Super-sensitive detectors on the West Coast of Canada and the U.S. -including one on Vancouver Island -may soon be picking up radioactive particles from Japan's crippled nuclear reactors, a forecast from a United Nations agency suggests.

Experts say the amounts will be so minuscule they'd pose no health risk, but the monitors could help show how the radiation disperses around the planet in coming days and weeks.

Maps generated by the UN agency that monitors for clandestine nuclear tests show the plume of radiation wafting across the Pacific, hitting the Aleutian Islands before sweeping down toward the West Coast by the weekend.

The forecast was produced by the Preparatory Commission for the Comprehensive NuclearTestBan Treaty Organization that runs a global network of 63 radionuclide detectors that constantly "sniff" the air. The network includes four stations in Canada. One of them is at Sidney. The others are in or near Yellowknife, Resolute in Nunavut and St. John's.

The detectors are so sensitive that the one in Yellowknife picked up traces of radioactive gas from a North Korean nuclear bomb test in 2006, said Lassina Zerbo, head of the agency's International Data Center and officer-incharge of the CTBTO.

The agency's forecast, calculated Tuesday, is likely to change as weather patterns shift; scientists expect the Japanese radiation will eventually disperse around the Northern Hemisphere.

The CTBTO is not allowed to release its data and forecasts directly to the public. But Zerbo said they are shared with 120 countries, including Canada. The agency's forecast showing the dispersal of radiation from Japan was released by scientists in Europe and was published in the New York Times on Thursday.

Health Canada refused to comment on the CTBTO's map or discuss the four monitors it operates for the UN agency. An official said that any radioactive material is expected to be dispersed over the ocean long before it reaches Canada.

Based on the forecast, the CTBTO detector located outside the federal Natural Resources Canada labs at Sidney might be among the first to detect radiation on this side of the Pacific.

Geophysicist Ulrike Schmidt operates the detector located near the Natural Resources Canada lab at Sidney and is responsible for making sure the detector's filter -a large sheet almost as big as a newspaper page -is changed daily.

"It has to be done at the same time every day," says Schmidt, who explained Thursday how she dons latex gloves to change the filter following procedures to ensure everything is kept "scrupulously clean."

The detector, a globeshaped device on a high point of land behind a secure fence, has a motor that sucks in 24,000 cubic metres a day through the filter. "It's a lot of air that gets pulled through," says Schmidt.

The filters she removes are folded into a device and compressed into a disc smaller than a hockey puck. It is placed in a special chamber and a spectrometer measures the radionuclides that have been captured. The data is relayed by satellite to Vienna for analysis.

Asked if she's been given any instructions in light of the Japanese accident, Schmidt said: "Just to keep everything running flawlessly."

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