Korean Radiation Workers Study (KRWS)

The Korean Radiation Workers Study (KRWS) was initiated to construct a cohort of Korean radiation workers for long-term follow-up.Reference Seo, Lee and Park ¹² The study population of the KRWS comprised Korean radiation workers registered with the NSSC under the Nuclear Safety Act and whose radiation exposure is monitored using individual dosimeters. The workers were categorized into 8 occupational groups depending on their workplace settings: nuclear power plants (NPP), industry, educational institutes, non-destructive testing facilities, medical institutes excluding diagnostic medical radiation workers with radiation generator (X-ray)-related occupations, research institutes, public institutes, and the military. The number of radiation workers has increased from 16 916 in 1984 to 48 853 workers in 2022, ¹³ whereas the annual mean effective dose decreased from 2.97 mSv to 0.34 mSv in 1984-2020.Reference Noh, Lee, Park, Ju, Kim and Seo ¹⁴

Organ Dose Reconstruction

The NSSC has maintained the Central Registry for Radiation Workers Information (CRRWI), which includes the dosimetry information quarterly measured personal radiation dose equivalent (H_p(10)) using a personal dosimeter for all radiation workers since 1984 (as of 2022, 196 379 individuals). To evaluate the risks of cancer and non-cancer diseases in specific organs, it is necessary to assess organ-specific radiation exposure, which might differ depending on the organ’s anatomical characteristics (e.g., volume, mass, and location). For organ dose reconstruction, representative exposure scenarios (i.e., radiation energy and exposure geometry) were determined according to occupational groups, and dose coefficients for converting H_p(10) to organ-absorbed doses were then appropriately taken based on the exposure scenarios.

In the KRWS, using dose data for 20 605 workers from the initial cohort, organ-specific absorbed doses were reconstructed for 27 organs.Reference Kwon, Jeong and Ha ²¹ To estimate organ-specific doses, individual doses were multiplied by 2 conversion coefficients (air kerma-to-H_p(10) (Sv/Gy) and air kerma-to-organ dose (Gy/Gy)) provided by the International Commission on Radiological Protection (ICRP). ^{22 , 23} The following equation expresses the organ dose reconstruction procedure:

$$ {D}_T={H}_p(10)\times \left[\frac{D_T}{K_a}/\frac{H_p(10)}{K_a}\right]. $$

Where D_T is the organ absorbed dose (Gy); H_p(10) is the personal dose equivalent (Sv); and H_p(10)/K_a and D_T/K_a are the dose conversion coefficients for air kerma-to-H_p(10) (Sv/Gy) and air kerma-to-organ doses (Gy/Gy), respectively. The conversion coefficients vary depending on the energy of the radiation and the body’s orientation in relation to the direction of the radiation, which was determined by the exposure scenarios for different occupational groups.

The highest values of individual annual and cumulative absorbed doses were observed in the thyroid doses with 0.77 mGy/y and 10.47 mGy, respectively. The mean values of individual cumulative absorbed doses in the red bone marrow, colon, and lungs were 7.83, 8.78, and 8.43 mGy, respectively. Most cumulative organ doses were highest for industrial radiographers, followed by nuclear power plant workers and medical workers.

In the future, organ dose reconstruction will be continuously refined through the development of more precise exposure scenarios based on individual tasks, the calculation of dose conversion coefficients using Korean reference phantoms, and the consideration of internal exposure. In addition, an uncertainty analysis of organ dose reconstruction will be conducted.

Expansion of the Cohort

In 2022, the cohort was expanded to include all radiation workers, including both retired and currently active workers.Reference Lee, Cha and Park ²⁴ This expansion aligns with the 2021 amendment to the Nuclear Safety Act, which facilitates the comprehensive collection and analysis of health data. The extended cohort comprised 196 379 workers whose radiation exposure between 1984 and 2021 was recorded by the NDR. The NDR data, including individual doses and work facility details, have been linked with various health datasets. These include cancer incidence data from the NCR, cause of death information from the National Vital Statistics Registry (NVSR), and data on non-cancer diseases and medical health check-ups from the National Health Insurance Service (NHIS) database. The NCR includes cancer diagnosis dates, cancer type (ICD-10 codes), Surveillance, Epidemiology, and End Results staging, and diagnostic methods. The NVSR includes data on causes of death. The NHIS database offers sociodemographic details (e.g., sex, address, and income), lifestyle information (e.g., smoking and alcohol intake), anthropometric measurements (e.g., weight, body mass index, and blood pressure), and laboratory results (e.g., fasting blood sugar and cholesterol).Reference Seong, Kim and Park ²⁵ These data are collected through the national health examination program at 1- to 2-year intervals, allowing for longitudinal analyses of comprehensive health information. The design and scope of the cohort study, including population, follow-up processes, and health outcome assessments, are illustrated in Figure 1.

