Air Changes Per Hour

A simple COVID mitigation for homeowners and facilities

image via the CDC

This will be a brief essay intended as a form of public advocacy for air ventilation related COVID mitigation measures that can be taken to reduce atmospheric viral particle loading inevitably found in high density gathering places. Examples of such impacted facilities could include airports, fitness centers, classrooms, churches, movie theaters, shopping centers, and other public gathering places. We will also discuss how these guidelines could be relevant to residential dwellings housing known infected individuals.


The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) is the standards board that publishes guidelines for heating, ventilation, and air conditioning (HVAC) equipment design and system engineering in the United States. One of the key metrics governing system ventilation design in new construction is the number of outdoor air changes per hour (ACH). Such air changes are generally achieved by drawing in and circulating additional outdoor air to supplement recycled indoor air with dedicated equipment like intake fans and ducting for this purpose. As part of this operation, some amount of indoor air is usually exhausted in a similar manner, resulting in the turnover of indoor air every few hours. The uniformity of turnover may be benefitted by ceiling or box fans and other forced air measures that increase circulation.

The relevant standard for ventilation design is ASHRAE 62.1 for commercial facilities and 62.2 for residential, which recommended thresholds may vary based on occupancy profile and facility use — where doubling the occupant density of a facility will translate to a doubled recommendation of outdoor air. A helpful summary of specific guidelines are provided here. (Note that outdoor air turnover is not the same as total air turnover, where the latter is associated with cumulative air passing through the central unit.) Medical facilities, which have the added design burdens of directional airflow characteristics, have other governing standards. The introduction of outdoor air is not without a cost. In addition to the additional fanning energy, any heating or cooling will have to handle this additional ambient heat exchange load. The purpose of the ASHRAE guidelines are to maintain recommended air quality associated with occupant CO2 exhale and other sources of impurity, and the design threshold will vary based on facility use and expected occupant density. This author studied and worked in HVAC engineering early in his career although he has not practiced in several years. The central concern of this essay will be logistics, implications, and justification for increasing the outdoor air change per hour as a simple form of COVID mitigation, which may be found particularly impactful when the prior rate was below ~2 ACH.

Residential Housing

Low density residential housing or apartment dwellings often do not have a formal air change mechanism. In practice most houses rely on outdoor air being introduced haphazardly from occasional opening and closing of doors, windows, and other passive measures. The primary exception is for exhaust fans commonly found in bathrooms and kitchens which introduce a negative pressure that may draw in some small amount of outdoor air through whatever part of the building envelope is not perfectly sealed. Unfortunately common efficiency measures of tight fitted and sealed windows and doors, which although may reduce electricity bills by retaining conditioned air, also have the effect of preventing intrusion of fresh outdoor air.

In the context of the various variants of COVID, residential housing ventilation measures may have the potential to benefit duration of recovery time of infected individuals dwelling within. After all with every cough and exhale an infected individual introduces additional viral particulates into the ambient surroundings that may collect on surfaces or otherwise be re-inhaled, increasing the viral loading that their immune system has to manage. Fortunately there are a few simple measures that should be of benefit when a dwelling is being used for quarantine:

  • Opening windows and doors (as appropriate based on presence of small children or pets)
  • Running ceiling fans to increase circulation
  • A dedicated fan directed to blow indoor air out a window, or otherwise running bathroom and kitchen exhaust fans (rephrased for clarity: fan directed to blow out the window)
  • Air purifiers with a “HEPA” class filter

Note that the HEPA class designation for air purifiers refer to having sufficiently small particle filtering size, which provide additional benefit. Most residential central air handler filters primarily are sized to catch dust particulates for keeping heat exchange coils and ductwork clean and will be less effective for this purpose. Unfortunately it is not as simple as just replacing to a better filter in an air handler unit, as the resulting additional inlet pressure drop may impact performance for the primary purposes of cooling and heating. That being said, for new construction or equipment replacements HEPA class filters can be incorporated into design of fan sizing for a built in central air purifier. Otherwise standalone portable air purifiers are relatively inexpensive and may be purchased from common retail outlets (rule of thumb, if they sell vacuum cleaners they probably also have air purifiers), and prices may vary slightly by addressable square footage and noise profiles. Ideally one should plan to replace an air purifier’s filter after a known extended virus exposure or based on the manufacturer recommended intervals, just as you would for your central air handler filters, otherwise their use may become counterproductive. (rephrased for clarity: it is important to change your filters)

