David Waltner-Toews is Professor in the Department of Population Medicine at the University of Guelph, and founding president of the Network for Ecosystem Sustainability and Health (www.nesh.ca). Waltner-Toews’ areas of expertise are the epidemiology of food and waterborne diseases, zoonoses, global change and emerging diseases, and ecosystem approaches to health. He is also author of a 2008 book on the ecological and cultural context for food-borne diseases, titled Food, Sex and Salmonella.
High levels of radiation have been found in some vegetables, milk, and even tap water in Northeast Japan due to radiation leaks from Fukushima power plant. How high are those levels, and how does it get into the food?
There are several ways to measure how strongly radioactive substances are in the environment. Two important ways are to measure the strength of the radioactive source, in becquerels (number of radioactive disintegrations per unit time) and to measure the ability of a particular dose to cause biological damage, in sieverts or millisieverts. For food, the latter is obviously the most important.
The International Commission on Radiological Protection (ICRP) has suggested that dose-equivalents, excluding natural background and medically-related radiation, should not exceed 5 millisieverts (mSv) in a given year, 1 mSv per year on average, and 70 mSv over a lifetime. Many countries have set standards based on this, but we should understand that these are based on averages and I, for one, don’t know any “average” people. What is safe depends on age, growth, variety of food sources, etc.
I have been unable to find reports of the actual levels found in Japanese food, only that they are “safe”, “unsafe for infants”, “dangerous”, etc. It would be useful to get actual numbers, although, to tell the truth, we are not sure what actual safe levels are, and we don’t know whether very low doses are dangerous. A lot depends not just on how much radioactivity is in the food, but also how big a part of the diet that food is. High levels in wildlife or fish may be of little concern to urban consumers but a huge issue for nomadic hunters and fisherpeople say in the north.
The radioactive elements (radionuclides) get into the food chain through water. That is, they either leak directly into water, or are thrown into the air by an explosion and come down in precipitation. How far they spread then depends a lot on weather patterns and water movements. Radionuclides in water tend to disappear pretty quickly after the exposure, and move into the biological sphere.
The two main radionuclides of concern in food after an accident like this are Iodine-131 and Cesium-137, and they behave quite differently.
I-131 behaves like iodine in the body. If you are eating a lot of iodine from other sources, then you get a lower exposure in the body. Once in the body, it tends to concentrate in the thyroid gland, because that gland uses a lot of iodine. The radioactive iodine can then cause cancer by damaging cell DNA; this is especially serious if the cells are multiplying quickly – hence the extra concern about infants, whose bodies are still developing, as well as pregnant women.
Cs-137 behaves like potassium in the environment and in the body. Unfortunately, for some reason, the body tends to actually prefer cesium to potassium. Because potassium is important for all cells, cesium can cause all different kinds of cancer.
The physical half-life of a radionuclide is the amount of time it takes to lose half its radioactivity. The biological half-life is the amount of time it takes an organism (person or animal) to get rid of half the radioactive substance. If you put the two together, you can get an idea of how long the radionuclide will persist in the environment.
I-131 has a physical half-life of 8 days, but has a biological half-life of months, so that prevention is important. Hence, the importance of iodine pills. Because it returns to non-radioactive forms relatively quickly, I-131 tends not to bio-accumulate up the food chain, and disappears if foods are kept in storage.
Cs-137 has a half life of 30 years, but has a biological half-life measured in weeks (a rapid turnover in the body). The danger here is long-term persistence in the environment. Cs-137 was still being picked up at high levels by sheep in Norway and the UK twenty years after the explosion at the Chernobyl Nuclear Power Plant near the town of Pripyat, Ukraine.
In the immediate time after an accident, the foods that have been directly exposed are most dangerous. This depends on rainfall, soil type (sandy soils being “worst”), and the plants (whether they take up surface water, or have deep roots, and the stage of growth at the time of exposure). After Chernobyl, spinach and mushrooms seemed to be the first and worst affected.
How might we see the radiation move through the food chain? How far could it spread, and how long will it affect the safety of food and water?
Right after an accident like this, the foods most affected are plants and animals near the bottom of the food chain. Carnivorous fish are safer to eat now than other fish. Because seawater has a lot of potassium in it, and because there is a lot of water in the ocean, creating a dilution effect, seafood tends to be safer than fish from fresh water. Cesium in particular will bio-accumulate over time, so that if the exposure stops, then animals higher in the food chain will tend to be more affected and those lower in the food chain will become safer. Animals like goats, who are browsers, and sheep, who eat close to the ground, will tend to pick up radioactivity faster than other animals. Animals that have been kept inside and fed feeds that were produced before the disaster will be safer than those at pasture.
Cesium will tend to concentrate in cheese over time, while iodine will disappear. This is a function of the half-life, and the drying out of the cheese.
What do you think the implications are for agriculture in the Northeast of Japan? Will farmers be able to plant again next season, considering the reservations so many have about contamination?
There are so many things to consider here—the actual safety, the consumer acceptance, and what other food is available. In some countries after Chernobyl, health and environment officials simply increased the threshold for safety. The rationale was that it was better to eat some fresh foods that had a bit of radioactivity than to eat only processed, imported foods. It will take a lot of monitoring to sort through this over time. Farmers may change practices, including keeping animals indoors more and importing feeds into the area, or at least being very selective in feeding. More plants might be grown in greenhouses rather than outside, as well. I don’t know what the soil types are in the north. Clay soils tend to sequester cesium, so the plants get less of it. Sandy soils are, from this point of view, more “dangerous”.
For more information see: Food, Sex and Salmonella: Why Our Food Is Making Us Sick