3.1. Pooling study to investigate associations between childhood leukaemia and combined exposure to magnetic fields and low-level ionising radiation

Objective

Epidemiological studies indicate that the development of childhood leukaemia could be associated to both weak low-frequency magnetic fields and to ionising radiation in the low-dose range (e.g. due to medical examinations or natural background radiation). A combinatorial effect of both types of exposure is also conceivable. It should therefore be examined whether there are epidemiological studies investigating the interaction of these two potential risk factors. If a sufficient number of suitable studies are available, a meta-analysis should be carried out.

Assessment of the current study situation

Although numerous studies have investigated the association between the risk of childhood leukaemia and either exposure to magnetic fields or exposure to low-level ionising radiation, only one study investigating a potential combinatorial effect of these two types of exposure was found (Pedersen et al. 2014).

This case-control study of 879 cases of leukaemia and 1,621 population-based controls investigated the effect of combined exposure to non-ionising radiation from power lines and ionising radiation from radon in residential buildings. The EMF exposure was approximated by the distance from the residential address to the nearest high-voltage power line and was evaluated in three categories (1–199 m, 200–599 m, and > 600 m). Using a validated model, domestic exposure to radon was estimated for the residential address of the test subjects, and exposures < 42 Bq/m2 and ≥ 42 Bq/m2 were considered.

The authors found weak evidence of an interaction between the two exposures with respect to the risk of childhood leukaemia: Compared with the group with the lowest exposure (> 600 m distance and less than 42 Bq/m2), the group with the highest exposure (0–199 m, ≥ 42 Bq/m2) showed an odds ratio (OR) of 2.88 (95 % confidence interval [CI]: 1.01–8.27). In addition, a significantly lower risk of leukaemia was observed in the subgroup with distances of 200–599 m from the high-voltage transmission line and radon concentration < 42 Bq/m3 (OR=0.24; 95 % CI: 0.07–0.83). However, these two findings are based on extremely low sample sizes and may have arisen by chance. A repeated analysis with a different threshold radon concentration (based on the 75th percentile) provided no evidence for the interaction. The authors concluded that this was more of an incidental finding.

Because this is so far the only epidemiological study dealing with the interaction between ionising and non-ionising radiation on the risk of childhood leukaemia, a comparison of the results with other studies is not possible. The strengths of this study are the inclusion of all eligible cancer cases and the use of population-based controls. The radon concentration was estimated using a validated model that includes many important parameters, including the floor on which the flat is located. In addition, the analysis controlled for potential confounding variables such as socio-economic status and fine dust/exhaust fumes.

Limitations of the study include the fact that exposure to non-ionising radiation was estimated only as the distance to the high-voltage power line. This does not take into account other important domestic sources of exposure to magnetic fields (e.g. the differences between different types of power lines that affect exposure). The largest pooled analysis on childhood leukaemia near power lines to date found no association between distance from the high-voltage power line and the risk of leukaemia (Amoon et al. 2018). In addition, exposures were calculated for the address at the time of the child’s birth; this address may not correspond to the address at the time of illness.

Because no further studies are available, it is not possible to conduct a meta-analysis.

The results from animal studies on the possible interaction between ionising and non-ionising radiation on the risk of cancer are inconsistent. In a study by Babbitt et al. (2000), there was no evidence of a combinatorial effect of exposure to low-frequency magnetic fields and ionising radiation on the frequency of haematological neoplasms. However, the study by Soffritti et al. (2016), significantly increased incidence rates were observed for several tumour types. The results of this study have not yet been reproduced.

Further procedure

The planned meta-analysis has been cancelled. This decision was presented in the expert workshop held on 25 January 2021 regarding epidemiological studies in the context of power grid expansion and was supported by the experts present. In the meeting, there was agreement that the scientific evidence for a possible interaction between low-frequency magnetic fields and ionising radiation, especially for radon, is quite low. Nevertheless, there is scientific and public interest in investigating these associations. The usability of original data regarding exposures to magnetic fields and weak ionising radiation from existing case-control studies on childhood leukaemia for a pooling study is thus currently being examined.

Literature

Pedersen C., et al. 2014. "Distance to high-voltage power lines and risk of childhood leukemia--an analysis of confounding by and interaction with other potential risk factors." PLoS One 9(9): e107096.

Amoon AT, Crespi CM, Ahlbom A, Bhatnagar M, Bray I, Bunch KJ, et al. 2018. Proximity to overhead power lines and childhood leukaemia: An international pooled analysis. British journal of cancer 119:364-373.

Babbitt JT, Kharazi AI, Taylor JM, Bonds CB, Mirell SG, Frumkin E, et al. 2000. Hematopoietic neoplasia in c57bl/6 mice exposed to split-dose ionizing radiation and circularly polarized 60 hz magnetic fields. Carcinogenesis 21:1379-1389.

Soffritti M, Tibaldi E, Padovani M, Hoel DG, Giuliani L, Bua L, et al. 2016. Life-span exposure to sinusoidal-50 hz magnetic field and acute low-dose γ radiation induce carcinogenic effects in sprague-dawley rats. International journal of radiation biology 92:202-214.