Ventilation in Buildings:

Air changes per hour, HEPA filters, Clean Air Delivery Rate

Excerpted from Center for Disease Control and Prevention (updated 5/12/2023)

How Much Ventilation Is Enough?

Aim for 5 Air Changes per Hour (ACH)

When possible, aim for 5 or more air changes per hour (ACH) of clean air to help reduce the number of germs (and allergens) in the air.

This can be achieved through any combination of central ventilation system, natural ventilation, or additional devices that provide equivalent ACH (eACH) to your existing ventilation. Supplying or exhausting an amount of air (use the larger of the two values but do not add them together) that is equal to all the air in a space is called an air change. Multiplying that amount by 5 and delivering it over one hour results in 5 ACH.

To calculate the ACH (or eACH):

  1. Determine (or measure) the airflow through the system in cubic feet per minute (cfm).
  2. Determine the area of the room = length (ft) x width (ft)
  3. Determine the height of the room (ft).
  4. Calculate ACH:
    ACH = (cfm x 60) / (Area x Height)
  5. When multiple strategies are used, repeat the ACH calculation for each system then add together for a total ACH value (which could be compared to the minimum 5 ACH recommendation).

Note: See What is a HEPA filter and why use an an-room HEPA air cleaner? (below)

for examples on how the ACH calculation may be applied.

While there is insufficient science to identify an optimum ventilation strategy for all spaces, 5 ACH is what portable air cleaners provide (as eACH) when properly sized following the Environmental Protection Agency’s guidance [2.9 MB, 7 pages] on the selection of portable air cleaners. Five ACH will not guarantee totally safe air in any space, but it reduces the risk of exposure to germs and other harmful air contaminants.

Rather than a hard-and-fast rule, the 5 ACH target provides a rough guide to air change levels likely to be helpful in reducing infectious particles. For example, increasing ventilation from 2 to 5 ACH substantially reduces the time to remove airborne contaminants.

  • Large volume spaces with very few occupants (e.g., a warehouse) may not require 5 ACH and spaces with high occupancy or higher-risk occupants may need higher than 5 ACH.
  • While ACH levels higher than 5 (e.g., those used in airborne isolation rooms in hospitals) may reduce infectious aerosols further, the potential benefits of increased ventilation should be balanced with the additional upfront, periodic maintenance, and energy costs that may be incurred.
  • Some limited studies have demonstrated this protective benefit of increased ACH, although an optimum number remains uncertain.
  • A Lancet Commission Report [249 KB, 33 pages] that draws on available scientific evidence proposes ACH levels of 4 as “Good,” 6 as “Better,” and >6 as “Best,” underscoring that ACH (to include eACH) represents a continuum.
  • It is unknown exactly how much this will reduce the risk of getting a viral infection in an indoor space.
  • However, the improvements are reasonable for indoor environments when additional protection is desired. More research is needed to evaluate the influence of central ventilation, portable air cleaning, and UV air treatment on respiratory infectious disease transmission.

† Some air cleaning and air treatment devices do not bring in outdoor air. Instead, they clean or treat the indoor air to reduce the concentration of infectious particles. Thus, they give eACH without the need or expense of conditioning outdoor air. Note that eACH depends on the contaminant. An air treatment device that provides eACH for particles may not be effective against other contaminants such as gases and vapors.

What is a HEPA filter and why use an in-room HEPA air cleaner?

By definition, a High Efficiency Particulate Air (HEPA) filter is at least 99.97% efficient at capturing particles 0.3 µm in size. This 0.3 µm particle approximates the most penetrating particle size (MPPS) through the filter. HEPA filters are even more efficient at capturing particles larger and smaller than the MPPS. Thus, HEPA filters are no less than 99.97% efficient at capturing human-generated viral particles associated with COVID-19.

Research shows that the particle size of SARS-CoV-2 is around 0.1 micrometer (µm). However, the virus generally does not travel through the air by itself. These viral particles are human-generated, so the virus is trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are larger. Most of the respiratory droplets and particles exhaled during talking, singing, breathing, and coughing are less than 5 µm in size.

