Measurement of Fukushima-related radioactive contamination in aquatic species

The Great East Japan (Tohoku) Earthquake and consequent tsunami of March 11, 2011, caused immense damage and loss of life in Japan. It is estimated that over 15,000 people died and in excess of 2,500 persons are still unaccounted for (1, 2). The tsunami that struck the Fukushima Dai-ichi Nuclear Power Plant (NPP) led to breakdown of the reactor cooling systems, resulting in overheating of fuel in the three reactors that were operating at the time of the earthquake (3). Substantial radionuclide releases ensued, both to the atmosphere (most of which was deposited on the Pacific Ocean) and directly to sea (4, 5). Concentrations of ¹³⁷Cs in seawater, which are an indicator of contamination of marine biota, were very high near the site of the accident for a few weeks afterward, but decreased to relatively low levels within a year or so (6) (Fig. 1, refs. 7, 8).

Fig. 1.

Measured concentrations of ¹³⁷Cs in seawater near the Fukushima Dai-Ichi NPP. JST, Japan Standard Time. Reproduced from refs. 7 and 8.

The paper by Okamura et al. in PNAS (9) builds on these estimates (6), and focuses on levels of contamination with ¹³⁴Cs and ¹³⁷Cs in a number of aquatic species, fitting a Weibull model to estimate the distribution of contamination in the sample. The paper uses extended aquatic monitoring data taken over the period from April 2011 to March 2015. More importantly, it takes account, as it has to, of the fact that many measurements are now below the limit of detection. To get around this problem, the authors substituted below-detection limit estimates with the value of the detection threshold. An additional problem was that parameter estimates produced by likelihood maximization did not converge; Okamura et al. (9) dealt with this issue by use of a two-stage fitting process. They first fitted a random-effects model, aggregating across groups of species and prefectures. Having estimated the necessary model parameters from the random-effects model, and fixing at the associated mean the ecological dispersion parameter, ρ, that controls the time decay in the scale parameter for the Weibull distribution, they then reestimated the remaining model parameters. Both of these modeling methods are less than optimal. Fixing measured estimates at the threshold limit will lead to bias, both in the central estimates (mean) and in their uncertainty. The mean is likely to be upwardly biased by this method, and estimates of uncertainties are likely to be downwardly biased. The problems introduced by the two-stage fitting process used by Okamura et al. (9) are less serious: It does not take full account of the variability in the data, but it would not be expected to bias the resulting parameter estimates. Bayesian methods of dealing with missing data are a better way of addressing both issues, although their computational demands can be substantial (10).

Okamura et al. (9) suggest that contamination levels of both ¹³⁴Cs and ¹³⁷Cs are low overall, with the probability of contamination at levels >100 Bq/kg very low in both marine and freshwater species. Contamination levels tend to decrease with distance from Fukushima, suggesting that “contamination of marine fish has rapidly dispersed even at the bottom of the sea” since the accident (9). Because the authors’ estimates of mean are likely to be upwardly biased, these statements are very likely still valid, although if the uncertainties were substantially underestimated, such would not be the case. There are additional grounds for caution in applying the results of this analysis. Statements made about the probability of given levels of contamination only relate to the particular sample of data used to fit the Weibull model. It would not be correct to make inference on the “distribution” of contamination in the environment.

The paper of Okamura et al. (9) makes the suggestion that improving “data collection plans with lower detection limits in the future,” to reduce the number of below-detection threshold measurements, would be beneficial (to a risk assessment). Although we would not quarrel with that statement, modern radiation measurement technology generally has quite low detection levels. Moreover, radioactivity detection levels cannot be improved by any refinements in data collection strategy because detection levels are solely a function of the measurement system and the sample preparation. It could be argued that a small number of precise measurements are less valuable than many measurements that are below detection limits. Bearing on the point discussed above on distribution of contamination in the environment, a population risk assessment needs, by definition, representative data of the entire range of exposure conditions. Knowing that samples from a wide range of locations and types of fish are below detection limits bounds the possible exposures much better than a few measurements with detectable radioactivity, but which are not necessarily representative of wide-ranging exposure conditions and locations.

What are the implications for radiation-related risks to people consuming contaminated aquatic food products in Japan (and elsewhere)? Subject to the above caveats, it would appear that contamination levels in most marine and freshwater species are very low. Consequently, radiation risks associated with consumption of such species by humans would appear to be very low as well, and such consumption does not pose a substantial issue for the protection of public health. Looking beyond the contamination of aquatic species, it should be recalled that other pathways of exposure to man may be more important, for example, those pathways of exposure associated with terrestrial deposition of radioactive iodine and cesium. Five years after the accident, there are still contaminated areas to the northwest of the Fukushima Dai-ichi NPP that remain evacuated, largely due to high levels of gamma radiation “groundshine” from deposited cesium. Dealing with such areas is proving to be an intractable problem. Whether health effects arising from radiation exposure as a result of the accident will be detectable seems unlikely on the basis of present evidence (7, 11). There have been suggestions that a reported excess of cases of thyroid cancer is attributable to exposure to radioiodine (12), but this conclusion is suspect (13). More generally, the psychosocial impact of accidents (and not just nuclear ones) on the health of those individuals affected is an aspect of medical response that needs greater attention (14, 15).