Aerosol particles proved to play a key role in airborne transmission of SARS-CoV-2
viruses. Therefore, their size-fractionated collection and analysis is invaluable.
However, aerosol sampling in COVID departments is not straightforward, especially
in the sub-500-nm size range. In this study, particle number concentrations were measured
with high temporal resolution using an optical particle counter, and several 8 h daytime
sample sets were collected simultaneously on gelatin filters with cascade impactors
in two different hospital wards during both alpha and delta variants of concern periods.
Due to the large number (152) of size-fractionated samples, SARS-CoV-2 RNA copies
could be statistically analyzed over a wide range of aerosol particle diameters (70–10
µm). Our results revealed that SARS-CoV-2 RNA is most likely to exist in particles
with 0.5–4 µm aerodynamic diameter, but also in ultrafine particles. Correlation analysis
of particulate matter (PM) and RNA copies highlighted the importance of indoor medical
activity. It was found that the daily maximum increment of PM mass concentration correlated
the most with the number concentration of SARS-CoV-2 RNA in the corresponding size
fractions. Our results suggest that particle resuspension from surrounding surfaces
is an important source of SARS-CoV-2 RNA present in the air of hospital rooms.