Figure 1.

Overview of the Korean Radiation Workers Study (KRWS) cohort and follow-up process.

The demographic characteristics of the extended cohort are summarized in Table 1. The cohort predominantly consisted of males, making up 83.1% (n=163 202), while female workers constituted 16.9% (n=33 177). The majority of the workers were born between 1970 and 1989, representing 54.4% of the total population. A significant proportion of the cohort worked in nuclear power plants (32.3%) and industrial radiography (14.7%), with most female workers employed in educational and medical institutes. Vital status data showed that 96.3% of the cohort are alive, and 6143 (3.1%) have been diagnosed with cancer. The cumulative radiation dose data indicated that over half of the workers (52.6%) had a cumulative dose of 0 mSv, whereas only 0.6% had received > 100 mSv. The mean cumulative dose in the cohort was 4.48 mSv.

Table 1.

Characteristics of the Korean Radiation Workers Study cohort, 1984-2021

SD, standard deviation.

Adapted from Int J Epidemiol. 2024;(53):3Reference Lee, Cha and Park ²⁴

Having successfully established the KRWS cohort, our study focuses on analyzing the health risks of cancer, non-cancer diseases, and mortality outcomes associated with occupational radiation exposure. We will calculate SIR and standardized mortality ratios to compare health risks within our cohort with those in the general Korean population, which served as the external control group. Risk estimates for radiation exposure are typically presented as excess relative risk (ERR), which reflects the magnitude of the radiation risk relative to the baseline and is calculated as the relative risk (RR) minus 1.0. We plan to employ statistical methods to estimate health risks per unit of organ absorbed dose, such as ERR and EAR per gray, to quantify the dose-response relationship. This model can potentially be adjusted for confounding factors such as birth cohort, lifestyle, and socioeconomic status. We plan to update the cohort every 5-10 years to include newly hired workers and update their work histories, including radiation doses. Moreover, health outcomes, such as disease incidence and mortality data, will be updated simultaneously. This cohort study is registered with ClinicalTrials.gov (NCT0538570), the Clinical Research Information Service in Korea (KCT0007149), and OECD-NEA Low-Dose Research to ensure transparency in the conduct of epidemiological research and to inform both domestic and international researchers about our study.

The KRWS is a large-scale national cohort study of radiation workers in Korea with a sustainable legal approach for long-term follow-up. In the future, by tracking a population of over 200 000 individuals, this cohort will enable the precise estimation of risks associated with both cancer and non-cancer diseases within the low-dose exposure range. Additionally, the scientific evidence from this study will contribute to the establishment of occupational safety standards and radiation protection guidelines for radiation workers.

Epidemiologic Study of Aircrews in Korea

The ICRP has issued essential guidelines for protecting aviation personnel from cosmic radiation. According to ICRP Publication 132, the Commission recommends that aircraft crews be informed about cosmic radiation, their individual and cumulative doses be recorded, and flight rosters be adjusted based on a specified dose reference level. ²⁶ Although there are no standardized dose limits for aircrew, each nation provides guidelines for management within the range of 5-10 mSv per year. ²⁶ In a study on space radiation exposure among pilots and cabin crew in Korea, the dose distribution showed that the highest frequency for pilots occurred at 0.7 mSv and 3.5 mSv, presumably depending on whether their primary aircraft are operated for domestic or international flights, while for cabin crew, the highest frequency was at 3.3 mSv.Reference Hwang, Lee and Kim ²⁷ In response to the growing need to regulate cosmic radiation and concerns about its health effects on aircrew, the Act on Protective Action Guidelines against Radiation in the Natural Environment was amended ²⁸ in 2022 to enable health impact assessments for aircrews in Korea.

