The World Is Completely Unprepared For Detecting Emerging Viruses/Bacteria In The Air

Abstract

N95 respirators are useful for reducing risk from a person infected with COVID-19 at both near-field (e.g. < 6 feet) and far-field exposures (e.g. > 6 feet). Although air filtration is not usually effective at near-field, emulating at least the far-field equivalent of N95 requires 95% relative reduction of particles (20x) from an airborne particulate source in each room. To mitigate COVID infection risk with air filtration, there is a wide range of air change per hour (ACH) recommendations from public health agencies (CDC, CDPH, etc.) ranging from 2 to 12 ACH. Tracking the removal of an inert airborne contaminant (e.g. salt water) as a function of time is often used to measure ACH but is cumbersome and generally not desirable in occupied rooms such as a classroom or home. Instead, we describe a procedure using an optical particle counter to track the decay of ambient aerosols (0.3 μm diameter) and fit it to an exponentially decaying curve to measure ACH from the exponential coefficient. Experiments were conducted both in a room (from surface deposition alone and with air filtration) and in a whole, multi-room house (with air filtration). First, the rate of surface deposition in an unventilated room without HVAC ventilation or air filtration enabled was measured to be 0.6 ACH. Second, ACH was measured (verified) with low-noise generating HEPA purifiers ($299-$999) and Do-It-Yourself (DIY) air purifiers ($55-$160, 1"-5", MERV 13-16) in each room. The ACH measured with these air purifiers ranged from 4 to 20. Using the measured ACH and volume of the room/house to estimate the clean air delivery rate (CADR) per $100, it varied by a factor of 3x from below 100 cfm for tested HEPA purifiers to above 300 cfm with tested DIY air purifiers. Using 0.6 ACH as a baseline for an unventilated room, at least 12 ACH is required to reduce exposures equivalent to protection of N95 respirators (95%) at far-field, which is verifiable in a room or building with ambient aerosols using either HEPA or DIY air filtration.