The United States is boosting radiation monitoring on the West Coast and Pacific territories, as officials forecast that low levels could be detected in California by Friday.

While authorities say they do not expect harmful levels, the Environmental Protection Agency (EPA) is sending the monitoring units to Alaska, Hawaii and Guam to boost an existing network, said an official Thursday.

Speaking on condition of anonymity, the official stressed that the move is precautionary. "We don't expect any significant amounts of radiation," he said, adding that the EPA was expected to brief reporters later in the day.

The US western states of California, Oregon, and Washington State have also been monitoring for any increase in radiation levels from Japan's Fukushima nuclear power plant, damaged by last Friday's earthquake and tsunami.

California's Emergency Management Department, meanwhile, noted that California could detect heightened radiation as early as Friday.

The forecast, by the UN's Comprehensive Nuclear Test Ban Treaty Organization, "indicates that monitoring stations in Southern California may be able to detect extremely low levels of radiation late Friday," it said.

"This projection is based on patterns of Pacific winds at the time of the forecast and is likely to change as weather patterns shift," it added.

It added that if a radioactive plume moved across the Pacific at its current strength, "it may not be detectable and would pose extremely minor health consequences even if trace amounts are detected."

Hawaii is some 4,000 miles (6,500 km) from Japan, while the California coast is some 5,500 miles (8,800 km).

California's emergency department also noted that the US Nuclear Regulatory Commission expects no harmful levels of radioactivity to reach the United States from Japan "due to the significant distance between the two countries."

President Barack Obama also sought to reassure Americans Thursday, saying: "We do not expect harmful levels of radiation to reach the United States, whether it is the west coast, Hawaii, Alaska or US territories in the Pacific."

The main US manufacturer of potassium iodide pills, which can protect against the effects of radiation, ran out of supplies within hours of the Japanese earthquake, according to the company's boss.

The decisions to deploy extra radiation monitoring units was made this week, according to the official, who gave no more details on when they would begin operating.

earlier related report
No drop in Chernobyl cancer risk: US study
Washington March 17, 2011 - The risk of thyroid cancer among people who were exposed as children to the nuclear fallout at Chernobyl has not declined nearly 25 years after the disaster, said a study released Thursday in the United States. The National Institutes of Health-led study examined more than 12,500 people who were under 18 at the time of the Chernobyl accident on April 26, 1986, and who lived near the accident site in one of three parts of Ukraine. Each person's thyroid radioactivity levels were measured within two months of the accident, and they were screened for thyroid cancer four times, beginning as early as 12 years after the disaster and continuing for 10 years. Sixty-five of those in the study were diagnosed with thyroid cancer. When researchers examined the cancer risk in relation to how much exposure to radioactive iodine-131 (I-131) each person received, they found a two-fold increase for each additional gray, an international unit of absorbed radiation. "The researchers found no evidence, during the study time period, to indicate that the increased cancer risk to those who lived in the area at the time of the accident is decreasing over time," said the study. Overall, the "clear dose-response relationship, in which higher absorption of radiation from I-131 led to an increased risk for thyroid cancer... has not seemed to diminish over time," it said. The study was carried out by an international team of scientists and was headed by the National Cancer Institute, which is part of the NIH. It appears in the March 17 issue of the journal Environmental Health Perspectives. Separate studies involving people who survived atomic bombs have shown that cancer risk begins to decline after 30 years, but remains high after 40 years compared to the general population. The study authors said more follow up is needed to judge if or when such a decline may be occurring among survivors of Chernobyl, the worst nuclear accident ever. The disaster, whose memory has been revived with Japan's attempts to contain overheating at its Fukushima nuclear plant after a 9.0-magnitude earthquake and massive tsunami, involved a full-scale nuclear explosion in the number-four reactor at Chernobyl as a result of human error during a botched testing procedure. Some five million people are believed to have been affected by the disaster in Belarus, Russia and Ukraine, where millions of acres (hectares) of agricultural and forest land remain contaminated. The disaster's death toll is hotly debated. UN agencies estimate up to 9,000 people could be expected to die as a direct consequence of the accident, and that the disaster will end up costing hundreds of billions of dollars. Environmental groups such as Greenpeace say up to 100,000 people could die.