Viruses
play important roles in microbial ecology and some infectious
diseases, but relatively little is known about the concentrations,
sources, transformation, and fate of viruses in the atmosphere. We
have measured total airborne concentrations of virus-like and bacterium-like
particles (VLPs between 0.02 and 0.5 μm in size and BLPs between
0.5 and 5 μm) in nine locations: a classroom, a daycare center,
a dining facility, a health center, three houses, an office, and outdoors.
Indoor concentrations of both VLPs and BLPs were ∼10<sup>5</sup> particles m<sup>–3</sup>, and the virus:bacteria ratio was
0.9 ± 0.1 (mean ± standard deviation across different locations).
There were no significant differences in concentration between different
indoor environments. VLP and BLP concentrations in outdoor air were
2.6 and 1.6 times higher, respectively, than in indoor air. At the
single outdoor site, the virus:bacteria ratio was 1.4
