Can we use ventilation with outdoor air during high pollution events outdoors?
In general, you should follow the guidance of local public health officials when high outdoor pollution events (such as wildfire smoke) occur in your building’s location. It is anticipated that persons who are either susceptible to or affected by COVID-19 may have health conditions that also make them more vulnerable to high outdoor pollution. To reduce your exposure to high outdoor pollution, you should stay indoors and take steps to clean the indoor air such as improving air filtration or setting up a clean air room. While increasing ventilation rates is good for reducing COVID-19 risk, this action is not recommended during high pollution events. However, a decision to turn off ventilation entirely can only be made after considering the details of your situation such as how severe the outdoor pollution is, whether building occupants have compromising health conditions, whether the filtration in the outdoor air path of the ventilation system can be upgraded, and whether turning off the ventilation will compromise designed building pressurization, which would allow additional outdoor pollution to enter via leaks in the building envelope.
Fortunately, the same filtration options, such as portable air cleaners with HEPA filters or MERV 13 (or equivalent) filters in HVAC systems, that will reduce risk of COVID-19 will also typically be effective at reducing indoor particle concentrations from high outdoor pollution. However, if the outdoor air pollution is not particulate but gaseous, typical portable air cleaners and HVAC filters will not help. If there are known or suspected COVID-19 infected persons in your building, you should create a separate clean air room with a portable air cleaner for them.
Steven Emmerich, NIST