Once a week Neil Rollinson changes a filter taken from a small monitoring station outside of his Environment Canada office in Whitehorse. The filter he now holds in his hands does not look out of the ordinary, and the type of grunge on its surface looks similar to what you would find on your standard home furnace filter.

But these filters are capable of trapping particles which none of us would want to find in significant amounts in our homes. The glass fibre filters are installed in air sampling stations that make up the Canadian Radioactivity Monitoring Network, set up in 1959 to determine the distribution of fallout across Canada from atmospheric nuclear weapons testing.

The network of 26 monitoring sites is now being updated, and new equipment was installed at the Whitehorse station this past June. The new stations will need less maintenance and will also be more accurate than the ones they are replacing.

There have always been background levels of radiation in the environment from natural sources such as rocks and soils, and information from the monitoring network will help improve understanding of how these naturally occurring radionuclides move through the environment.

But the main purpose of the system will continue to be tracking artificial radioactive substances, which began to enter the environment about 50 years ago primarily because of atomic bomb tests.

In 1980 China was the last country to explode an atomic bomb in the atmosphere. In 1996 all the major powers have agreed in principle to a treaty forbidding the testing of atomic bombs. Even though radiation levels are now negligible, that does not mean that monitoring is not needed.

"Now our primary purpose is to measure fallout from any new tests or from an actual nuclear explosion or an accident at a nuclear power station," explains Sonia Swenson, a technologist with the monitoring network.

Results and samples generated by the network are also used for research into climate change and for atmospheric dispersion modeling.

A separate worldwide network of monitoring stations is also being built to ensure that the nuclear test ban treaty is not broken. The 80 stations in this network, which will include locations in Yellowknife and Resolute, will have equipment capable of immediately detecting any increases in radiation.

Filters from equipment in the Canadian network are changed weekly, and shipped via regular mail to a laboratory in Ottawa for analysis. But when a nuclear event does take place, the pace is stepped up.

After the 1986 explosion at the Chernobyl nuclear power plant, the filters were changed daily and monitoring took place around the clock. Milk was also tested to ensure that the public was being protected from radioactivity.

In Canada, the Chernobyl explosion increased concerns among many northerners about the safety of caribou meat, one of their most important foods. Caribou eat large quantities of lichens, which act like sponges and collect radioactive substances that fall on them from the air. This radioactivity is then passed on to people who eat the meat.

The levels of radiocesium in caribou were quite high in the early 1960's when many atomic bombs were being tested, and northerners were concerned that fallout from Chernobyl would raise levels again. In 1989 many northern residents, including people in the communities of Old Crow and Fort McPherson, took part in a special study designed to measure the amount of radiocesium in their bodies.

The levels of radiocesium detected in people were negligible, and it was determined that only 25 percent of the radiocesium in Canadian caribou originated from the Chernobyl accident; the rest was residual fallout from earlier atomic weapons testing.

Sonia Johnson, the acting head of the Canadian Radiation Monitoring Network, says that falling radiation levels mean that they are finally getting information on normal background levels of radiation. "It is nice to be able to track what is happening now because when the program started off we were getting high levels, and now we are getting the baseline information."

For more information on the Canadian Radioactivity Monitoring Network, contact Neil Rollinson at Environment Canada in Whitehorse, 633-4443.