SEATTLE -- Radiation sensors in the United States have not detected radioactive material from Japan's damaged nuclear plants, though one computer simulation suggests that traces could show up along the West Coast as early as Friday.

"All air-monitoring stations are indicating background levels," said Jonathan Edwards of the Environmental Protection Agency's radiation program.

Airborne plumes of radioactive particles will be diluted and diminished as they travel thousands of miles over the Pacific Ocean, Edwards said.

"Based on the type of material involved in this particular scenario, and based on the long distance ... from Japan, we just don't anticipate that any kind of contamination that would get to the U.S. would be harmful," he said.

A lack of accessible radiation-monitoring data has been frustrating to people on both sides of the Pacific -- but particularly in Japan, where the dangers are real and residents are losing faith in government assurances.

"It not clear they even have monitors located all around the circumference" of the nuclear-accident sites, nuclear physicist Marvin Resnikoff said in a briefing sponsored by Physicians for Social Responsibility.

The U.S. Air Force has deployed a jet equipped with sensors to track the radioactive plumes from Japan. On Thursday, other U.S. airborne sensors confirmed harmful radiation levels at the reactors but found the danger dropped off sharply beyond 19 miles.

EPA bolstered its U.S. network of radiation sensors this week with seven additional instruments deployed in Alaska, Hawaii and Guam -- closer to the unfolding crisis. The network of 100-plus stations, which originally monitored fallout from nuclear tests, provides near-real-time reports on radiation levels in the air. Levels also are measured in precipitation and milk, which can be vulnerable to contamination by radioactive iodine.

Data from the EPA monitors is available online, though it's difficult for the layperson to decipher. The agency is working to provide a more user-friendly version, a spokeswoman said. EPA also has started posting daily updates on the monitoring results.

Washington has four sensors: in Seattle, Tumwater, the Tri-Cities and Spokane.

The Tumwater station on Thursday reported levels of overall radiation, measured as counts per minute of gross beta radiation, ranging from 11 to 30. On March 7, before the earthquake and tsunami that damaged the nuclear plants, the same instrument logged beta counts between 11 and 61.

Background levels vary with wind, atmospheric conditions and local soil conditions, said Donn Moyer, spokesman for the Washington Department of Health.

"Levels would have to be tens of thousands of times higher than what we're seeing now before we would even think there's a potential health risk," he said.

A spike in levels triggers an automatic alarm. "We would be able to see any increase," Edwards said.

A United Nations computer simulation predicted that radiation released from a nuclear plant in Japan on March 12 could reach Alaska's Aleutian Island chain Thursday -- but sensors in Alaska didn't register an uptick, Edwards said.

The model, developed by the U.N. organization that tracks fallout from nuclear-weapons tests, projected that the leading edge of the plume could reach Southern California by Friday.

But computer models are only as good as their underlying assumptions -- most of which are unknown in this case, University of Washington atmospheric scientist Dan Jaffe said.

The New York Times reported Thursday that the U.N. simulation was purely theoretical, based on weather forecasts as of March 15. The model did not include data on the amount of radiation released in Japan, nor did it forecast radiation levels in the United States.

Jaffe suspects the modelers assumed that radiation from Japan would be injected high into the atmosphere, where transit time to North America would be shorter. "The winds blow a lot faster at higher elevations."

So far, radiation releases have occurred close to the ground. The explosions that rocked several reactors were not powerful enough to fling radioactive material into the zone of strong, steady winds, at least 10,000 feet up, Jaffe said. Even in the case of a larger explosion, he said it's unlikely radioactive particles would make it into the jet stream, 25,000 feet high.

Which doesn't mean radioactive material won't reach the United States. Jaffe, who has studied trans-Pacific transport of air pollutants for 15 years, said low levels are very likely to reach U.S. shores. He calculates a transit time of about two weeks for particles traveling in the lower atmosphere. During the 4,500-mile trip, particles will fall out, be washed out by rain, and be vastly diluted, Jaffe said.

"I've talked to five other meteorologists and I would say none of us have a concern about significant amounts of radiation reaching the West Coast."