# What Do Radiation Measurements Mean?

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.

• On average, 80% of exposure (2400 µSv) is due to naturally-occurring sources, 19.6 % (almost 600 µSv) is due to the medical use of radiation and the remaining 0.4% (around 10 µSv) is due to other sources of human-made radiation such as medical isotope production.
• For reference, a long, cross-country flight could expose a person to about 30 µSv of radiation, diagnostic procedures such as a dental X-rays may provide 10 µSv of radiation, and CT scans may deliver an approximate radiation dose of 5,000-30,000 µSv.

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 (µSv) so that the data is meaningful.

1Sv = 1,000mSv = 1,000,000 µSv
0.000001Sv = 0.001mSv = 1 µSv

The data reported by Health Canada from the FPS Network in response to the Japan nuclear incident is given in µSv/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.