When assessing chemical exposure at Superfund sites, the Agency for Toxic Substances and Disease Registry (ATSDR) sometimes encounters volatilization of chemicals from household water, a pathway that might have a significant impact on families. Historically, ATSDR evaluated this pathway using a one-compartment model (Andelman, 1985). The one-compartment model estimates exposure to volatilized chemicals from showering only, however, it does not include exposure from 1) showers by other household members, 2) household appliances that use water, and 3) time spent in the house throughout day. ATSDR needed a better model.
To meet this need, ATSDR developed a three-compartment Shower and Household Water-Use Exposure (SHOWER) model that captures inhalation exposure from not only showering but also being in the bathroom and in the house throughout the day. The model includes contributions from showers and tub baths taken by other family members, as well as the contribution from other water sources in the house such as clothes washers, dishwashers, toilets, and faucets. The model can account for persons being away from home during the day and for using a bathroom fan. The SHOWER model is a more comprehensive model that includes multiple pathways of exposure (i.e., inhalation and dermal) from the most common indoor water sources and usage for households with up to four persons. ATSDR released the SHOWER model in May 2018.
The SHOWER model mathematically characterizes volatilization from multiple water sources in each compartment: shower water in the shower stall; the toilet, sink, faucet, and bathtub in the bathroom; and kitchen faucet, clothes washer, and dishwasher in the main house (McKone, 1987). Using air-mixing formulas, the model predicts the indoor air contaminant concentrations in each compartment within the house as a function of time by solving a set of constantly changing mass balance equations (Kim, Little, & Chiu, 2004):
[mathematical expression not reproducible]
[V.sub.i] = volume of compartment i,
[C.sub.i](t) = air concentration in compartment i at time t,
[[??].sub.ij] = air exchange rate from compartment i to j,
[[??].sub.ji] = air exchange rate from compartment j to i,
[C.sub.j](t) = air concentration in compartment j at time t, and
[S.sub.ik](t) = contaminant source in compartment from a chemical volatilizing from a water source, removal of contaminated air by the exhaust fan, or migration to outdoor air.
A detailed description of the model is available elsewhere (Agency for Toxic Substances and Disease Registry, 2018). The model has...