The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
2002); no information on internal dose from iodine isotope was available in this study.
It is noteworthy that no increase in the incidence of leukemia or thyroid cancer has been reported to date among Baltic country liquidators (Kesminiene and others 1997; Rahu and others 1997). These findings do not contradict the findings reported in Belarus, Russia, and Ukraine in that the number of liquidators in the Baltic countries is small, and the results are also consistent with a radiation-related increase.
At this time, no conclusion can be drawn concerning the presence or absence of a radiation-related excess of cancer—particularly leukemia—among Chernobyl accident recovery workers. There is a pressing need for well-designed, sound analytical studies of recovery workers from Belarus, Russia, Ukraine, and the Baltic countries, in which special attention is given to individual dose reconstruction and the effect of screening and other possible confounding factors.
Studies of Chernobyl cleanup workers offer an important opportunity to evaluate the effects of protracted exposure in the low- to medium-dose range. No reliable risk estimates can be drawn at present from studies of these workers, however, because of the difficulties of follow-up and lack of validated individual dose estimates.
AIRLINE AND AEROSPACE EMPLOYEES
Airline pilots and flight attendants are exposed to increased cosmic radiation during flights. In 1991, the ICRP recommended that exposures to natural cosmic radiation should be considered occupational exposures for aircrews (ICRP 1991). Although aircrew members are not thought to exceed the National Council on Radiation Protection and Measurements (NCRP 1995) recommendation for occupationally exposed workers of 20 mSv per year averaged over 5 years, they do exceed the safety level set for the general public (1 mSv per year). The exposure varies with altitude, latitude, and solar flare activity. Solar activity varies on an 11-year cycle; however, prediction of short-term intense periods of activity is not possible. At 41,000 feet over the poles, the equivalent dose may vary from a norm of about 12 μSv to an extreme of 100 μSv (Friedberg and others 1989). The mean annual dose from galactic cosmic radiation can be modeled using knowledge of altitude, latitude, solar activity, and the Earth’s geomagnetic field. Friedberg and colleagues (1989) estimated the annual equivalent doses that would be received on 32 U.S. domestic and international flight routes as 0.2–9.1 mSv, considerably less than recommended annual adult occupational exposures.
Several review articles have been published recently on epidemiologic studies of the occupational cancer risk for pilots and flight attendants (Blettner and others 1998; Blettner and Zeeb 1999; Boice and others 2000). The ability of studies to detect an association with ionizing radiation has been limited by several factors. Few studies have included internal comparisons, basing results instead on proportional mortality ratios, SMRs, or standardized incidence ratios. As a group, pilots and flight attendants differ appreciably from the general population. Pilots and other aircrew members are required to be very healthy and undergo frequent medical checkups, leading to the possibility of enhanced early detection of cancers in this occupational group. Disrupted circadian rhythms and, in females, relatively late age of first parity are other characteristics that complicate the choice of a suitable comparison group. Increased sun exposure, exposure to elevated ozone levels, fuel exhaust fumes, and electromagnetic fields are factors that may also confound any relationship observed between adverse health effects and cosmic radiation. Moreover, small study group sizes and the relatively low exposure levels of restricted range are further obstacles to the precise quantification of any risk.
Whether epidemiologic studies of airline personnel can have sufficient power and precision to detect so small an association has been questioned. Based on published values of annual radiation exposure of aircrew flying at high altitudes, Boice and colleagues (1992) estimated that a flying career of 20–30 years duration would result in only an 80–180 mSv cumulative dose, corresponding to a relative risk (RR) of only about 1.06, if causal. The cosmic radiation to which aircrews are exposed is predominantly in the form of high-LET neutrons and low-LET γ-radiation, the former of which can contribute as much as half of the total equivalent dose at typical flight altitudes (Boice and others 1992; Hammer and others 2000). The choice of an appropriate weighting factor for the conversion of neutron dose estimates to equivalent doses is thus crucial for dosimetry in this occupational group and for assessment of the contribution of low-LET γ-radiation to any adverse health effects. At present, the evidence for an adverse health effect in aircrews due to ionizing radiation is inconclusive.
Studies of airline and aerospace employees do not currently provide estimates of radiation-related risks because dose estimates have not been used in the studies to derive quantitative risk estimates.
MEDICAL AND DENTAL OCCUPATIONAL EXPOSURES
Early studies of patterns of mortality among radiologists and other physician specialists produced a suggestion of an excess risk of specific cancers. Excess mortality from leukemia and lymphoma, especially multiple myeloma, and also from skin, lung, pancreatic, and prostate cancer (e.g., Matanoski and others 1975a, 1975b; Smith and Doll 1981; Logue and others 1986; Wang and others 1988) have been suggested, although findings were not consistent across