There will be an increase to heating or cooling load from drawing in ambient air, which obviously will have an impact on electrical bills. A homeowner may find that in certain conditions just drawing in ambient air in place of air conditioning may be sufficiently comfortable, and with larger central exhaust equipment this may even be suitable in hotter climates, at least during some months of the year. Otherwise timing of ambient air intake can be adjusted by hours of the day to take advantage of cooler morning and evening temperatures for instance. As an asterisk, one should avoid ambient air intake during rainy conditions as excessive indoor humidity can promote mold or mildew growth.

Commercial Facilities

In the context of commercial facilities with higher density occupancy, it is a simple impact of randomness that there will periodically be occupants present who are active virus carriers, after all in some cases it may be possible for an unknowingly infected individual to transmit the virus before symptoms are noticed. In the current context of loosening guidelines for public mask wearing, this leaves building operators with a need to protect their workers and visitors, for which we propose similar measures as residential housing noted above, particularly by increasing the target number of outdoor air changes per hour.

Commercial facilities will often have the benefit of dedicated infrastructure for purposes of external air intake. And because it is common for design engineers to initially size equipment based on calculations or heuristics with safety factors on top, there will often be some amount of margin in equipment capacity for handling the additional heat exchange needed to serve and condition additional external air, especially in non-peak load conditions. The obvious tradeoff is that there will be an energy cost for increasing external air intake. This can partly be mitigated by measures noted above such as considering seasonal ambient temperatures and time of day. Another option could be to base extent of outdoor air on peak operational hours, such as increasing air intake during known rush hours or proportionately based on a real time measurement or estimate of current occupant density. We recommend adjusting operating profiles initially in small increments to help detect any adverse operating impacts to system performance or energy cost, and it may be appropriate to consult with a professional for guidance.

In medical facilities, engineers ideally design directional airflow characteristics to channel known riskier ambient conditions towards exhaust outlets. For instance, you wouldn’t want to allow airflow from patient rooms to pass through a visitor waiting area. This is generally achieved by setting airflow characteristics for a negative pressure gradient between e.g. patient rooms and the surrounding hallways (with a lower pressure in patient rooms in comparison to the hallway). In the context of commercial facilities, we expect the riskiest conditions will likely occur in public restrooms owing to the smaller enclosure and aerosol effects of flushing mainstream toilet designs. (If you are looking for an excuse to wear your mask in public restrooms we just gave you one.) In the short term, we suggest ensuring there are active exhaust fans in public restrooms to promote a negative pressure gradient in comparison to the surrounding occupied space, particularly in high traffic environments like airports. In the long term we hope organizations like ASHRAE may seek to improve standards for flushing mechanisms to mitigate this effect, perhaps with the support of domain experts like those at the Bill Gates Foundation.

There is no reason that the introduction of HEPA filtered air purifiers we suggested for residential housing can’t be extended to commercial facilities. These may be particularly impactful in smaller spaces such as individual storefronts and classrooms for instance. This author was raised the child of a public school teacher who often had to take the initiative of purchasing her own school supplies to ensure her students were given the opportunity to succeed. We hope local governments can step up in offering funding for such simple measures as classroom HEPA air purifiers, however there may also be a benefit of federal support by way of individual teacher grants or aggregate facility grants for funding similar measures. As the first lower dose vaccines approved for children aged 5 and up recently became publicly available, we are optimistic that the last major high density occupancy transmission vector of K-12 schooling may soon be better mitigated.


This essay was intended as a form of public advocacy in support of increasing the outdoor air changes per hour metric for both commercial and residential buildings as a form of COVID transmission mitigation. Such measures may include simple tactics like opening windows, running fans, and HEPA air purifiers, or for more sophisticated facilities may include targeted deviations from initial design profile of building operation. In many cases such operational deviations may be possible with existing equipment owing to safety factors present from initial system design.

For additional guidance we recommend consulting with your facility operations team, a local professional mechanical engineer, your utility company, or otherwise reaching out to the professional membership of ASHRAE. Further reading is available in the various reports assembled by the ASHRAE Epidemic Task Force.

For further readings please check out the Table of Contents, Book Recommendations, and Music Recommendations. For more on Automunge:



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