As we all socially distance and stay home amid the COVID-19 pandemic, some may be asking themselves what else can they do to ‘flatten the curve’. While handwashing and staying home remain the most effective means of limiting the spread of the virus, there is conflicting information surfacing about the role and efficacy of air-filters. “It is important for people to understand how air-filtration systems are supposed to work,” says Prof. Jeffrey Siegel, a professor in the Department of Civil & Mineral Engineering at the University of Toronto and an expert in indoor air quality. “Some may be frightened with the situation we are all facing right now, but the best way to protect ourselves, our families, and our communities is with evidence-based information.”

Filtration in building heating, ventilation, and air conditioning (HVAC) systems can be a 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. In order for filters to have any impact on infectious disease transmission, transmission has to occur through the airborne route, filters have to be properly installed and maintained in appropriate systems to treat recirculated air, and filters have to be appropriately designed for the building in which they are used. More importantly, in most buildings and in most situations, filters may be considerably less effective than other infection control measures including social distancing, isolation of known cases, and hand-washing.

There is no obvious answer to this question given unknowns about the nature of SARS-CoV-2 containing particles and droplets, as well as the broader issues raised above. We do know that low-efficiency filters (e.g., less than MERV 8 according to ASHRAE Standard 52.2 or less than ePM2.5 20% according to ISO 16890-1:2016) are very unlikely to make a difference. Properly installed higher efficiency filters can remove particles of a relevant size depending on their installed capture efficiency, but current information does not allow for specific recommendations.

High-efficiency filters may be appropriate for your building, but they can also be counterproductive. A high-efficiency filter may have a high initial pressure drop and/or load with dust and particles very quickly, thus requiring frequent filter changes. A high-pressure drop filter (either because it is that way when it is new or because it loads quickly) can also cause more air to bypass the filter if it is not properly installed and well-sealed. Depending on the design of your system, 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, in many residential and some light commercial systems, the fan in the system does not run very often and the efficiency of the filter may not be as important as it could be.

Hospitals (and many healthcare facilities) have specially designed mechanical systems that can accommodate the levels of filtration that they need. They often rely on other systems and control strategies (e.g., UV lamps, humidity control, airflow management) to maximize the benefit from filtration. Most importantly, they have dedicated staff who operate and maintain this equipment so that it provides maximal benefit.

A properly designed and maintained UV system, often in concert with filtration, humidity control, and airflow management, has been shown to reduce infections from other viruses. The details of the system are very important (e.g., design of fixtures, lamp type, lamp placement airflow amount and mixing, etc.). Simply adding UV to an existing system without consideration of these factors has not been demonstrated to have a benefit.