Ventilation in the COVID Era – Filters for Outside Air Ductwork?

Author: Lou Vogel, PE, LEED AP, President
Published: January 26, 2021
Last updated: January 26, 2021

There has been some concern with bringing in the outside air in cities and a question of whether or not there should be filtration of that outdoor air. This is a separate issue from reducing COVID transmission because it involves primarily PM2.5 (particulate matter smaller than 2.5 microns), also termed inhalable particulates, and there is very little possibility that the outside air would contain active coronavirus virions.

The problem with PM2.5, or fine particulate matter, is that they can be breathed in deep into the lungs with effects including  breathing difficulties and even impacts on the heart. The 2.5 micron size is in that same range of aerosols that can float in the air, and is also in the range of particles that can be captured by the more effective filters, such as a MERV 13 filter.

You may be familiar with the deadly smog that occurred in England in the 1940s and 50s, but there was also a major event in Pennsylvania in 1948 that led to the creation of the Clean Air Act. Nearby steel and zinc production plants produced chemicals and PM2.5 that caused at least 20 deaths,  and likely many more over the years. There is no doubt that these particulates need to be taken seriously.

Denora, PA at noon in October 1948

These fine particles primarily come from internal combustion exhausts, the burning of fuels such as wood, heating oil or coal, as well as natural sources such as forest and grass fires. They can also form as a result of the reaction of gases or droplets in the atmosphere coming from facilities such as power plants and then carried miles from the original source.

From 2017-2019, New York’s air quality index (AQI) has remained less than 50, or “good.” The US Environmental Protection Agency (EPA) defines “good” air quality as air that poses little to no risk to health. 

AQI and PM2.5 can be converted, but it is a weighted equation, so it is perhaps simpler to just use the PM2.5 numbers when comparing. NYC has brought down its PM2.5 rating in recent years at least partly due to the reduction in use of fuel oils #6 and #4 for heating boilers. New York City air quality has consistently fallen within the “good” target, averaging about 7 μg/m3. These annual PM2.5 levels are comparable to the air quality of Taos, New Mexico and Waco, Texas (6.9 and 6.8 μg/m3, respectively). PM2.5 3 year average values for Buffalo (7.2), Rochester (6.7), Syracuse (5.4), and Albany (6.4) were not markedly less than NYC values.

In summary, a case could be made that a filter should be provided in all outdoor air systems in the US because breathing in any of these particles is not a good thing. But there isn’t a significant difference between the PM2.5 levels in NYC vs the rest of the state and assuming the EPA rating has value, it doesn’t indicate that action should be taken.

There still haven’t been any studies that demonstrate that COVID19 has been distributed through a building via HVAC ductwork. But aerosols definitely contribute to transmission and we can spread the virus even if we don’t feel any symptoms, so mask up everybody!


New York City Air Quality Index (AQI) and New York Air Pollution | AirVisual
NYS Ambient Air Quality Report for 2019 – NYS Dept. of Environmental Conservation
AirNow Interactive Map