Slight amounts of radioactive iodine from the Soviet Union have been detected in rainwater in the Pacific Northwest, but U.S. officials said the levels pose no health threat and residents need not take special precautions.

The first measurement of radioactive rain from the nuclear disaster in Chernobyl came in Richland, Wash., where technicians detected iodine 131 at the rate of 500 picocuries per liter in a light sprinkling of rain that fell Monday. Slightly higher levels, 630 picocuries per liter, were reported later in Portland, Ore.

In either case, a person who drank a quart of the water would receive less than half as much radiation as from a standard chest X-ray, according to a federal interagency task force monitoring the fallout. External contact with the water would result in even lower doses, the task force said.

"We are way, way below anything that would trigger action in the United States," said Lee M. Thomas, Environmental Protection Agency administrator and task force chairman.

At 500 picocuries, the rain contains more than 150 times as much radioactive iodine as permitted by EPA drinking-water standards. The federal standards are aimed at protecting someone who drinks water with that level of radioactivity for a lifetime, however, and officials said there is no danger in higher levels for short periods. Iodine 131 has a half-life of eight days, which means that its radioactivity is reduced by 50 percent every eight days.

Robert Mooney, supervisor of the Washington state Office of Radiation Protection, said state officials would not request people to wash leafy vegetables or take other precautions unless rainwater reached levels of 10,000 picocuries or more per liter.

The federal task force released no data on airborne radiation at ground level, but state officials said they had measured .06 picocuries per cubic meter -- about one-seventieth of the radiation that the Pacific Northwest received from nuclear tests in China in 1979.

Radiation levels may rise later in the week as additional radioactivity wafts over the United States at lower altitudes, according to Lester Machta, head of the National Oceanic and Atmospheric Administration's air resources laboratory.

Federal officials are monitoring air and water daily, and have stepped up milk sampling to twice a week. The safety of milk is considered of key importance to public health because dairy products represent a significant portion of the diet, especially for small children who are most vulnerable to radiation.

Guidelines drawn up by the Food and Drug Administration four years ago and released yesterday by the task force indicate that no emergency action to protect the milk supply will be deemed necessary unless levels of radioactive iodine reach at least 130,000 picocuries at the ground level.

At that level, dairy cows might be removed from pasture. According to the guidelines, milk contaminated with 15,000 picocuries of iodine might be destroyed or withheld from market long enough to let some of the radioactive material decay.

Machta said current weather patterns suggest that the high-altitude cloud, by now broken into patches and diluted, is moving toward Minnesota and Michigan.

Aircraft also have detected radioactivity at lower altitudes, but those air masses have yet to reach the United States.