For the baseline study in 2023, a systematic review and meta-analysis of previously published research on cancer incidence among aircrew was conducted to synthesize current evidence regarding the health effects of cosmic radiation. From 2024 onward, a cohort of aircrews in Korea is being constructed using a sustainable long-term strategy, and currently approximately 21 000 aircrews are available to be enrolled in the cohort. To achieve this, the crew registry and flight records obtained from each domestic airline are collected and linked to health datasets, including cancer incidence data from the NCR, cause of death information from the NVSR, and data on non-cancer diseases and medical health checkups from the NHIS database. Based on flight records, the individual radiation doses received by aircrew were calculated using space radiation estimation programs such as CARI-6, CARI-6M, and CARI-7.Reference Hwang, Lee and Kim ^{27 , 29} Ultimately, to assess the health effects of space radiation, SIRs were compared with those of the general population and dose-response relationship indices such as RRs and ERRs for cancer and non-cancer diseases will be calculated. Moreover, because aircrew can be exposed to other occupational hazards, such as shift work, in addition to space radiation, special health examination data for workers exposed to harmful factors, such as shift work and noise, will be collected in the future to assess the combined effects of these factors along with radiation.

This cohort study is the first long-term epidemiological study of aircrew in Korea and stands out as one of the few to explore the health effects on flight attendants beyond Europe and the US, where most previous research has been concentrated. This is particularly meaningful because it provides scientific evidence on the relationship between radiation dose and health effects by estimating the dose based on aircrew flight records. Additionally, while previous studies on radiation workers have primarily focused on males, this research is significant for its investigation of occupational radiation exposure and its health effects, specifically in female workers in more vulnerable working condition related to circadian rhythm disruption.

Epidemiologic Study of Residents Living Near Nuclear Facilities in Korea

As of 2024, Korea has operated a fleet of 26 nuclear reactor units at 5 nuclear power complexes, which provide approximately one-third of Korea’s electricity. In addition to these power plants, 1 research reactor over 2 megawatts is also in operation. Korea has the highest density of nuclear reactors in the world. ³⁰ With such a significant reliance on nuclear energy, concerns about the potential health impacts on residents living near these facilities have grown over the years. Several incidents and subsequent investigations have drawn attention to the health impacts on residents living near nuclear power plants (NPPs) in Korea. A notable prospective cohort study (1991-2011) indicated a higher incidence of thyroid cancer among female residents near nuclear power plants during normal operations of facilities.Reference Ahn and Li ⁹ Although the cohort study could not establish a direct causal link between the elevated cancer rates and radiation exposure, it underscores the necessity for further investigations of the potential health impacts on these communities with consideration of possible screening effects (e.g., thyroid ultrasound).Reference Lee, Park, Jang, Park, Jun and Seo ⁷ These concerns prompted an amendment to the Nuclear Safety Act that broadened the scope of health assessments to include all current and former residents near nuclear facilities. ¹¹ In response, the KIRAMS initiated health evaluations in 2023 to assess the potential risks associated with proximity to nuclear facilities.

The baseline research conducted in 2023-2024 aims to establish the groundwork for assessing the health risks to residents near nuclear facilities in Korea. First, comprehensive literature reviews on the cancer risks associated with residing near nuclear facilities were conducted to evaluate the existing scientific evidence. Moreover, a quantitative synthesis of the health effects using a meta-analysis methodology is ongoing. Second, an ecological study is underway to compare cancer incidence rates at varying distances (<3, 3-4.99, 5-9.99, 10-19.99, 20-30 km) from each nuclear facility to those of the general population. Based on the findings of this baseline study, long-term plans will include cohort or case-control studies at the individual level.

This epidemiological study of residents near nuclear facilities can provide information on the surrounding population’s baseline risk and radiation dose levels, thereby contributing to rapid and accurate preparedness for health risk assessments and the establishment of response strategies in nuclear emergencies. Moreover, it establishes a foundation that facilitates the identification of the population at risk of nuclear accidents and enables the efficient implementation of health surveillance and epidemiological investigations targeting these individuals.

Challenges, Resources, and Opportunities for Future Work

In low-dose epidemiological studies, low statistical power is a key limitation. For a given sample size, the statistical power of a study is significantly affected by the dose levels of the exposed group and the number of outcomes. ^{31 ,} Reference Gilbert, Little and Preston ³² Given that the average cumulative dose among radiation workers in our study was approximately 5 mSv, with >90% of workers receiving <20 mSv, enhancing the statistical power is crucial to draw concrete conclusions. Furthermore, because 96.3% of the workers in our study are currently alive with an average age of 40.4 years,Reference Lee, Cha and Park ²⁴ it is expected that cancer deaths and cases will be concentrated in the next 20-30 years. However, the presence of a young cohort in this study offers some advantages as more complete information on health outcomes is available, given that our national resources for disease status have been more secure since the 1990s. Therefore, the continued follow-up of the KRWS cohort will be an important opportunity to refine risk estimates through the accumulation of cumulative doses and an increase in the number of observed cases over time.

