Traditional HVAC systems, when used in conjunction with other methods like surface cleaning, social distancing, and face coverings, can help manage airborne disease transmission in offices, schools, and other indoor work locations where people are present. In our jobs, we usually use ventilation for indoor comfort and to control exposures to below some acceptable concentration, such as ACGIH threshold limit values and OSHA permissible exposure limits. These standards allow us to tailor the flow rates to achieve acceptable concentrations in the air people breathe. However, airborne limits for disease-transmissible particles are not yet found in such standards. Therefore, the recommendations in this article represent the best we can do to reduce airborne concentrations of disease vector particles.

A FEW AIR BASICS Air is made up of gases (for example, oxygen, nitrogen, argon, and carbon dioxide), vapors (mainly water), and tiny particles (such as dust, mold spores, shed skin particles, condensation nuclei, droplets, aerosols, bio-aerosols, and others). Air in typical buildings is in constant random turbulent motion, naturally drifting, mixing and stirring itself at instantaneous velocities of about 10–30 feet per minute (fpm) or 2–6 inches per second, even in “still air.” We don’t typically feel air at indoor temperatures moving at these speeds. We only begin to feel the air movement on our hands when the air velocity is about 100 fpm, and on our necks and ankles when it is moving at about 50 fpm.

In schools, offices, and workrooms, the air is also usually moving from one place to another—for example, from an open window to another window on the other side of the space, or from an air supply register in the ceiling to an air return louver.

In an office space, look up at the ceiling, for example, and imagine how the air could be moving from the air supply register to the air return louver, all the while mixing with itself. Take note of potential problems: are there any obstructions such as cubicle walls or curtains? Is anyone sitting immediately downwind from someone else?

How fast would the air be moving? In a typical school classroom with about eight air changes per hour and air supply and return louvers in the ceiling at opposite ends of the classroom, the main body of air would move from the supply registers to the return louvers at an average velocity of about 5–10 fpm, coupled with the natural air mixing and stirring that occurs in all air at room temperatures. (See checklist item 10 for more information on evaluating air movement in a space.)

DISEASE TRANSMISSION THROUGH AIR Virus- and bacteria-contaminated particles can transmit diseases from a source to a human respiratory system through the air, given the right conditions. Air-transmittable diseases include colds, influenza, measles, tuberculosis, chicken pox, COVID-19, and many others. People who carry disease can be sources of infected particles when they sneeze, cough, talk, or even breathe.

Some airborne infected particles are moist like mucus droplets and others are dry; some are very small (aerosols, which have a diameter of about 5 microns or less) and some are bigger (droplets, whose diameter is typically bigger than 5–10 microns). See the resources for typical particle nomenclature, sizing, and composition details, as they pertain to a specific disease vector particle. TB, for example, is thought to be transmitted mostly by small airborne aerosols while colds are often transmitted by larger droplets that settle on a surface, which someone then touches.

Very small particles (aerosols, bioaerosols) tend to disperse and dilute as they float along with the turbulent air. We can help the air move, dilute, disperse, or remove such air contaminants at greater rates if we optimize the ventilation in the space for these purposes. Most bigger particles (droplets, for example) typically settle after being emitted to the air, sometimes within six feet of the source, but there are important exceptions, such as, when a wet droplet dries and becomes a smaller aerosol, or when the air is moving through the space at higher than normal speeds (for example, downwind from a free-standing fan).

Air can facilitate transmission when not properly controlled—for example, when contaminated air travels from an infected person’s breathing zone into the breathing zone of someone nearby, or when insufficient airflow allows the buildup of disease particles in a space. Air can also help reduce transmission by diluting or conveying an exhaled breath contaminant away from others nearby, or by keeping airborne concentrations of contaminated particles as low as possible.

THE CHECKLIST The following checklist and the accompanying explanations don’t address all of the complex issues involved in ventilation, but they offer reasons, ideas, and suggestions for using ventilation to best support the control of airborne disease transmission in non-healthcare workplaces.

☐ 1. We have identified the personnel responsible for the operation and maintenance of the HVAC system and talked with them about the system. It is always helpful to get to know the people in charge of the day-to-day operation of the HVAC system. These individuals typically have titles such as building manager, building mechanic, service manager, furnace operator, and so forth. They should be able to help you understand each of the checklist items.

☐ 2. The HVAC system complies with or exceeds appropriate codes, standards, guidelines, and supplier instructions. Following codes, standards, and supplier instructions helps us achieve the minimum performance a system must provide. But exceeding such minimum standards can often make the system perform even better. Many checklist items suggest how to comply with or exceed minimums.

According to ASHRAE 62.1, the traditional default minimum standard for OA delivery in offices has been, for example, “at least 17–20 cfm/person.” Simply complying with this minimum standard would likely not be sufficient for disease transmission control, where that is one of your goals. We should provide as much OA as feasible, limited by technical and economic concerns, as discussed in Item 6.

☐ 3. When possible, the air supply fan at the HVAC system is set to the “on” position during occupancy. To meet current standards and codes, the HVAC system should be running whenever someone is present in the building. Also, the air supply fan in many systems can be set to the “on” position, thus moving air through the HVAC system even when thermostats are not calling for heating or cooling. This can help maximize the dilution, dispersion, and filtering of small airborne particles that may be present in the air. In many systems, you can manually set the fan to “on” or “circulate” at the thermostat. Note: in warm, humid climates, manually setting the fan to “on” could cause the indoor humidity to become too high when the AC system is not calling for cooling. If indoor humidity exceeds 60–65 percent, return the system to its original settings. Also, check with your HVAC operator to see how your system might handle this potential problem.

