On the public transit point: fun fact, BART (a large SF/Bay Area transit system) trains replace all of the air in each train car every 70 seconds, which is a mu

On the public transit point: fun fact, BART (a large SF/Bay Area transit system) trains replace all of the air in each train car every 70 seconds, which is a much higher rate of airflow than most other public spaces. Since they also use MERV 14 filters, there may be a disconnect between CO2 readings on BART (or other transit systems with aggressive filtration) and the true risk of catching a respiratory virus.