Radium dial workers: back to the future

Abstract

Purpose: This paper reviews the history of the radium dial workers in the United States, summarizes the scientific progress made since the last evaluation in the early 1990s, and discusses current progress in updating the epidemiologic cohort and applying new dosimetric models for radiation risk assessment.

Background: The discoveries of radiation and radioactivity led quickly to medical and commercial applications at the turn of the 20th century, including the development of radioluminescent paint, made by combining radium with phosphorescent material and adhesive. Workers involved with the painting of dials and instruments included painters, handlers, ancillary workers, and chemists who fabricated the paint. Dial painters were primarily women and, prior to the mid to late 1920s, would use their lips to give the brush a fine point, resulting in high intakes of radium. The tragic experience of the dial painters had a significant impact on industrial safety standards, including protection measures taken during the Manhattan Project. The dial workers study has formed the basis for radiation protection standards for intakes of radionuclides by workers and the public.

Epidemiologic approach: The mortality experience of 3,276 radium dial painters and handlers employed between 1913 and 1949 is being determined through 2019. The last epidemiologic follow-up was 30 years ago when most of these workers were still alive. Nearly 65% were born before 1920, 37.5% were teenagers when first hired, and nearly 50% were hired before 1930 when the habit of placing brushes in mouths essentially stopped. Comprehensive dose reconstruction techniques are being applied to estimate organ doses for each worker related to the intake of ²²⁶Ra, ²²⁸Ra, and associated photon exposures. Time dependent dose-response analyses will estimate lifetime risks for specific causes of death.

Discussion: The study of radium dial workers is part of the Million Person Study of low-dose health effects that is designed to evaluate radiation risks among healthy American workers and veterans. Despite being one of the most important and influential radiation effects studies ever conducted, shifting programmatic responsibilities and declining funding led to the termination of the radium program of studies in the early 1990s. Renewed interest and opportunity have arisen. With scientific progress made in dosimetric methodology and models, the ability to perform a study over the entire life span, and the potential applicability to other scenarios such as medicine, environmental contamination and space exploration, the radium dial workers have once again come to the forefront.

Keywords: Million Person Study; Radium; dial painter; mesothorium.

Figure 1.

Decay chains for ²²⁸Ra (top) and ²²⁶Ra (bottom). Arrows pointing up and to the right indicate beta decay. Arrows pointing down and to the right indicate alpha decay. Stars indicate decays that are associated with meaningful gamma emission. Radiological half-lives are listed below each nuclide (International Commission on Radiological Protection (ICRP) 2008).

Figure 2.

Available initial systemic intakes of radium (μCi) for individuals in the current cohort by year of first exposure, generally taken to be the year an individual was first hired. (×) represent females and (o) represent males. Data were drawn from the Comprehensive Epidemiologic Data Resource (DOE 2021). (Top panel) Mean initial systemic intakes (linear scale) of ²²⁶Ra (left) and ²²⁸Ra (right). Error bars represent the standard error of the mean in reported intakes and do not include consideration of measurement or modeling uncertainty. (Bottom panel) Individual initial systemic intakes (log scale) of ²²⁶Ra grouped by age at first exposure. Vertical dashed lines represent the year 1926.

Figure 3.

Schematic of the selection and vital status tracing results as of 31 December 2019 for the study population of 3,276 radium dial painters and radium dial handlers employed prior to 1950. COD indicates Cause of Death.

Figure 4.

Pelger Huët percent (mean % ± SEM) observed in a cohort of 166 radium dial painters and ancillary workers versus date of entry into the workforce.

Figure 5.

Structure of the ICRP’s current biokinetic model for systemic radium (International Commission on Radiological Protection (ICRP) 2017) with the addition of the brain (shaded).

Figure 6.

Plots of the fraction of the energy absorbed in a target tissue per alpha energy emitted from a skeletal source region. Data points (circles) are from ICRP Publication 133 (2016) and dashed lines are the values in ICRP Publication 30 (1979). The shaded region represents the range of alpha particle energies emitted in the ²²⁶Ra and ²²⁸Ra decay chains.

Figure 7.

1,558 members of the radium dial painter cohort by sex and age at time of first employment. The vertical dashed line is at age 25 years and is the age of the adult in systemic biokinetic models for radium and its progeny.

Figure 8.

Absorbed dose rates to selected target regions for ingestion of 1 Bq of ²²⁶Ra in a reference 15-year-old female (top) and reference adult female (bottom). The plots include contributions from radium progeny created post-ingestion.

Figure 9.

Absorbed dose rate to selected targets versus time following the ingestion of 1 Bq of ²²⁶Ra (top) and ²²⁸Ra (bottom) including contributions from radium progeny created post-ingestion.