  Airborne diseases can be transmitted by infectious aerosols in the near
field, i.e., in close proximity, or in the far field, i.e., by infectious
aerosols that are well mixed within the indoor air. Is it possible to say which
mode of disease transmission is predominant in large indoor spaces? We
addressed this question by measuring the transport of aerosols equivalent to
the size of human respiratory particles in two large hardware stores (V>10000
m$^3$). We found that aerosol concentrations in both stores decreased rapidly
and almost independently of aerosol size, despite the different ventilation
systems. A persistent and directional airflow on the order of a few cm/s was
observed in both stores. Consequently, aerosol dynamics in such open settings
can be expected to be dominated by turbulent dispersion and sweeping, and the
accumulation of infectious aerosols in the indoor air is unlikely to contribute
significantly to the risk of infection as long as the occupancy of the store is
not too high. Under these conditions, well-fitting face masks are an excellent
means of preventing disease transmission by human aerosols.
