Risk of infection due to airborne virus in classroom environments lacking mechanical ventilation

Abstract

The COVID-19 pandemic highlighted the role of indoor environments on disease transmission. However, our understanding of how transmission occurred evolved as the pandemic progressed. Enclosed spaces where pathogen-laden aerosols accumulate were strongly linked to increased transmission events. Most classrooms, particulalry in the U.S., do not have any mechanical ventilation systems but do have many people congregating indoors for long periods of time. Here we employ a safe, non-pathogenic surrogate virus, the bacteriophage phi6, to interrogate aerosol transmission in classroom environments that do not have any natural or mechanical ventilation in order to provide baseline understanding of how effectively aerosols facilitate new infections. We measure exposure risk using a modified passive monitoring technique compliant with applicable standards, including ISO 14698-1:2003. We find that virus-laden aerosols establish new infections over all distances tested within minutes and that the time of exposure did not change transmission rate. We further find that relative humidity, but not temperature nor a UV-based disinfection device, significantly lowered transmission rates. Our data suggest that, even without mechanical ventilation, relative humidity remains an inexpensive and highly effective mitigation strategy while UV air treatment may not.

Fig 1. Airborne transmission rate in non-conditioned classrooms.

Relative exposure rates represent the rate of exposure per unit time (in 15-minute blocks) normalized to 4x 10⁷ total phage aerosolized in order to allow for comparison of multiple trials with variability in total number of phages aerosolized. At all distances we observe a drop off in the relative exposure rate when RH > 40%. Relative exposure rates are empirically higher within 1.8 m (6 ft) of viral source. Error bars represent 95% confidence intervals.

Fig 2. Aerosol transmission by relative humidity.

Relative Exposure Rates for all Trials. A. The relative exposure rates for all trials are arranged by RH. A drop in transmission is evident from RH ≥ 33.4%, however, there is a high degree of variability until RH ≥ 40.8%. B. The inset panel represents all trials with RH > 40% with the y-scale changed for easier viewing. Color indicates any UV treatment/fan speed. Red bars are no UV control conditions, green bars are UV treatment with “low” fan settings, blue bars represent UV treatment with “medium” fan setting, and purple bars show UV trials with fans set to “high” speed.

Fig 3. UV treatment by time.

Mean number of phage plaques (normalized) by time of exposure and UV treatment. Attempts to disinfect the classroom air through use of portable UV-C generating devices had no effect upon the number of transmission events observed, regardless of the UV device fan speed or the amount of time of UV treatment. Some evidence suggests that the specific type of airborne virus as well as the wavelength of UV light along with various environmental conditions could affect disinfection results [21], although no impact was observed with the units used in this study.