  How may exposure risks to SARS-CoV-2 be assessed quantitatively? The material
metabolism approach of Industrial Ecology can be applied to the mass flows of
these virions by their numbers, as a key step in the analysis of the current
pandemic. Several transmission routes of SARS-2 from emission by a person to
exposure of another person have been modelled and quantified. Start is a
COVID-19 illness progression model specifying rising emissions by an infected
person: the human virion factory. The first route covers closed spaces, with an
emission, concentration, and decay model quantifying exposure. A next set of
routes covers person-to-person contacts mostly in open spaces, modelling the
spatial distribution of exhales towards inhalation. These models also cover
incidental exposures, like coughs and sneezes, and exposure through objects.
Routes through animal contacts, excrements, and food, have not been quantified.
Potential exposures differ by six orders of magnitude. Closed rooms, even with
reasonably (VR 2) to good (VR 5) ventilation, constitute the major exposure
risks. Close person-to-person contacts of longer duration create two orders of
magnitude lower exposure risks. Open spaces may create risks an order of
magnitude lower again. Burst of larger droplets may cause a common cold but not
viral pneumonia as the virions in such droplets cannot reach the alveoli.
Fomites have not shown viable viruses in hospitals, let alone infections.
Infection by animals might be possible, as by cats and ferrets kept as pets.
These results indicate priority domains for individual and collective measures.
The wide divergence in outcomes indicates robustness to most modelling and data
improvements, hardly leading to major changes in relative exposure potentials.
However, models and data can substantially be improved.
