FAQ on Radiation

What you have read is not correct. Although Chernobyl did not have increased malformations (i.e. birth defects), there were birth defects associated with the atom bomb survivors. However, it is correct that even for atomic bomb survivors the incidence of radiation-induced birth defects is low, for several reasons:

• Birth defects are only produced when acute doses are high enough to kill a relatively large proportion of cells in the embryo/fetus (>500 mSv).
• If the embryo is exposed to such high doses very early in the pregnancy (before the 10th day of pregnancy, when there are very few cells), it fails to develop, spontaneously aborts, and the woman often doesn’t even know that she was pregnant.
• If the fetus is exposed late in the pregnancy (after 40 days of pregnancy), its radiation resistance has increased to the point that the only observable health effect is typically low birth weight, delayed growth, or possible mental deficits.
• The sensitive time for radiation malformation during gestation is during the organogenesis period. Organogenesis is very short (from day 10 to day 40, over a 280 day total gestation period). Thus, the time window for producing malformations is very narrow (just 30 days out of a 9-month pregnancy).
• Whole body doses above 4,000 mSv will likely be lethal to the mother as well as the fetus.

So if you put all the above factors together, you can see that the target population for radiation-induced birth defects is usually quite small: Women in their 10th to 40th day of pregnancy receiving a radiation dose more than 500 mSv but less than 4,000 mSv. So in any catastrophic radiation accident there are typically few women that fall within this category, so the number of malformations produced by the radiation is usually quite small.

The issue with the power plant release was radioactive cesium (Cs-137). This radioisotope has a long physical half-life (30 years) and, therefore, persists in the environment. But two factors mitigate its risk in seawater:

• Cesium is highly soluble in sea water so it mixes quickly and dilutes quickly. The amount of water released at Fukushima was equal to the volume of 5 Olympic swimming pools. The volume of the Pacific ocean is equivalent to 300 trillion Olympic swimming pools. So you can see that the dilution factor for cesium in the ocean is huge.
• There is some concern that fish exposed in the area of the radioactive water release (before the cesium has time to dilute) will take up high levels of cesium and then migrate to other areas and be caught for food consumption by fishermen. But this concern is mitigated by what’s known at as the “biological half-life“. Although sure you’re familiar with the physical half-life of a radioisotope, you may not know that radioisotopes also have a biological half-life. The biological half-life is how quickly a radioisotope is eliminated from an animal’s body by normal physiological processes (e.g. sweat, urine, feces, etc.). For fish, the half-life for cesium is less than 30 days, even for the large pelagic fish that could ingest large amounts of cesium contaminated food (e.g. tuna) and then can migrate great distances. Typically, by the time the fish is caught, many biological half-lives have passed and the cesium levels are quite low. In fact, much was made recently of a scientific report that showed that tuna that had migrated from Japan and were caught off the coast of California had detectable levels of cesium from Fukushima in their flesh. But these levels were as predicted based on the biological half-live of cesium, and the cesium radioactivity levels were much much lower the natural radioactivity that is present in all fish (primarily from radioactive potassium), so there would be no significant increase in radiation exposure to individuals eating those tuna.

This is a difficult question to answer in a short statement because it depends upon many factors and the particular biological endpoint you are concerned about. But generally speaking, radiation sensitivity for cell killing is remarkably similar among all species when adjusted for DNA content. This is because DNA is the target for radiation cell killing. If a cell genome has more DNA then it has a larger target, and it is, therefore, more likely to be “hit” with a photon of radiation. This is why organisms with very small DNA contents (e.g. viruses, bacteria, yeast) “appear” to be very radioresistant. Their DNA content is so low that it is hard for the radiation to hit its target, so a higher radiation dose is required to kill them. But if you account for their differences in DNA content, these organisms are no more radiation-resistant than mammalian cells. There are some organisms that typically don’t fit this rule, and have radioresistances greater than would be expected based on DNA content alone. But again, their radiation resistance can usually be explained based on DNA. Either they have multiple copies of their genomes, like the bacteria Deinococcus radiodurans, and can, therefore, afford to lose a genome to radiation, or they have chromosome replication processes that mitigate the effects of radiation damage (e.g. insects cells).

The butterfly story needs to be taken with a grain of salt. There were several problems with that study that were not addressed properly and, therefore, raise serious doubts about the validity of its findings:

• A major finding of the study is that forewing size was inversely correlated with distance from Fukushima, resulting in the conclusion that radiation from Fukushima had stunted forewing development. However, the more distant butterfly sampling sites were all progressively further south of Fukushima, so that latitude was also changing with the distance. This is a problem because it is well established that the forewing size of a number of insect species is dependent upon the latitude of their microhabitat. The potential latitudinal influences on forewing size were completely ignored in this study. Had the data been adjusted for sampling site latitude, it is likely there would have been no significant forewing findings to report.
• The second major problem is that the decreased butterfly survival rates reported to be associated with proximity to Fukushima are claimed to be reproducible in the laboratory with external beam irradiation. This claim stretches credulity since it has long been established that insects, including butterflies (Order: Lepidoptera), are resistant to radiation effects. The concept that the low environmental radiation exposures (less than 15 mSv per year) that are being attributed to the Fukushima accident could be killing off butterflies, or any other insect species, is simply not credible. It should further be noted the external radiation doses that were used to reproduce the results from field-collected individuals were 100 times higher than any radiation doses in the field that could possibly be attributed to Fukushima. Thus, it can even be seen from the investigators’ own laboratory experimental data that no measurable killing would be expected at the radiation doses that were encountered in the field.
• The third major problem regards the time to eclosion (emergence of an adult insect from a pupa). Eclosion times were claimed to be associated with proximity to Fukushima. Yet irradiation has been employed as a pest control measure for a number of insect species for decades, and the effects of radiation on various insect biological endpoints have already been well characterized. It typically takes as much as 30,000 mSv of Lepidoptera egg irradiation to extend eclosion times by the 4 to 5 days reported in this study. It is, therefore, astounding that effects on eclosion of a similar magnitude can be seen at radiation doses that are just a few fold above natural background doses. So the claim that eclosion times were extended due to these environmental radiation exposures is also incredible when compared to the literature. Perhaps it is more plausible that eclosion time of the pale grass blue butterfly, like forewing size, might also be related to microhabitat latitude or temperature. [Average daily temperatures differ by as much as 9 degrees Celsius between Fukushima and Tokyo during April (hatching season).]

In conclusion, the results reported in this study should be considered highly suspect due to both their internal inconsistencies and their incompatibility with earlier and more comprehensive radiation biology research on insects. The study’s central assertion is that “artificial radionuclides from the Fukushima Nuclear Power Plant caused physiological and genetic damage to [the pale grass blue butterfly]”. This statement is incredulous and goes well beyond anything that the study data can actually substantiate. Therefore, this study’s sensational claims should not be used to scare the local population into the erroneous conclusion that their exposures to these relatively low environmental radiation doses put them at significant health risk.