How dishwashers pollute the indoor air

Science News, July 10, 1999 by J. Raloff

Using an automatic dishwasher to clean up plates, glasses, and cutlery may dirty the kitchen's air, a new study finds.

Tap water contains trace quantities of potentially toxic organic chemicals, often a result of water-system chlorination. When such water is heated and sprayed, some of the waterborne pollutants enter the air.

Cynthia Howard-Reed of the Environmental Protection Agency in Reston, Va., and her colleagues now report that the heating and spraying actions of modern automatic dishwashers make these ubiquitous kitchen aids the home's most efficient means of releasing waterborne chemicals into indoor air.

The researchers conducted 29 experiments, running a residential dishwasher through its paces under a range of conditions--with varying cycles, temperatures, and numbers of dishes. Each time, they spiked the incoming water with four volatile organics. The researchers chose these chemicals to represent typical pollutants having different water solubilities. The more soluble a compound, the less likely it is to escape into the air.

Within a minute or two, the first cycle in the dishwasher stripped the water of 96 to 100 percent of toluene, ethylbenzene, and cyclohexane, the researchers report in the July 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY. Because dishwashers continuously vent some 5 to 7 liters of air per minute into the kitchen, the volatilized pollutants almost immediately begin circulating within the house.

Acetone, the most volatile of the tested chemicals, is also the most soluble in water. That's why, depending on the conditions, dishwashers spewed into air just 18 to 55 percent of the waterborne acetone, explains coauthor Richard L. Corsi, a civil engineer at the University of Texas at Austin.

Per gallon, showers release only about three-quarters as much waterborne toluene as dishwashers; hot cycles in a clothes washer, only about half as much; and kitchen faucets and washing-machine cold cycles, a mere 20 percent. However, owing to the smaller volume of water used in dishwashers, Howard-Reed's team estimates that these devices contribute only about 10 percent of the waterborne pollution spewed into indoor air, while showers contribute about 45 percent.

The amount of contamination in the incoming water largely determines how much all this water use contributes to a home's indoor-air pollution. Under some conditions, however, Corsi's team has found that appliances can add troublesome chemicals to the water. Their washing-machine tests showed that chlorine bleach reacted with organic materials in dirty clothes to generate chlorinated organic chemicals.

Since many dish detergents contain chlorine, Corsi suspects that as they react with food scraps, they "will form some pretty nasty chlorinated organics in the dishwasher, and they will volatilize." This will add to whatever chemicals had been in the water, he says.