One chemical alone may do no harm in low doses, but in conjunction with a few of its peers, even in doses that are individually safe, it can inflict serious harm. New research in frogs shows that a mixture of nine chemicals found in a seed-corn field in York County, Nebraska, killed a third of exposed tadpoles and lengthened time to metamorphosis by more than two weeks for the survivors.

Biologist Tyrone Hayes and his colleagues at the University of California, Berkeley, have spent the past four years testing four herbicides, two fungicides and three insecticides commonly used in American cornfields. Individually, the chemicals had little effect on developing tadpoles at low concentrations, such as about 0.1 part per billion. But when Hayes exposed them to all nine at the same low level in the laboratory--the lowest level actually found in the field--the future frogs fell prey to endemic infection. Those that survived ended up smaller than their counterparts raised in clean water--despite taking longer to mature into adults. "In humans, this is like saying, 'The longer you are pregnant, the smaller your baby will be,' which means the womb is no longer a nurturing environment," Hayes notes.

Hayes's study joins a growing body of work showing that chemicals in combination can produce a wide range of effects even at low concentrations. Rick Relyea of the University of Pittsburgh has shown in several studies that tadpoles exposed in their water to low levels of a single pesticide and the smell of a predator will face significantly higher mortality rates. For instance, about 90 percent of bullfrog tadpoles died from exposure to the pesticide carbaryl when the smell of predatory newts was present, whereas no tadpoles perished if exposed to each individually. The pesticide may be inducing a general stress in the tadpole that, when combined with another stressor, becomes deadly, Relyea argues.

It is not just pesticides that show a mixture effect. Phthalates--chemical softeners that make polymers flexible--can interfere with the sexual development of male rats. "We have males treated with phthalates where the testes are under the kidneys or floating around in the abdominal cavity," explains L. Earl Gray, Jr., a biologist at the Environmental Protection Agency and co-discoverer of this deformity, which has been dubbed phthalate syndrome. Gray has also found that various kinds of phthalates in combination either with one another or with certain pesticides and industrial effluents exert ever more powerful effects. For example, two phthalates at concentrations that on their own would not produce much deformity combined to create defective urethras (hypospadias) in 25 percent of exposed rats.

Besides adding to the issue of endocrine disruption--whether industrial chemicals are mimicking natural hormones--the findings on mixtures pose an incredible challenge for regulators. With tens of thousands of chemicals in regular use worldwide, assessing which combinations might prove harmful is a gargantuan task. "Most of the offices in the agency recognize that we cannot operate via the idea of 'one chemical, one exposure' to an individual anymore. We need to look at broader classes of compounds and how they interact," says Elaine Francis, national program director for the EPA's pesticides and toxics research program. But such testing has a long way to go to reach any kind of regulation, particularly given industry's qualms about the validity of existing research.

Marian Stanley, who chairs the phthalates panel for the American Chemistry Council, notes that at least one study showed that rodents suffering from phthalate malformations could still mate and have litters. "The additivity of phthalates alone are on end points that may not have any biological relevance," she says.

Nevertheless, evidence continues to accumulate that mixture effects are a critical area of study. In its National Water Quality Assessment, the U.S. Geological Survey found that a sampling of the nation's streams contained two or more pesticides 90 percent of the time. "The potential effects of contaminant mixtures on people, aquatic life and fish-eating wildlife are still poorly understood," states hydrologist Robert Gilliom, lead author of the study. "Our results indicate, however, that studies of mixtures should be a high priority."