Published: Jan 2022 | doi.org/10.1093/cid/ciac006
Quantifying Environmental Mitigation of Aerosol Viral Load in a Controlled Chamber With Participants Diagnosed With Coronavirus Disease 2019
Hooman Parhizkar, Leslie Dietz, Andreas Olsen-Martinez, Patrick F Horve, Liliana Barnatan, Dale Northcutt, Kevin G Van Den Wymelenberg
This study set out to examine the effects different environmental control strategies had on the viral load suspended in the air, with a view to recommending which strategies should be prioritised to minimise viral transmission.
An airtight, modular chamber was equipped with air samplers, humidifiers, dehumidifiers, HEPA filters, settling plates, particle counters, a stand-up desk and a treadmill. Over a period of two months, 11 University of Oregon students who had been diagnosed with COVID-19, entered the unit one at a time and were invited to sit, stand, talk, talk loudly, cough on purpose and walk on the treadmill during a three-day set of experiments.
Throughout the course of each study day, the researchers conducting the study would measure viral RNA in the air and on surfaces, as well as directly from the research participants’ nose and mouth. The idea was to measure how virus particles move through the air, controlling for three variables: ventilation, filtration and humidity.
- Increased viral load in nasal samples is associated with higher viral loads in environmental aerosols and on surfaces captured in both the near field (1.2 m) and far field (3.5 m).
- Aerosol viral load in far field is correlated with the number of particles within the range of 1–2.5 µm.
- Increased ventilation and filtration significantly reduced aerosol and surface viral loads
- Higher relative humidity resulted in lower aerosol and higher surface viral load, consistent with an increased rate of particle deposition at higher relative humidity.
- Data from near field aerosol trials with high expiratory activities suggest that respiratory particles of smaller sizes (0.3–1 µm) best characterize the variance of near field aerosol viral load.
The findings indicate that building operation practices such as ventilation, filtration, and humidification substantially reduce the environmental aerosol viral load and therefore inhalation dose, and should be prioritized to improve building health and safety.
Prof. Kevin Van Den Wymelenberg, who carried out the study, commented, “The most exciting result was that higher humidity caused viral particles to drop out of the air and onto surfaces. From a particle physics standpoint, it makes perfect sense,” he said. “Increased water content in the air with higher relative humidity makes for more rapid particle deposition.
“Mid-range humidity, between 40 and 60 percent, is probably optimal for removing viral particles from the air”, Van Dem Wymelenberg continued. “If the air is too humid, it increases the risk of mold and very dry air allows dry particles to float longer. Humid air is also good for the human immune system because it helps keep mucous membranes healthy and moist.”
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11 students diagnosed with COVID-19 spent 3 days each in the controlled chamber.
Participants carried out a variety of activities at different levels of ventilation, air filtration and humidity.
Viral loads were monitored in the air and on surfaces, across the different activities and environmental conditions. (photo credit: University of Oregon)
Prof. Kevin Van Den Wymelenberg, "The most exciting result was that higher humidity caused viral particles to drop out of the air and onto surfaces."