SAN FRANCISCO — By scientific standards, the radiation found over the last week in batches of milk on the West Coast was minuscule and, moreover, not dangerous to humans.
But the mere mention of any contamination in that most motherly of beverages still stirred concern in people like Marilyn Margulius, an interior designer from Berkeley, Calif., who called her daughter on Thursday and told her not to let her 10-year-old son drink milk.
“There is a big trust issue with this,” said Ms. Margulius, 71, who was shopping at a Whole Foods Market in Berkeley.
“The health department does not want people to panic. Milk is probably O.K., but who the heck knows?”
There have been repeated public assurances this week — officials said an adult would need to drink thousands of liters of the milk containing radiation at the levels found so far before it would be remotely dangerous. Officials also tried to tamp down anxiety from dairy farmers concerned about bad press.
“I’ve had members call to ask whether we’ve seen the media, and media calling to ask how this is impacting our members,” said Michael Marsh, the chief executive of Western United Dairymen, the milk industry’s West Coast trade association. Mr. Marsh said he had repeated the assurances given by officials, but he also understood the fears in the supermarket’s refrigerated aisle.
“Consumers, doubtless, when they hear about something like this, the cautionary principle kicks in,” he said. “Even if you’d have to drink an oil tanker of this.”
Dr. Elizabeth N. Pearce, an associate professor of medicine at the Boston University School of Medicine and an expert on the thyroid, where radioactive iodine is often absorbed, said there was no health risk, even for small children, from milk on American shelves.
Still, she said, “If people feel safer not drinking it, there are no long-term health effects from abstaining for two weeks, either.”
The alarm was sounded on Wednesday, when federal officials announced that tests had detected a trace amount of iodine 131 — a radioactive byproduct released by leaks at the Fukushima Daiichi nuclear plant in Japan — in a sample taken on March 25 in Spokane, Wash. The level of radiation was tiny and would have to be more than 5,000 times higher to reach the “intervention level” set by federal officials.
Jason Kelly, a spokesman for the Washington State Department of Agriculture, said the positive sample came from a gallon of pasteurized whole milk produced at the Darigold plant in Spokane, which processes milk from a number of farms in Washington and Idaho.
Jim H. Klein, a spokesman for Darigold, said milk from the Spokane plant was distributed in the Northwest, but said there was no reason for concern.
Some people even saw a potential silver lining in their bottom line. Mike Vieira, owner of a small dairy west of Spokane that produces about 250 gallons of milk a day, said the scare might be good for his business because customers know where his milk comes from, as opposed to milk from larger dairies that is pooled at a processing plant.
“We’ve had regular customers come to the farm buying milk,” he said. “And they’re buying six or eight gallons.”
The California health department also confirmed Wednesday that it had detected a tiny amount of radioactive iodine in a sample collected Monday from a dairy in San Luis Obispo County on the state’s Central Coast.
Milk in San Luis Obispo is regularly tested because the Diablo Canyon nuclear power plant is on the county’s southern coast. Officials said that monitoring — done weekly since the crisis began — was accountable for finding the contamination.
Dr. Penny Borenstein, the county’s public health officer, said that the tests would continue and that dairy products would continue to be safe. “The situation in Japan continues to evolve, but we are still 6,000 miles away.”