The readings, picked up by highly sensitive detectors set up to monitor clandestine nuclear blasts, were the first evidence of the feared radioactive plume that has gradually travelled across the Pacific with the prevailing winds over the past week and has now reached the continental United States, The New York Times reports.
Health experts, however, assured that the plume's radiation had been diluted enormously in its journey across thousands of miles, adding that its very low concentrations would have no health consequences in the United States.
The paper quoted Lars-Erik De Geer, research director of the Swedish Defense Research Agency, a part of the monitoring system, as saying that: "What we can measure is almost a single atom, which has absolutely no danger" for human health.
"It has to be very sensitive because we are looking for people who are trying to hide the testing of weapons," he added.
Another expert Mike Sicilia, a spokesperson in Sacramento for the California Department of Public Health, also emphasised that the danger of being affected by the radiation very low.
"We're monitoring the situation. All data from state and federal sources show that harmful levels of radiation won't reach California," Sicilia added.
Environmental and watchdog groups, however, have cited a growing anxiety in the United States and complained of a lack of adequate information from American officials, adding: "The U.S. government clearly has information that the public has a right and need to know."
President Barack Obama had said on Thursday that he was aware of American's concern over the matter, but had assured his countrymen saying: I want to be very clear, we do not expect harmful levels of radiation to reach the United States, whether it's the West Coast, Hawaii, Alaska or US territories."