Mortality experience was evaluated for the approximately 40,000 U.S. Navy personnel who participated in Operation CROSSROADS, a 1946 atmospheric nuclear test series that took place in the Bikini Atoll in the Marshall Islands. To judge whether that mortality experience was influenced by CROSSROADS participation, those personnel were compared to a control group assembled to be similar to the participants in all ways (age, pay grade, military experience, time of service, location of service) possible except for the Operation CROSSROADS participation.
A roster of Operation CROSSROADS participants was assembled and provided to the IOM committee, which found in a validation study that the final roster captured between 93 and 99% of participants. The mortality data gathered from the Department of Veterans Affairs (VA) records were validated by sample comparisons with other national data sources. By the study cut-off date (12/31/92) 31.3 of the participants and 30.8 of the comparison cohort were known to have died. Cause of death was available for 86.3% of the participants, and 89.3% of the controls.
Using proportional hazards analysis, and adjusting for remaining differences between the cohorts, the survival times of the two groups were compared. The committee looked at three principal causes of mortality: all-cause, all-cancer, and leukemia, hypothesizing that increases in the latter two could result from radiation exposure.
Findings were as follows:
- Among Navy personnel, the primary analysis group for this study, Operation CROSSROADS participants experienced higher mortality than a comparable group of non-participating military controls. However, the increase in all-cause mortality did not appear to concentrate in any of the disease groups considered: of the specific cancers and disease categories examined, there were no statistically significant increases in mortality. The overall elevation of mortality rate ratios for malignancies and leukemias in the participants was not statistically significant, and in fact, was lower than for many other causes of death.
- Navy mortality due to all malignancies and leukemia did not vary substantially among the exposure surrogate groups.
- Participants who boarded ships were thought to be more highly exposed than the rest of the participant group, and experienced a 5.7% increase in all-cause mortality relative to the controls. However aside from all-cause mortality, risks for boarding participants did not significantly exceed those for controls for any of the disease categories, and risks relative to controls were similar for boarding and nonboarding participants. In all cases the 95% confidence intervals overlap, suggesting the difference between boarders and nonboarders could well be due to chance.
- Participants holding an Engineering & Hull (E&H)occupational specialty were thought to be more highly exposed to radiation than their non-E&H counterparts. However, the E&H participants had essentially the same risk of mortality from all causes as non-E&H participants. Risk ratios for leukemia and malignancies among E&H controls showed a similar elevation relative to non-E&H, suggesting that a factor specifically associated with CROSSROADS was not likely to have been the cause.
- These findings do not support a hypothesis that exposure to ionizing radiation was the cause of increased mortality among CROSSROADS participants. Had radiation been a significant contributor to increased risk of mortality, we should have seen significantly increased mortality due to malignancies, particularly leukemia, in participants thought to have received higher radiation doses relative to participants with lower doses and to unexposed controls. The committee did not observe any such effects. They note however, that this study was neither intended nor designed to be an investigation of low-level radiations effects, per se, and it should not be interpreted as such.
In comparing the findings and methods employed in this study with those of other similar investigations, the committee identified a possible self-selection bias in the participant cohort. Participants who died of a disease (particularly cancer) may have been more likely than healthy participants to have identified themselves, and hence become a part of this study. Such a bias would have resulted in an apparent increase in death rates among the participants. The committee did not have the data to make a good quantitative estimate of this potential bias. However, as the roster of participants was nearly complete, and mortality from all malignancies and leukemia was lower, not higher than the increase in all-cause mortality, the committee found that a self-selection bias was not entirely responsible for the findings of increased all-cause mortality in study participants.
The elevated risk of all-cause mortality relative to a comparable military group is probably the result of two factors. The first is an unidentified factor, other than radiation, associated with participation in, or presence at, the CROSSROADS test. The second is a self-selection bias within the participant roster. However, the relative contributions of these two explanations cannot be accurately determined within the available resources for this project.