Modeling a shower's toxic threat.

Studies have documented the presence in drinking water of many potentially toxic volatile organic chemicals (VOCs) -- from chloroform and pesticides to carbon tetrachloride. Such findings have spurred investigations into the inhalation hazards these compounds may pose when released into the air during baths and showers. However, because shower and tub equipment, as well as other design features, differed widely in these experiments, air releases for a single VOC could vary up to 10-fold from one studied system to another, observes John C. Little of Lawrence Berkeley (Calif.) Laboratory.

A general model incorporating known factors such as a chemical's volatility, water temperature, and the flow rates of water and air would help gauge the relative impacts of other shower variables. Little developed such a model, described in the July ENVIRONMENTAL SCIENCE AND TECHNOLOGY, by applying a classic engineering theory to data from five shower studies that others conducted. The trick to modeling a shower's turbulent air-water interface, he explains, is to treat the material on either side of that interface as a stagnant thin film.

The most volatile chemical studied in the experiments was a Freon known as CFC-11. Little's worst-case analyses indicate that people showering for 10 minutes in water containing this VOC would inhale 50 percent more of the potential toxicant than they would ingest by drinking two liters of that water. Indeed, his analyses indicate, under the same shower conditions, exposures to CFC-11 would be three times higher than those to the lowest-volatility VOCs studied -- the pesticide 1,2-dibromo-3-chloropropane, a suspected carcinogen.

The more volatile chemicals also take longer to saturate indoor air. "I don't want to sound alarmist," Little says, "but this could have serious implications for institutional shower facilities, like health clubs," that are in use all day and don't provide a lot of ventilation. While air concentrations of the low-volatility compounds peak in enclosed spaces in about three hours, Little's model indicates that concentrations of very volatile compounds can keep growing for up to 16 hours.

By confirming that water turbulence represents one of the biggest factors affecting a VOC's release, Little says, the model suggests that ways to minimize a shower head's creation of turbulent mists may offer a fertile field for study.

The new model makes possible "much better predictions of VOC-inhalation exposures from showers" for chemicals and conditions not yet studied, Little says. At a minimum, that should help drinking-water regulators, he adds, since it makes no sense to regulate contaminants solely on ingestion risks if daily showers can present comparable or greater dangers.