Participants who boarded target ships were thought to be a more highly exposed than the rest of the participant group. Relative to the controls (nonparticipating comparison group), boarding participants experienced a 5.7 percent increase in all-cause mortality, RR equal to 1.057 (1.014–1.10), p-0.0093, whereas the nonboarders (less exposed participant group) experienced a 4.3 percent increase (RR = 1.043 [1.015–1.073], p = 0.0028). 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. The increase in risk for all-malignancies among the participants was 2.6 percent (RR = 1.026 [0.94–1.12], p = 0.55) for boarders and I percent (RR = 1.010 [0.95–1.068], p = 0.73) for nonboarders. For leukemia, the increase in mortality risk for boarders was, 0.7 percent (RR = 1.007 [0.61–1.66], p = 0.98) and for nonboarders, 2.4 percent (RR = 1.024 [0.737–1.422], p = 0.89). In all cases the 95% confidence intervals overlap, suggesting the difference between boarders and nonboarders could well be due to chance.

Those Navy 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 (RR = 0.99 [0.95–1.038], p = .81). For all malignancies and leukemias, the rate ratios were somewhat higher, 1.051 [0.97–1.14] and 1.51 [0.94–2.44] respectively, but both could be attributed to chance (p = 0.25 and 0.088 respectively). Risk ratios for leukemia and malignancies among E&H controls showed a similar elevation relative to non-E&H controls, 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. We did not observe any such effects. We note, however, that this study was neither intended not 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 investigations of atomic veteran mortality, we have 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 to the NTPR, and hence become a part of this study. Such a bias would have resulted in an apparent increase in death rates among the participants. We do not have data with which to make a good quantitative estimate of this potential bias. However, the roster of participants is nearly

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