However, the results of testing reflected expectations by International Atomic Energy Agency officials that radiation had dissipated so much by the time it reached the U.S. coastline that it posed no health risk whatsoever to residents.
The U.S. Department of Energy said minuscule amounts of the radioactive isotopes iodine-131, iodine-132, tellurium-132 and cesium-137 had reached a Sacramento monitoring station tied to the U.N.'s Comprehensive Test Ban Treaty Organization, but the readings were far below levels that could pose any health risks.
A detector at the Pacific Northwest National Laboratory in Washington State earlier this week also detected trace amounts of xenon-133 -- a gas produced during nuclear fission -- the DOE said.
The doses that a person normally receives from rocks, bricks, the sun and other natural background sources are 100,000 times the dose rates detected at either location, the DOE and the U.S. Environmental Protection Agency said in a joint statement.
The statement confirmed statements from diplomats and officials in Vienna earlier in the day.
Air pollution regulators in Southern California said they have not detected increased levels of radiation. The South Coast Air Quality Management District said radiation measured at its three sites was not higher than typical levels.
The agency's monitors are part of the EPA's network of more than 100 sensors across the nation that track radiation levels every hour.
In Alaska, Dr. Bernd Jilly, director of state public health laboratories, also said monitoring had shown no readings of above-normal levels of radiation.
The same was true in the state of Washington, health department spokesman Donn Moyer said. The levels would have to be hundreds of thousands of times higher than current readings before health officials would recommend any response, he said.
Graham Andrew, a senior official of the Vienna-based International Atomic Energy Agency, said that after consultation with the IAEA, the International Civil Aviation Organization found there was no reason to curtail normal international flights and maritime operations to and from Japan and "there is no medical basis for imposing additional measures to protect passengers."
The CTBTO presentation Friday showed radiation levels peaking in Tokyo and other cities in the first days of the disaster at levels officials said were well below risk points before tapering off.
"The rates in Tokyo and other cities ... remain far from levels which require action, in other words they are not dangerous to human health," Andrew said.
While set up to monitor atmospheric nuclear testing, the CTBTO's worldwide network of stations can detect earthquakes, tsunamis and fallout from nuclear accidents such as the disaster on Japan's northeastern coast that was set off by a massive earthquake and a devastating tsunami a week ago.
Since then, emergency crews have been trying to restore the Fukushima Dai-ichi nuclear plant's cooling system and prevent overheated fuel rods from releasing greater doses of radioactivity.
Japanese officials on Friday reclassified the rating of the accident at the plant from Level 4 to Level 5 on a seven-level international scale, putting it on a par with the 1979 Three Mile Island accident. The International Nuclear Event Scale defines a Level 4 incident as having local consequences and a Level 5 as having wider consequences.
Nuclear experts have been saying for days that Japan was underplaying the severity of the nuclear crisis.
Andrew refused to be drawn on that issue, saying severity assessments would be the task of a post-emergency investigation. Describing the situation as very serious, he nonetheless noted no significant worsening since his last briefing Thursday, when he used similar terminology.
Things are "moving to a stable, non-changing situation, which is positive," he said. "You don't want things that are rapidly changing."
