The Food and Drug Administration almost never tests products itself. But in May, the Journal of the American Medical Association published the results of a randomized trial, conducted by F.D.A. researchers, to determine whether the chemicals in four commercially available sunscreens are absorbed through the skin into the bloodstream. Four times daily, subjects were coated in one of the formulas in an amount determined to be the maximum a person might use: two milligrams per square centimeter of skin over 75 percent of the body. Later, blood samples were drawn and analyzed. All of the sunscreen chemicals were detected in concentrations that exceeded an F.D.A. threshold past which manufacturers are required to do further toxicology tests. “People who use sunscreens very reasonably presume they have been tested and are safe and effective,” says Kanade Shinkai, a dermatologist at the University of California, San Francisco, and an author of an editorial accompanying the JAMA study. “And we don’t really have that evidence.”

Legally, the U.S. regards sunscreen as a drug, meaning a substance “intended for use in the diagnosis, cure, mitigation, treatment or prevention of disease” — in this case, sunburn and skin cancer — and/or one that affects “the structure or any function of the body.” Until 1962, drugs could be sold in the U.S. without any data to support claims of their efficacy. But that year, reports that a sedative called thalidomide had caused severe birth defects in thousands of babies in Western Europe led to the Kefauver-Harris Amendment to the Federal Food, Drug and Cosmetic Act, which requires drug makers to satisfy the F.D.A. that their products are safe and effective before they go on sale.

But more than 100,000 over-the-counter drug products were already on the market, including sunscreens, each of which, under the new law, needed review. To streamline the process, in 1972 the F.D.A. sorted them into therapeutic categories (antacids, for example) as assigned “monographs,” which included lists of active ingredients. If publicly available data demonstrated that these ingredients were generally safe and effective, they could be used in current and future products under conditions specified in the monograph without further review.