According to researchers, traces of Cesium-137 and Cesium-134 – radioactive materials that leaked from Japan’s Fukushima Daiichi nuclear power plant when the Tsunami hit in June of 2011 – have been found to the waters of British Columbia. Without knowing anything further, one might be tempted to panic. Scientists who are studying the issue however, are reassuring, indicating that the Cesium-137 levels are well below Canada’s maximum allowable concentrations in drinking water.
But what is the possible significance of these numbers and these mysterious units called Becquerels? How does one relate Becquerels to the amount of radiation a human is normally exposed to, and the possible risk from excess exposure?
First some background: A Becquerel is an internationally-agreed-upon unit used to measure radioactivity. Radioactivity is also measured in REMs (for Roentgen Equivalent Man). One REM is a unit of ionizing radiation equal to the amount that produces the same damage to humans as one roentgen of high-voltage x-rays. Acute radiation syndrome (ARS) is the result of damage to DNA and other key molecules in cells and tissues, which affects the cells’ ability to divide correctly; symptoms begin within an hour or two of an acute dose and can last up to several months. Chronic radiation syndrome may result from continuous exposure to doses of radiation that are too low to cause the acute syndrome, and can manifest in other diseases, primarily cancers.
According to a Massachusetts Institute of Technology website, the average background exposure for a human is about 300 millirems (mrem)/year. A search online turned up a calculator whereby one can calculate the dose-rate in mrem/hr according to the isotope, the activity units (in this case, Becquerels), and the distance from the source of contamination.
Looking only at radioactive Ce-137, which is the bigger threat, because it has a longer half life, the levels measured in the waters of British Columbia at one Becquerel per cubic meter (Bq/m3) translate into 7.64×10-5mrem/hr. Estimates put future levels of Ce-137 at 27 Bq/m3 which translates into 0.00206mrem/hr. One can choose 1 centimeter as the distance from the contamination, to replicate as closely as possible the effect of actually drinking the radioactive water. Some simple algebra enables one to calculate the millirems per hour into millirems per year, in order to get a comparison to the average background exposure. Yes, er, hum…cancellation of terms, multiplication, etc…and one gets an entire 18 millirems per year, which, with some more basic calculations works out to only six percent of the normal background dose.
Another interesting tidbit: Radiation exposure is slightly higher at higher elevations-thus the exposure in Denver averages 400 millirems per year. Meaning one could literally drink a cubic meter of the contaminated water that has arrived on the shores of British Columbia, and still be below the baseline of background exposure if one lived at a higher elevation.
According to Canada’s Department of the Interior, the maximum permissible concentrations in drinking water is 10,000 Becquerels per cubic meter, which works out to a total of 0.764mrems/hr, or an unhealthy 6,692mrem/yr – more than 22 times the natural background exposure.
All this does not mean that nuclear power plant disasters should be treated with any levity at all. Measurements of water taken from the plant’s immediate vicinity measured at 10 million becquerels per cubic meter (bq/m3) – this comes out to 764 mrems/hr, or about two and a half times the yearly dose in the space of an hour. Another visit to the online calculator gives this dose in Grays, which is a way to measure the dose of radiation that has been absorbed by human tissue. 10 million becquerels translates into a little more than two Grays, which will produce nausea and vomiting, mild to moderate leukemia, fatigue, and weakness, and death in up to five percent of people who are exposed at this level within six to eight weeks. Now take into account that this dosage is figured per hour, and imagine there is no practical way of getting away from the source of the radiation – and the numbers get considerably scarier. At six to eight Grays, mortality is up to 95 to 100 percent with death occurring in two to four weeks.
If there is a take-home lesson, it is this: Planet Earth has a wonderful buffering system called the Pacific Ocean. It will continue to protect humanity from leaks of radioactive materials for only so long. But like any biological system, if it is damaged beyond repair, it loses its protective capacity, threatening every living thing on Earth. Carpe diem.
By Laura Prendergast