dcyphr | Measurement of background gamma radiation in the northern Marshall Islands


The U.S. used the northern Marshall Islands to test nuclear weapons between 1946 and 1958. The radioactivity may still be present from the decay of cesium-137. The authors studied six islands. All islands had radioactive islands that were within safety standards except for Bikini Island. Some islands still had some radioactivity. Bikini Island had excessive and unsafe gamma radiation. Further research needs to find if radiation exposure through food ingestion or other ways make habitation unsafe.


The authors aim to determine the amount of background radiation that still exists in the Northern Marshall Islands.


The United States conducted 67 nuclear tests on the Enewetak and Bikini atolls. Enewetak, Medrin, and Runit are on Enewetak Atoll. Bikini and Nam are on Bikini Atoll, and Rongelap is on Rongelap Atoll. The U.S. severely underestimated the radioactivity of the weapons. Specifically, they detonated Castle Bravo, which was the largest thermonuclear weapon ever tested. The easterly winds also further spread radioactive substances to the Rongelap and Utirik Atolls. The inhabitants did not know to evacuate, so many suffered great health consequences.

Inhabitants prematurely resettled in Rongelap in 1957 and Bikini in 1968. They had to be re-evacuated because of the high amounts of radiation. But, there have been cleanup efforts in Enewetak and Medren. Radioactive topsoil was removed in 1977 to 1980. Now, a population lives on Enewetak. In 1982, the Marshallese and U.S. government made an agreement on acceptable levels of radiation. Only some contractors for these governments currently live on Bikini and Rongelap Islands.

Most Marshallese people live on Majuro in the Majuro Atoll and Ebeye in the Kwajelein Atoll. There is now severe overcrowding. Relocation to other Marshall islands would solve the problem.

The cesium-137 from the nuclear testing goes through beta decay to become barium-137. The barium decays, giving off gamma radiation. The cesium often settles on the soil. Thus, the food supply takes up the radioactive isotope, leading to massive health crises.

Results and Discussion

Raw Observations

Marjuro Island was the control island. The mean amount of gamma radiation was 9.5 millirem/year (mrem/y). The mean amount of radiation on Enewetak Island after removing an accurate outlier was 6.9 mrem/y. The mean in Medren Island was 7.1 mrem/y. There was an even distribution of radiation on Enewetak and Medren. The mean on Runit was 13.1 mrem/y. But, the Runit Containment Dome contains radioactive waste from the weapons testing program. Measurements around the dome were likely inaccurate due to some emission of alpha particles from the waste.

Bikini Island had a lot of variation in gamma values. Much more radiation was at the center of the island. The mean amount of radiation is 184 mrem/y, and the median is 137 mrem/y. Many areas still were not measured. 

Not enough measurements were collected on Nam Island  due to excess vegetation in the interior of the island. The mean amount of radiation was 38.4 mrem/y. The mean amount of radiation on Rongelap was 19.8 mrem/y. The mean amount of radiation in Central Park in New York City was 100 mrem/y.

Comparison Across Islands

Bikini has more radiation than Rongelap, which has more radiation than Enewetak. This is likely because of the large cleanup on Enewetak that occurred in the 1970s. Bikini was the direct site of testing. 

Bikini, Nam, Runit, and Rongelap all had radiation levels higher than the control of Majuro. Enewetak and Medren had lower levels than Majuro, but the difference is statistically significant. The lower levels indicate that maybe Majuro did experience some of the fallout radiation.

Comparison with Previous Studies

The measurements from this study are much higher than projections from previous studies. The 1994 report of radiation levels to resettle Rongelap were lower, taking into account the amount of decay that should have occurred by now. This report also did not include possible exposure from diet. The Robison and Hamilton 2010 projections were also much lower.

Comparison with Standards

All islands had external gamma radiations below 100 mrem/y except for Bikini Island (Figure 4). The value of 100 mrem/y was agreed upon as the maximum level of exposure to gamma radiation. This includes all different types of exposure including consumption or absorption. But, this study only looks at external measurements. So, more studies need to be done to measure levels of exposure from other pathways before inhabitation.

The lower values of radiation at the beaches may be because the waves will disperse the radiation. However, the waste site is at Runit, and Castle Bravo was detonated in Nam. So, inhabitation will likely never happen on Runit or Nam.

Comparison with Central Park

Central Park had higher radiation than some of the Marshall Islands. This is likely due to background radiation from the granite as part of the Manhattan Schist. But, the authors stress that the measurements from the island are only external measurements rather than total exposure.


They detected gamma radiation with detectors. They traveled on a scuba diving boat and collected data in August 2015. They used a Gaussian model and semivariogram models to interpolate the distribution of radiation on each island. They conducted the Kruskal-Wallis test and the Shapiro-Wilk test for statistical analyses.


There is still a significant amount of radiation present as a result of the U.S. nuclear testing. Bikini Island has radiation levels that well exceeds the safety standard. The Enewetak and Rongelap Atolls have levels that are less than the safety standard. But, more research is needed on other exposure pathways to determine if inhabitation is really safe.