The excellent capture efficiency of HEPA filters comes at a cost, namely the significant pressure drop (and energy) required to move air through the HEPA filter. For this reason, most traditional HVAC systems are not able to use HEPA filters and are limited to the use of less-efficient filters. To account for the increased pressure requirements, HEPA filtration units often combine a HEPA filter with a dedicated fan system.

In-room HEPA air cleaners that combine a HEPA filter with a powered fan system are a preferred option for auxiliary air cleaning, especially in higher risk settings such as health clinics, vaccination, and medical testing locations, workout rooms, or public waiting areas. Other settings that could benefit from in-room HEPA filtration can be identified using typical risk assessment parameters, such as community incidence rates, facemask compliance expectations, and room occupant density. While these systems do not bring in outdoor dilution air, they are effective at cleaning air within spaces to reduce the concentration of airborne particulates, including SARS-CoV-2 viral particles. Thus, they give equivalent air exchanges without the need for conditioning outdoor air.

In choosing an in-room HEPA air cleaner, select a system that is appropriately sized for the area in which it will be installed. This determination is made based on the air flow through the unit, which is typically reported in cubic feet per minute (cfm). Many portable HEPA filtration units are assigned a Clean Air Delivery Rate (CADR) (See EPA’s Guide To Air Cleaners In The Home), which is noted on a label in the operator’s manual, on the shipping box, and/or on the filtration unit itself. The CADR is an established standard defined by the Association of Home Appliance Manufacturers (AHAM). Participating portable air cleaner manufacturers have their products certified by an independent laboratory, so the end user can be assured it performs according to the manufacturer’s claims. The CADR is generally reported in cfm (cubic feet per minute) for products sold in the United States. The paragraphs below describe how to select an appropriate air cleaner based on the size of the room in which it will be used. The procedure below should be followed whenever possible. If an air cleaner with the appropriate CADR number or higher is not available, select a unit with a lower CADR rating. The unit will still provide incrementally more air cleaning than having no air cleaner at all.

In a given room, the larger the CADR, the faster it will clean the room air. Three CADR numbers are given on the AHAM label, one each for smoke, dust, and pollen. The smoke particles are the smallest, so that CADR number applies best to viral particles related to COVID-19 and other viral respiratory diseases. The label also shows the largest room size (in square feet [ft2]) that the unit is appropriate for, assuming a standard ceiling height of up to 8 feet. If the ceiling height is taller, multiply the room size (ft2) by the ratio of the actual ceiling height (ft) divided by 8. For example, a 300 ft2 room with an 11-foot ceiling will require a portable air cleaner labeled for a room size of at least 415 ft2 (300 x [11/8] = 415).

The CADR program is designed to rate the performance of smaller room air cleaners typical for use in homes and offices. For larger air cleaners, and for smaller air cleaners whose manufacturers choose not to participate in the AHAM CADR program, select a HEPA unit based on the suggested room size (ft2) or the reported air flow rate (cfm) provided by the manufacturer. Consumers might take into consideration that these values often reflect ideal conditions which overestimate actual performance.

For air cleaners that provide a suggested room size, the adjustment for rooms taller than 8 feet is the same as presented above. For units that only provide an air flow rate, follow the “2/3 rule” to approximate a suggested room size. To apply this rule for a room up to 8 feet tall, choose an air cleaner with an air flow rate value (cfm) that is at least 2/3 of the floor area (ft2). For example, a standard 300 ft2 room requires an air cleaner that provides at least 200 cfm of air flow (300 x [2/3] = 200). If the ceiling height is taller, do the same calculation and then multiply the result by the ratio of the actual ceiling height (ft) divided by 8. For example, the 300 ft2 room described above, but with an 11-foot ceiling, requires an air cleaner that can provide at least 275 cfm of air flow (200 x [11/8] = 275).