# Thyroid doses estimated for a cohort of people exposed to fallout from atmospheric nuclear weapons testing at the semipalatinsk nuclear test site, Kazakhstan

## TL;DR

This study estimates thyroid radiation doses for people exposed to nuclear test fallout in Kazakhstan, showing high exposure levels and potential health risks.

## Contribution

The study provides detailed thyroid dose reconstructions for a cohort exposed to Semipalatinsk nuclear test fallout, highlighting unique exposure pathways and dose levels.

## Key findings

- Thyroid doses from ingestion of 131I and other isotopes were significantly higher than external irradiation doses.
- The highest average thyroid doses were observed after the 1951 and 1949 nuclear tests.
- Fresh milk consumption was the main pathway for internal thyroid exposure in the cohort.

## Abstract

Thyroid doses were estimated for a cohort of 3,183 individuals who were exposed to fallout from atmospheric nuclear weapons tests conducted at the Semipalatinsk Nuclear Test Site (SNTS), Kazakhstan, between 1949 and 1962. The study participants were mostly younger than 21 years of age at the time of their first exposure and lived in settlements near the SNTS. Individual thyroid doses from external irradiation from gamma-emitting radionuclides deposited on the ground as well as internal irradiation from intake of 131I and short-lived radiotellurium and radioiodine isotopes (132Te+132I, 133I, and 135I) with locally produced foodstuffs and inhalation of contaminated air during the passage of the radioactive cloud were reconstructed for the cohort. Estimated thyroid doses from external irradiation ranged from 4.9 × 10−5 Gy to 0.58 Gy (arithmetic mean (AM) dose was 0.048 Gy, median dose was 0.023 Gy), internal thyroid doses from ingestion of 131I, 132Te+132I, 133I and 135I ranged from 0 to 13.3 Gy (AM: 0.34 Gy, median: 0.062 Gy), and internal thyroid doses from inhalation of 131I, 132Te+132I, 133I and 135I ranged from 0 to 0.28 Gy (AM: 0.046 Gy, median: 2.8 × 10−3 Gy). The AM of thyroid doses from all exposure pathways was 0.43 Gy (range from 3.5 × 10−4 Gy to 13.7 Gy) and the median was 0.13 Gy. The highest thyroid doses were received by cohort members after test #2 conducted on 24 September 1951 (AM: 1.1 Gy, geometric mean (GM): 0.70 Gy), followed by test #1 conducted on 29 August 1949 (AM: 0.49 Gy, GM: 0.047 Gy) and the thermonuclear test #4 conducted on 12 August 1953 (AM: 0.16 Gy, GM: 0.14 Gy). The predominant pathway of thyroid exposure in the cohort was intake of 131I with fresh milk from mares and cows, and dairy products made from these types of milk. Although the uncertainties in the dose estimates were not quantified, it was estimated that they are characterized by a geometric standard deviation from 2.0 to 4.0 for most individuals. The study cohort received quite high thyroid doses compared to other populations exposed to fallout from the Chernobyl accident and atmospheric nuclear weapons tests conducted elsewhere. The cohort included individuals exposed in utero, as children and as adolescents to high doses of radiation to the thyroid gland. Consequently, it provides a unique opportunity to assess radiation-related risks of thyroid cancer, thyroid nodules, and other structural and functional non-cancer thyroid diseases.