Additionally, concerns have been raised regarding the reliability of the doses recorded for regulatory purposes in occupational epidemiological studies.Reference Wakeford ³³ Specifically, there have been concerns regarding the potential underestimation of doses for workers employed in earlier periods.Reference Gilbert, Little and Preston ³² Furthermore, it is necessary to assess the neutron doses and internal exposure from radionuclide intake comprehensively, along with the external exposure to photon radiation.Reference Wakeford ³³ In our study, there is a possibility of underestimation or uncertainty in the dose estimation because of the lack of dose data before 1984 and limited information related to internal depositions and neutrons. In Korea, nondestructive testing using radiation began in the 1960s,Reference Chae ³⁴ a research reactor was first introduced in 1962, and the first nuclear power plant opened in 1978.Reference Lim, Kim and Park ³⁵ The number of workers with dose records starting from 1984 (including those who were employed before 1984, but whose dose data exists only from 1984 onwards) was 4892, representing 2.5% of the total cohort. When reconstructing doses for the initial cohort participants, historical radiation doses for workers between 1961 and 1983, which were not available in the NDR, were estimated to range from 4.88-11.15 mSvReference Lee, Lee and Jin ³⁶. It may be worthwhile to either apply these reconstructions of historical doses to the extended cohort or analyze the data separately for workers who started before and after 1984. Additionally, to identify these uncertainties better and estimate the radiation-induced risk more accurately, sensitivity analyses will be considered by regrouping workers into categories based on internal exposure or the potential for high linear energy transfer exposure, such as from neutrons.

Despite the previously discussed limitations of low-dose epidemiological studies, ongoing studies on radiation workers, airline crew, and residents near nuclear facilities in Korea are expected to enhance our understanding of the health effects of chronic exposure to low-dose radiation. The completeness of cancer registration in Korea is estimated at approximately 98.2%,Reference Hong, Won and Lee ³⁷ and nearly all Koreans (97%) are registered in the NHIS database.Reference Kyoung and Kim ³⁸ This ensured that the health information used in these studies was accurate and complete. Moreover, the NHIS database provides information on the diagnosed diseases, treatments received, prescribed medications, potential confounding factors (smoking, drinking, physical activity, income level, and family disease history), and health examination data (cholesterol, blood glucose, and liver enzyme test results). The availability of comprehensive healthcare utilization information is a significant strength of our study, allowing the quantification of radiation-induced health risks. This includes assessing cancer risk and evaluating all types of non-cancer diseases and even preclinical symptoms that precede disease manifestation. While previous radiation epidemiology studies were often limited to assessing mortality risks due to constraints in data sources, ongoing studies in Korea are uniquely positioned to assess not only mortality risk but also the incidence of both cancer and non-cancer diseases. The large cohort size and integration of multiple data sources enhance the ability to conduct subgroup analyses, allowing for the identification of potential effect modifiers such as age, sex, occupation, lifestyle factors, and family history. This comprehensive approach improves the accuracy and depth of risk assessment, contributing to a better understanding of radiation-induced health effects.

The ICRP has continued to refine its recommendations, particularly in the context of new research on the effects of low-dose radiation. Initially, the recommendations aimed to avoid threshold effects; however, as measurement systems improved, more precise dose limits were introduced. The annual dose limit of 50 mSv for workers, established in 1956, was maintained until 1990. In 1990, based on the reassessed risk of effects from the Life Span Study of Hiroshima and Nagasaki atomic bomb survivors, the annual dose limits for workers and the public were further reduced. ³⁹ Based on the ICRP 103 recommendations, ⁴⁰ the limit for the occupational effective dose is 20 mSv per year, averaged over 5 years (i.e., a limit of 100 mSv in 5 years), with no single year exceeding 50 mSv. Scientific evidence from ongoing research and programs on radiation epidemiology in Korea will enhance our understanding of the health effects of low-dose radiation and serve as a basis for monitoring contemporary regulatory guidelines. Moreover, these findings are crucial for communicating to the public about the health effects of radiation exposure in our daily lives.