Ventilation & Filtration for COVID-19
A little speck of protein wrapped in fat has killed over 400,000 people in the United States, and it is not going to go away. So while we hope that the vaccines will bring an end to this pandemic sometime this year, we still have a ways to go until we get to that point. We need to learn from this tragedy in order to protect ourselves in the meantime and to implement strategies that will protect us from other virus strains in the future.
There are three major variables that affect the risk of exposure: occupant density, ventilation rate, and exposure time. Minimum ventilation rates are established by ASHRAE Standard 62.1, and are based on three principal factors: the space size, the density of occupants in that space, and what the occupants do in that space. What they do is relevant because of their respiratory rate. The minimum amount of outside air required in an office is 5 Cubic Feet of outside air per minute (CFM) per person, plus an additional 0.06 CFM per square foot. The occupant density levels are greater in spaces like break and conference rooms, so these spaces need higher ventilation rates or reduced occupancy levels.
Outside air reduces the concentration of virus particles by dilution. Secondarily, high efficiency filters can capture contaminated particles. Virus particles themselves are extremely small (~140 nm), but they are typically attached to mucous or other larger particles. Therefore there is a wide range of particles which we want to capture. Most typical HVAC units have MERV 7 or lower filters, which are not even rated for smaller micron particle ranges. MERV 13 and higher provide increasingly better rates of efficiency in these smaller ranges. The best are HEPA filters which have a minimum efficiency of 99.5%, but these are generally only used in operating rooms and other critical spaces, and are now also being deployed in portable units to provide supplemental air cleaning.