The elevated mercury levels that were found pose little, if any, health risk for adults, but may increase the risk of neurological damage in fetuses and infants whose mothers pass on the mercury through their bloodstreams during pregnancy or through breast milk.

“These are not risks that are significant at all or existent for adults,” said Daniel Kass, the assistant commissioner for environmental surveillance and policy at the Health Department. “These are really issues for the developing brain and nervous system.”

Mercury is released into the atmosphere largely by coal-fired power plants and by solid-waste incinerators. In the form of methylmercury, it passes into lakes and rivers, where it is absorbed by fish and shellfish.

For years, the State Department of Health has issued warnings about eating fish caught in the Hudson River, because of mercury and other contaminants.

Nationally, said Mr. Kass, research showed that about 10 percent of women of childbearing age had blood mercury levels at or above five micrograms per liter, the threshold considered the low end for potential health risks.

Given that, Mr. Kass said, city health officials wanted to assess mercury exposure in a city with a diverse population, popular seafood restaurants and one of the world’s largest wholesale fish markets.

The survey, conducted among 1,811 adults in 2004, found that one-quarter of women ages 20 to 49 had a blood mercury level at or above five micrograms per liter, while nearly half of Asian women had a blood mercury level at that threshold.

The department’s report linked the higher mercury levels among Asians to eating more fish, finding that foreign-born Chinese New Yorkers eat an average of three fish meals a week, compared with about one a week among New Yorkers over all. About one-quarter of Chinese New Yorkers eat fish five or more times a week, the study found, compared with fewer than one in 15 over all.

The department also found that adults eating fish three or fewer times a week had average mercury levels below the threshold, while average readings for adults who ate fish four or more times a week were above it.

The report attributed elevated mercury levels among higher-income New Yorkers to possibly eating more expensive varieties, like swordfish or sushi-grade tuna, that are higher in mercury.

Robert Goldstein, general counsel at Riverkeeper, an environmental group that focuses on the Hudson River and upstate watershed areas, said he was not surprised by the results of the study, as more people seek alternatives to mass-produced poultry and beef.

“You’re looking for the healthy alternative you think of fish, especially when you’re trying to get all those fish oils,” he said. “So you see people turning to this, and lo and behold, there’s no question that all of these chemicals work their way into the food chain.”

Health officials emphasized that fish was still important to a healthy diet and said that the elevated levels should not pose much risk to most adults.

Still, officials said that young children and pregnant and breast-feeding women should avoid high-mercury species like Chilean sea bass, orange roughy and tilefish, while eating limited quantities of low-mercury fish.

In a brochure called “Eat Fish, Choose Wisely,” the department suggests this group eat no more than two six-ounce servings a week of low-mercury varieties like cod, mullet, scallops and canned light tuna, and no more than five such servings of very-low-mercury fish like clams, shrimp, tilapia and salmon. The brochure, in English, Spanish and Chinese, is available through the 311 telephone line.