It is also a good practice to have the HVAC system running 30–60 minutes before and after occupants are present in the building.

☐ 4. The HVAC system is checked, inspected, cleaned, and maintained on a regularly scheduled basis. System performance (for example, airflow rates) should be checked on a regular basis. System checks are usually performed by the in-house HVAC operator, but an occupational and environmental health and safety professional (OEHSP) might help with airflow measurements. See the resources list for more information about system testing and cleaning procedures.

☐ 5. Air supply and return louvers are open, clean, and operating properly. When people are present in a space, air should always be flowing through air supply and return louvers (also known as registers, grilles, and vents), which are usually located in the ceiling. You can check flow using an anemometer or velometer; more simply, the presence and direction of airflow can be determined by placing a piece of paper at the louver. Air coming through the supply register should push the paper away; air entering the return louver should suck the paper toward the louver.

☐ 6. The system provides fresh, clean outdoor air near the maximum acceptable flowrate. Try to set the OA controls to the maximum OA the HVAC system is capable of providing. The maximum OA usually depends on air quality, weather, season, operating costs, and system capabilities, such as the ability to maintain appropriate humidity levels in the air. When disease transmission is a concern, providing more OA can enhance dilution of disease vectors. If a school’s HVAC system, for example, moves air through a library at a rate of about four air changes per hour (ACH), and half of that is fresh clean OA, it can reduce the concentrations of air contaminants by 50 percent every 15 minutes, assuming no emission sources are present and air in the space is mixing effectively. See the list of resources for information about and techniques for estimating ACH.

☐ 7. OA intakes are dry, clean, open, and operating properly. Check the areas around OA intakes to ensure that they are clean, open, not blocked by bushes or defective louvers, and not near sources of mold, bacteria, or exhaust air from the building.

☐ 8. System air filters are clean. Check system air filters: are they clean? Have they been replaced recently? When someone is cleaning or replacing filters, be sure they have appropriate personal protection, such as respirators and gloves. Bacteria, virus, and mold spore counts can sometimes be reduced in the air using appropriate UV-C lighting in the return air ductwork upstream of the filters. See the resources for details and applicability to your system.

☐ 9. Air filter ratings are at or near the highest level feasible for the system. Are HVAC system filters rated at the highest efficiency the system is capable of handling? Most HVAC systems installed in the past ten years are capable of using MERV 13–16 filters, which can remove very small particles that could be laden with bacteria or virus. When HVAC systems cannot provide sufficient filtration, portable HEPA filter units can sometimes be used in occupied spaces to help control airborne concentrations of particles (for example, in office cubicles). See the resources for more information.

☐ 10. Supply registers provide appropriate airflow to all occupied spaces, and the air flows fairly uniformly through all spaces so that everyone is provided with airflow through their occupancy areas. Airflow rates at louvers can usually be checked using a velometer or balometer. In an occupied zone (for example, near an employee’s chair), smoke tubes can be used to check on the behavior of the airflow at that location. (Before using smoke tubes, be sure the occupants are not in the space. You may need a respirator.) Standing upwind, slowly release a small amount of smoke at the location of concern and observe how the plume behaves. It should mix with the air, expand, and move in the intended direction. Small amounts of smoke can also help identify areas of low or no airflow (smoke may linger in an office cubicle, for example). Purge the environment of detectable smoke before occupants are readmitted. This may take 10–30 minutes, depending on conditions.

☐ 11. Restrooms are under negative pressure, the exhaust fan is on all the time, and air exhausts to the outside. Because restrooms can sometimes be the source of bacteria and virus, be sure exhaust fans in restrooms are operating and that the restrooms are under negative pressure—that is, air always flows into the restroom and is exhausted to the outside of the building. Be sure airflow into the restroom does not interfere with the ventilation of adjacent spaces.

☐ 12. Any free-standing fans are not blowing air from one person’s breathing zone directly into the breathing zones of other persons nearby. Free-standing fans (such as pedestal fans, floor fans, wall fans, and desk fans) are sometimes used for cooling or to help mix the air in a space. For example, small fans can help move the air into and out of a cubicle. If air movement or air replacement in a space is low and the occupant is infected, infectious particles could quickly build up.

☐ 13. Occupants feel comfortable and satisfied with the performance of the HVAC system. Regularly check with occupants to see if they feel comfortable with the ventilation system at their location. Always follow up on any complaints, such as “The air seems stale here in my cubicle,” “It’s too dry (or too humid) in here,” “I smell something in the air,” “I get asthma attacks here,” or feelings of agitation, irritation, or discomfort with the environment in the building.

☐ 14. We have studied the latest data on the disease transmission behaviors of particles of concern. The literature is constantly being updated on the important modes of transmission of various disease vectors in air, especially the virus causing COVID-19. Tailor your ventilation program to best utilize its capabilities in assisting transmission control.

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Editor's note: This article was updated on Jan. 21, 2021, to provide additional context regarding item 3 in the checklist. The new information addresses the possibility that setting the HVAC air supply fan to “on” could potentially lead to indoor humidity problems in warm, humid climates.