The quantities of radiation reaching Canada are very small and do not pose any health risk to Canadians. In the first few weeks of the nuclear incident in Japan, we have seen very slight increases in radiation across the country, smaller than the normal day to day fluctuations.
The unit generally used to measure a dose of radiation in material is the Gray (Gy). For example, the data routinely reported for Health Canada's Fixed Point Surveillance (FPS) Network is given in total nGy per month (1 Gy = 1,000,000,000 nGy).
However, the health impact of radiation depends on both the type of radiation and the part of the body exposed. The unit generally used to express this dose is the Sievert (Sv), which takes these variations into account. A fraction of a Sievert is also commonly used to represent smaller doses. The doses we are measuring are so small that we are using the unit microSievert (ÁSv) so that the data is meaningful.
The data reported by Health Canada from the FPS Network in response to the Japan nuclear incident is given in ÁSv/day, in order to be consistent with other estimates of doses to humans calculated by Health Canada. In the case of the data from the FPS network, 1 Sv is equivalent to 1 Gy, therefore the data can be directly compared.
Measured doses will vary from one location to another. This is mainly due to the difference in the local geological composition and altitude of the locations. Doses measured will also vary over short periods of time (day to day) depending on the weather conditions and seasonal changes. Rain, as an example, can raise the readings from 2 to 4 times and in northern stations such as Yellowknife we can expect to see gradual increases in radiation levels as the snow melts because the snow shields the radiation in the ground.
Radiation is produced when an unstable atom spontaneously disintegrates and releases excess energy. The activity of a radioactive sample is measured by the number of disintegrations per second, in units called Becquerels. One Becquerel (Bq) is equal to one disintegration per second. The radiation concentration data are reported in milliBecquerels per cubic meter (mBq/m3) or Becquerels per cubic meter (Bq/m3).