Researchers in the UK have developed a new method for evaluating plutonium workers’ historical internal radiation exposure. They focused their study on workers employed at the start of plutonium operations at the Sellafield (formerly Windscale) nuclear reprocessing facility (J. Radiol. Prot. 10.1088/1361-6498/ab1168).
Internal exposure to plutonium, which decays via alpha particle emission, is a recognised health hazard. But with little specific information available, potential risks from plutonium exposure have largely been assessed through knowledge of radiation exposure risks in general, much of which comes from external exposure to photon radiation such as gamma and X-rays. However, due to its high linear energy transfer rate, alpha particle radiation exhibits significantly enhanced biological effects at the cellular level, creating a specific need to investigate the associated exposure risks.
“To obtain more direct estimates of potential internal exposure risks, epidemiological studies of plutonium workers need to be conducted,” explains lead author Tony Riddell, from Public Health England’s Centre for Radiation, Chemical and Environmental Hazards. “These studies require individual plutonium exposure estimates that are as accurate and unbiased as possible.”
The Sellafield workforce includes one of the world’s largest cohorts of plutonium workers. Through the support of the workforce, this group has been comprehensively monitored for internal exposure to plutonium, primarily through inhalation.
However, for 630 workers employed there at the start of plutonium operations, from 1952 to 1963, the historical urinalysis results available do not provide sufficiently accurate and unbiased exposure assessments. These results were based on a threshold level of urinary plutonium excretion, which was suitable for operational protection purposes at the time, but tended to overestimate exposure, leading to underestimation of any risks if used in epidemiological analyses.
“This means these early workers are excluded from epidemiological studies of exposure risks, which significantly reduces the power of these studies,” says Riddell. “Early workers are important for assessing potential exposure risks because they usually received some of the highest plutonium exposures and, due to the passage of time, health outcomes for these workers will now be largely known.”
To solve this problem, Riddell and colleagues employed an approach called a job-exposure matrix (JEM). The JEM approach uses exposure data from other sources to estimate the average exposure that a typical worker (in the same work group) would have received in a given period. Substituting the missing data with these JEM estimates allowed the researchers to build a more reliable picture of the early workers’ radiation exposure.
“To overcome the problem of missing or deficient exposure data, we used more reliable data from other relevant workers (‘exposure analogues’) along with statistical, mathematical and empirical analyses to estimate the average exposures for a typical worker at Windscale/Sellafield for all combinations of specific occupation and year required to build the JEM,” explains principal investigator Frank De Vocht from the University of Bristol.
The authors note that the exposure analogues approach developed in this study provides a generic methodological advance that is potentially transferable to other internally exposed workers, and which may permit other epidemiological cohorts to include significant groups of workers who otherwise might have been excluded due to the lack of reliable exposure information.
“It’s likely that replacing the missing or unreliable exposure data with JEM-derived values in future epidemiological studies could have considerable impact on the risk estimates which can be produced,” adds De Vocht.
