaminated than the air to be diluted.
In some cases, a major source control, such as local exhaust ventilation, also provides dilution ventilation through the makeup or replacement air required for the local exhaust systems. HVAC systems routinely provide outdoor air (OA) dilution ventilation while heating and cooling the environment.
STANDARDS AND GUIDELINES Three resources in particular can be very useful for dilution ventilation. ANSI/ASHRAE Standard 62.1-2019, Ventilation for Acceptable Indoor Air Quality, specifies minimum ventilation rates and other measures for new and existing buildings and is intended to provide indoor air quality that is acceptable for humans. ANSI/ASSP Z9.10-2017, Fundamentals Governing the Design and Operation of Dilution Ventilation Systems in Industrial Occupancies, covers most occupational environments. The ACGIH publication Industrial Ventilation: A Manual of Recommended Practices provides standards and guidelines for industrial occupancies. See the resources list on page 11 for information on these and other standards.
TYPICAL ISSUES OEHS professionals often face many questions when applying dilution ventilation controls. For example, how much dilution air is necessary to achieve the desired air conditions? How much dilution air is flowing, and what does it actually accomplish?
Suppose a small amount of solvent is evaporating into an occupied space, so you open some windows and fresh air flows in. What is the solvent vapor emission rate? At what volume flow rate is fresh air entering the space, and how well does it mix with and dilute the vapor? What are the final vapor concentrations in the air?
The following examples introduce some basic approaches to dilution ventilation and equations that can answer some of our typical questions. Equation derivations and other information are available in the resources.
Estimation of fresh OA being delivered to a space. Almost always, some percentage of the supply air in an HVAC system is outdoor fresh air, which provides dilution ventilation for fugitive emissions into the air that occur in the space during regular manufacturing, school, or office activities.
Equation 1: In a small warehouse that also includes office spaces, carbon dioxide concentrations in the return air (RA), supply air (SA), and outdoor air ducts are Cra = 750 ppm, Csa = 650 ppm, and Coa = 420 ppm. These measurements can be used to estimate the percentage of OA in the SA:
If the total amount of HVAC air being supplied to the space (Q) is 2,300 cubic feet per minute (cfm), then about 30 percent, or 700 cfm, is fresh outdoor air, which is useful for dilution. About 1,600 cfm of the air is being recirculated within the warehouse and office spaces.  Equation 2: There are 160 people (P) in a small school where the steady-state average concentration (Cs) of carbon dioxide near noon is 890 ppm, and the concentration of carbon dioxide outside (Coa) is about 420 ppm. How much outdoor air is being delivered? How much per student?
Using carbon dioxide concentrations to estimate the amount of OA reaching a specific area in a building. During the day, the concentration of CO2 usually builds up to some asymptote—for example, 1,000 ppm. Measuring the decay rate of the CO2 after all people leave a building (or HVAC zone) can provide an estimate of the amount of fresh OA being delivered to a location in the space. For example, suppose the initial average carbon dioxide concentration (Ci) at an office suite is approximately 1,000 ppm at 5:00 p.m. when everyone has departed from the building. The ventilation system continues to run. By 6:00 p.m. the concentration (Ca) in the office suite has been reduced to 550 ppm. The outdoor concentration of CO2 is about 420 ppm. How many fresh air changes per hour (N) does this suggest? (Note: “ln” is the natural log.)
What would the dilution airflow rate (Qd) be for an office suite volume of 16,000 cubic feet?
Estimation of emission and dilution rates. What is the emission rate (q) if 0.5 gallon of toluene is evaporated uniformly from a wide area of process equipment into a small factory’s air over an eight-hour shift? Assume standard temperature and pressure (STP). The air density correction factor (d) is 1.0. For toluene, the specific gravity (SG) is 0.866 and the molecular weight (MW) is 92.1. Note: at STP, the volume of vapor formed from the evaporation of 1 lb.-mole of a liquid is 387 cubic feet. To convert volume of toluene to weight, use the weight of a gallon of water in pounds (8.31 lbs.):
With an estimate of the emission rate, an appropriate dilution rate can be estimated. What dilution ventilation airflow rate (Qd) is required for dilution of the airborne vapor to an average concentration (C) of 10 ppm if the air mixing factor (Kmix) is 1.4? (See the March 2021 Synergist for an article on mixing factors.)
Estimation of dilution airflow into buildings due to wind blowing. The wind can provide dilution ventilation of spaces that can open upwind and downwind windows or doors. For the factory mentioned above, and with a wind speed (Vw) of 5 mph blowing against the building, what open window or door areas are necessary to achieve the estimated dilution airflow (Qd) of about 4,400 cfm through the building due to wind? Note: The angle factor for wind blowing on a building (Kw) is 0.3 for oblique winds. Aw represents the smaller of the total open areas, upwind or downwind. Use the constant 88 to convert mph into feet per minute (fpm).
Dilution ventilation is a complex subject that I will expand on in future articles. In the meantime, see the resources for more information, equations, and explanations.

ACGIH: Industrial Ventilation: A Manual of Recommended Practices, 29th edition, chapters 4 and 10.
AIHA: IAQ and HVAC Workbook, 4th ed., chapters 2 and 13 (2017).
AIHA: Industrial Ventilation Workbook, 7th ed., chapters 4, 11, 17.
AIHA: Useful Equations: Practical Applications of OH&S Math, chapters 8 and 13.
American National Standards Institute: ANSI/ASHRAE 62.1-2019, Ventilation for Acceptable Indoor Air Quality (2019).
American National Standards Institute: ANSI/ASSP Z9.10-2017, Fundamentals Governing the Design and Operation of Dilution Ventilation Systems in Industrial Occupancies (2017).
ASHRAE: ASHRAE Handbook (2021).
ASHRAE: Standard 62.1-2019, Ventilation for Acceptable Indoor Air Quality (2019).
Building Science Corporation: “Air Flow Control in Buildings” (October 2007).
The Synergist: “Six Ways to Approximate Airflow” (June 2018).
World Health Organization: Natural Ventilation for Infection Control in Health-Care Settings, part 2: Designing for Natural Ventilation, chapter 4: Understanding Natural Ventilation (2009).