1. conservative estimates of the maximum individual dose from Fukushima was less than 0.0003 (1/ 3,000) of the typical annual dose for a Canadian owing to natural background sources
2. the additional dose resulting from Fukushima derived contamination is far less than the normal variation in dose from place to place in Canada
3. there are likely to be no health impacts related to this small, incremental dose

The new report details the monitoring activities by Canadian agencies during the period March — June 2011 and can be downloaded here.  Full data tables are available in the original document and I urge those interested to read the entire report.

The Monitoring Network

In the case of Health Canada, scientists working for the Radiation Protection Bureau increased the frequency with which they made measurements. Observations are/were made using three systems:

1. The Canadian Radiological Monitoring Network (CRMN) The CRMN includes 26 monitoring stations located across Canada that measure radionuclides in air and precipitation and provide measurements of external ionizing radiation dose.  Stations are typically equipped with an air sampler that pumps air through a filter to sample airborne particles and a cup that collects rain and snow. Thermoluminescent dosimeters (TLDs) are installed at CRMN sites to measure the total exposure to gamma radiation. In the aftermath of the Fukushima disaster sampling frequency was increased from weekly to daily so that air filters could be measured to identify specific radionuclides and determine their concentrations in air and higher frequency measurements of contamination in rain and snow and external does could be made.  The location of CRMN stations is shown in the figure below.

2. The Fixed Point Network (FPN) which measures radiation levels in air in real time.  Since 2000, Health Canada has complemented its network with 77 automated radiation monitoring stations where each station is equipped with a high sensitivity sodium iodide (NaI) detector which measures gamma radiation levels in real time.  NaI detectors measure radioactivity of airborne and ground deposited contaminants. These stations are located near major population centres, around nuclear power plants sites across Canada. These stations operate 24/7 sending data back to Health Canada every 24 hours under routine conditions but can send every 15 minutes during emergency situations.

3. A more sensitive system of detectors that represents Canada’s contribution to the Comprehensive Nuclear Test Ban Treaty (CTBT) monitoring network can sense much lower levels of radionuclide activity in air but not in real time.  Measurements are made on air samples integrated over a 24h period of time and while not normally reported to the public were posted for citizens in response to Fukushima between March-August 2011.

Tracking the Contamination from Fukushima

As an example of the monitoring data collected by the network following the first major releases from the Fukushima Dai-ichi plant on March 2011 the figure below tracks the atmospheric levels and arrival times of Cesium-137 (¹³⁷Cs half life ~30 years) at various locations across Canada.

The levels of radiologically significant isotopes, that could pose a health risk (e.g. ¹³¹I, ¹³⁴Cs, ¹³⁷Cs), measured in air from Fukushima were about an order of magnitude lower than levels measured in 1986 post-Chernobyl and many orders of magnitude lower than levels during the height of atmospheric weapons testing in the 20^th century.  The following figure puts the levels measured into perspective.

Impact of the FDNPP Disaster on the Health of Canadians

Based on measurements made by the monitoring network the external dose of ionizing radiation experienced by an individual in Canada was not significantly elevated in the wake of the Fukushima disaster compared to variability in natural background exposure.

By using measurements of contamination in food, air and water additional internal dose can be determined and the total additional dose from the accident to Canadians can be estimated in units called Sieverts (Sv).  Total additional doses from Fukushima are in millionths of a Sievert or microSv (μSv = 10^-6 Sv).   μSv is a very small fraction of the normal background radiation dose where the total dose from normal background radiation in Canada is 2,000 – 3,000 μSv per year (μSv yr^-1).  Note that a typical chest x-ray gives an additional dose of about 20 μSv.  The estimated doses from Fukushima contaminants in the first year following the disaster showed very little variation across Canada with the doses to an adult falling in a range from 3.8 – 4.4 μSv, doses to a 10 year old child ranging from 1.8 – 2.4 μSv, and doses to a 1 year old infant from 1.1 – 1.9 μSv. The maximum calculated dose of 4.4 μSv is a very small fraction of the 2,000 – 3,000 μSv dose from natural background ionizing radiation in Canada.

Given how small the incremental dose from Fukushima contaminants in the Canadian environment was in the first year following the accident when compared to natural background ionizing radiation doses, health physicists at Health Canada conclude there will be no health impacts in Canada from the disaster.

Detailed information on individual isotopes and their exposure pathways can be found in the original Health Canada report.