Appropriate song to play while reading this post: Mat Kearney – Breathe In, Breathe Out
This paper came out last month, and I thought it would be nice to briefly mention it here, even though many other papers have looked at the concentrations of airborne bacteria and viruses as well.
In this study, done by Aaron Prussin et al. from the Department of Civil and Environmental Engineering at Virginia Tech, air samples were collected in triplicate at 9 locations in Blacksburg, VA. The sample sites were: a classroom, a daycare center, a dining facility, a health center, three single-family houses, an office, and an outdoors at an unspecified university campus.
For each sample, about 100 liter of air was pumped through a 0.2 Î¼m pore size filter, and nucleic acids on the filters were stained with a fluorescent dye and counted. Pinprick sized particles (between 0.02 and 0.50 Î¼m) were counted as virus-like particles (VLPs), while the larger signals (between 0.50 and 5.00 Î¼m) were counted as bacteria-like particles (BLPs).
Both BLP as well as VLP concentrations were found to be between 105 and 106 per cubic meter, and not significantly different between the indoors and outdoors locations, although there was a trend for higher concentrations outdoors. Notably, the lowest concentration of both particle types was in the health center, and the highest concentration indoors was found in the classroom and homes. Overall, the differences between the locations were very small.
The authors state that not many other studies have looked at virus concentrations in air. Unfortunately, filtering through a 0.2 Î¼m filter is not the best choice to catch viruses (Update: see below). Many viruses will be able to pass that filter, and recent work from the Banfield lab has shown that even some very small bacteria will pass through these filters (Luef et al., Diverse uncultivated ultra-small bacterial cells in groundwater, Nature Communications 6: 6372, 2015). So it is likely that the actual virus concentration both indoors and outdoors is much higher.
Total Concentrations of Virus and Bacteria in Indoor and Outdoor Air – Aaron J. Prussin, II, Ellen B. Garcia, and Linsey C. Marr – Environmental Science & Technology Letters 2015, 2, 84âˆ’88. Work founded by the Alfred P. Sloan Foundation, the National Science Foundation, and the National Institutes of Health.
Update: I stand corrected. Filtering air and water are two very different processes, and in the conditions used in the paper, filtering air over a 0.2 Î¼m filter will actually retain most of the viruses. For a detailed explanation, see the comments section below.