Filtration in building heating, ventilation, and air conditioning (HVAC) systems can be part of an overall risk mitigation approach, but is not generally regarded as a solution by itself.

There is no direct scientific evidence of benefit, but some reduced exposure can reasonably be inferred based on the ability of some filters to remove particles that contain a SARS-CoV-2 virus.

More importantly, in most buildings and in most situations, filters may be considerably less effective than other infection control measures including physical distancing, masking, isolation of known cases, and hand washing.

There are several reasons why Laurier has not installed HEPA filters in our air filtration systems. The most important reason is that our systems were not designed to handle HEPA filters.

If we were to put HEPA filters in these units, the pressure drop would be so great that our rooms would no longer be getting the air exchanges needed to support employees and classes. The air would be clean, but the carbon dioxide levels would be much higher and the temperature would be too high in the summer or too low in the winter. Our HVAC units also cannot physically handle HEPA filters due to the increased size.

Commercial buildings are not designed to handle HEPA filters and Laurier systems are no different.

Similar to building filtration, there is no direct clinical evidence of the benefit of portable air cleaners for reducing infectious disease risk, but some benefit can be reasonably inferred for appropriately sized (e.g., their removal rate is appropriate for the room), maintained, and operated portable HEPA filters.

As with building filtration, the details are important (e.g., efficiency and airflow rate of the air cleaner, sizing and placement within the space, maintenance and filter change, nature of space that is being cleaned), and appropriate portable filtration is only likely to be effective in concert with other measures.

Laurier received some commercial HEPA units that will be used in some classrooms to ensure that ASHRAE standards are met.

High-efficiency filters may be appropriate for your building, but they can also be counterproductive. The higher the MERV rating, the higher the resistance to allow proper airflow through the filter. A high-pressure drop filter can also diminish the amount of air supplied into the environment, making the filter less effective as well as causing other problems with other parts of the HVAC system.

Most importantly, the fan in the system may not run often and the efficiency of the filter will be less effective.

None of these technologies have been proven to reduce infection in real buildings, even if they have promise based on tests in a laboratory or idealized setting. Some of them have substantial concerns about secondary issues (such as ozone production).