Making the Air in the Office Cleaner

In May, the Centers for Disease Control and Prevention set five air changes per hour (ACH) as the target amount of ventilation needed in occupied indoor spaces, such as offices and stores, to better reduce the risk of airborne infections such as Covid-19. This level will better protect everyone and is especially important for immunocompromised and other at-risk groups who can’t be adequately protected by vaccines. (The CDC still advises both immunocompromised and pregnant people to avoid crowded or poorly ventilated indoor spaces.)

Beyond this recommendation, there are additional reasons that organizations should take this target seriously. One is the federal Pregnant Workers Fairness Act, which took effect on June 27. Similar to the Americans with Disabilities Act’s required protections for the immunocompromised, it mandates that employers provide “reasonable accommodations” for pregnant workers. (Like those who are immunocompromised, pregnant people and their developing babies are also at greater risk of serious complications from Covid-19.) Another is a phenomenon that seems to be occurring much more frequently: forest fires. The recent intense air pollution from Canadian wildfires that extended across much of the United States is a glaring case in point.

Upgrading central air systems to achieve five ACH would, for many buildings, require extensive and costly renovations. Even once completed, these systems would incur exorbitant energy costs to cycle large amounts of air through them. Alternatively, businesses could take three steps to readily achieve five ACH by using cost-effective in-room, portable air cleaners.

1. Install portable air cleaners (purifiers).

Portable, low-noise air cleaners such as HEPA purifiers and do-it-yourself (DIY) air cleaners assembled from box fans, filters, and other off-the-shelf materials are a cost-effective method to implement the CDC target by recycling the air in each room. Based on our experience in San Francisco schools, three DIY purifiers can exceed the CDC target for as little as $250 to $400 (about $0.25 to $0.50 per square foot) for a typical 900-square-foot classroom with 10-foot ceilings — and a monthly energy bill of less than $10. Since HEPA can be store bought and DIY quickly constructed, they can be implemented as an effective clean air solution within a matter of days — results that have been verified by the CDC and the U.S. Environmental Protection Agency. DIY air cleaners are being offered for free to organizations that serve the disadvantaged. Even more energy-saving DIY models using personal computer cooling fans have recently come to market.

In general, the number of purifiers needed and their total cost rises in proportion to both room volume and air exchange target. Many purifiers come with variable speeds. They need to be set to run at a speed needed reach the CDC target of five ACH; otherwise they will be less effective and may not reach the intended target. Once installed and running, the air exchange rate can be checked using handheld particle counters to measure the change in air particles present in a room.

2. Exceed the new CDC target with 12 ACH in crowded situations.

To limit bacterial growth, fish tanks typically use filtration systems to clean water at least four times an hour (every 15 minutes), analogous to four air ACH for air in a room. Until recently, there has been a lack of clarity on the degree of air cleaning needed to effectively reduce viral transmission in buildings. So in 2022 we wrote to the CDC in the Journal of the American Medical Association, asking it to say clearly how many ACH are necessary to stop Covid. CDC researchers replied that although they were not aware of Covid-19 outbreaks in spaces with five to six ACH, 12 ACH or more (like an airplane inflight or airborne isolation room in a hospital) may be needed in some situations. As the number of people in the space increases, the probability of transmission increases, which could require more ACH and distancing to reduce risk. For example, the California Department of Public Health recommends a minimum of six and ideally 12 ACH in schools.

3. Run air cleaners during business hours to reduce costs and replace filters periodically.

Filters and fans can either be replaced on a regular schedule recommended by the manufacturer or, if an organization wants to be conservative, after testing shows that their performance has dropped below acceptable levels. Testing can reveal when filter or fan lasts longer than expected or if its performance dips sooner than expected (e.g., a filter has clogged up with particles or has lost its ability to filter particles after an intense wildfire smoke event). In some places, such as hospitals, they may need to be run 24×7, but for many businesses, if turned on only during typical business hours (e.g., 9 a.m. to 5 p.m.) when people are present, they can last up to four times longer and consume four times less power. They can either be turned on and off manually by someone in the room or via low-cost motion sensors, which can be set to turn them off automatically after 20 minutes of inactivity.

Businesses can implement these three steps to cost effectively support the new CDC standard and protect people from Covid, wildfire smoke, allergens, and any other airborne dangers that might arise. These simple steps to clean the air will end up benefiting everyone and create a much more inclusive environment for at-risk groups of people, including the immunocompromised, pregnant people, the elderly, children, and those with chronic heart and lung conditions.