Higher doses of radiation, more health harm

Chernobyl was by far the worst reactor accident of all time. A total of 127 reactor workers, firemen and emergency personnel on site sustained radiation doses sufficient to cause radiation sickness (over 1,000 mSv); some received doses high enough to be lethal (over 5,000 mSv). Over the subsequent six months, 54 died from their radiation exposure. And it’s been estimated that 22 of the 110,645 cleanup workers may have contracted fatal leukemias over the next 25 years.

In contrast, at Fukushima, there were no radiation doses high enough to produce radiation sickness, even among the reactor core workers. Two Fukushima workers who had leaky respirators received effective doses of 590 mSv and 640 mSv. Due to their exposure, the two workers’ lifetime cancer risks will increase about 3 percent, but they are unlikely to experience other health consequences.

Beyond just the plant workers, over 572 million people among 40 different countries got at least some exposure to Chernobyl radioactivity. (Neither the United States nor Japan was among the exposed countries.) It took two decades to fully assess the cancer consequences to these people. Finally, in 2006, an international team of scientists completed a comprehensive analysis of the dose and health data and reported on the cancer deaths that could be attributed to Chernobyl radioactivity.

Using statistical models, the scientists predicted a total of 22,800 radiation-induced cancers, excluding thyroid cancers, among this group of 572 million people.

So that’s 22,800 non-thyroid cancers in addition to the approximately 194 million cancer cases that would normally be expected in a population of that size, even in the absence of a Chernobyl accident. The increase from 194,000,000 to 194,022,800 is a 0.01 percent rise in the overall cancer rate. That’s too small to have any measurable impact on the cancer incidence rates for any national cancer registries, so these predicted values will likely remain theoretical.

Chernobyl’s iodine-131 thyroid effects far worse

Unfortunately, at Chernobyl, the one type of cancer that could have easily been prevented was not. The population surrounding Chernobyl was not warned that iodine-131 – a radioactive fission product that can enter the food chain – had contaminated milk and other locally produced agricultural products. Consequently, people ate iodine-131-contaminated food, resulting in thyroid cancers.

For the local population, iodine-131 exposure was a worst-case scenario because they were already suffering from an iodine-deficient diet; their iodine-starved thyroids sucked up any iodine that became available. This extremely unfortunate situation would not have happened in countries such as the United States or Japan, where diets are richer in iodine.

Beginning five years after the accident, an increase in the rate of thyroid cancers started and continued rising over the following decades. Scientists estimate that there will ultimately be about 16,000 excess thyroid cancers produced as a result of iodine-131 exposure from Chernobyl.

At Fukushima, in contrast, there was much less iodine-131 exposure. The affected population was smaller, local people were advised to avoid local dairy products due to possible contamination and they did not have iodine-deficient diets.

Consequently, typical radiation doses to the thyroid were low. Iodine-131 uptake into the thyroids of exposed people was measured and the doses were estimated to average just 4.2 mSv for children and 3.5 mSv for adults – levels comparable to annual background radiation doses of approximately 3.0 mSv per year.

Contrast this to Chernobyl, where a significant proportion of the local population received thyroid doses in excess of 200 mSv – 50 times more – well high enough to see appreciable amounts of excess thyroid cancer. So at Fukushima, where iodine-131 doses approached background levels, we wouldn’t expect thyroid cancer to present the problem that it did at Chernobyl.