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• Occupational exposure to 50 Hz magnetic fields and risk of non-Hodgkin lymphoma.
  Bruce Hocking
  Published on: 27 April 2016

• Published on: 27 April 2016

  Occupational exposure to 50 Hz magnetic fields and risk of non-Hodgkin lymphoma.

  • Bruce Hocking, Specialist in occupational medicine

  Karapidis etal (1) conducted a population based case-control study of 694 cases of NHL and 694 controls in relation to exposure to occupational 50 Hz magnetic fields. They found an increased risk for workers in the upper quartile of exposure of 1.48 (95% CI: 1.02 -- 2.16) compared to referents and a statistically significant trend of increasing risk with increasing exposure. The risk increased after lagging for five year...

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  Karapidis etal (1) conducted a population based case-control study of 694 cases of NHL and 694 controls in relation to exposure to occupational 50 Hz magnetic fields. They found an increased risk for workers in the upper quartile of exposure of 1.48 (95% CI: 1.02 -- 2.16) compared to referents and a statistically significant trend of increasing risk with increasing exposure. The risk increased after lagging for five years and was not affected by adjusting for other occupational exposures associated with NHL. However their estimates of risk should be taken as minimal estimates due to problems of non-differential misclassification of exposure in their study.

  Exposure was based on applying a job-exposure matrix (JEM) to the subject’s occupational history of jobs held. The subject’s occupational history was assigned occupational codes corresponding to the Standard Occupational Classification and the corresponding JEM applied. The JEM is based on samples of exposures in many occupations and then calculated using “the geometric mean of the magnetic field measurements for one work day within an occupation, measured in microTesla”. Applying such an average is liable to cause considerable misclassification of exposures. This is illustrated by consideration of some common occupations such as cleaners and welders as well as their own data.

  Cleaners (janitors) are involved in domestic and commercial cleaning; they commonly wipe-down surfaces, empty rubbish bins, etc with little exposure to magnetic fields. However a small number vacuum-clean using an industrial back-pack with an electric motor causing intense exposure to electromagnetic fields (particularly in the retro-peritoneal area which is a common site for NHL). Thus in the occupation ‘cleaners’ there is likely to be a bimodal distribution of exposure to electromagnetic fields and application of a geometric mean exposure is inappropriate and a potential cause of misclassification. Welders usually carry the electric cables over their left shoulder (near the neck which is a common site of NHL)and across their body to their welding torch. Their exposure to magnetic fields varies greatly depending on the amperage (50 – 400A) used in welding and the instantaneous magnetic field values range from 10 -- 100 microtesla (2). To simply apply an average exposure to this group is likely to be erroneous. Many other examples could be given. Magnetic fields from electric motors decrease as the square of the distance. Therefore the distance of an operator such as a seamstress, wood-worker, metalworker, etc from the electric motor powering their machine is crucial to their individual exposure. Using average exposures is little better than using job titles and highlights the need for individual exposure assessment.

  In Table 2 it is shown that 130 of the cases and 144 controls were "sales workers". Their exposure to magnetic fields will depend on what they were selling, for example, books or equipment powered by an electric motor. It is likely that also within this group there is a bimodal distribution of exposure and to apply an average on the assumption of unimodal exposure is inappropriate and liable to lead to misclassification of exposure.

  The study did not take into account non-occupational 50 Hz magnetic field exposures which may arise from home duties and hobbies. These may include using power tools, welding machines, sewing machines, electric stoves, etc.

  The several deficiencies of estimation of exposure as highlighted above would apply to both cases and controls leading to non-differential misclassification of exposure and bias the risk estimates towards the null. The International Agency for Research on Cancer has classified 50 Hz magnetic fields as a possible carcinogen: the results of the study, which should be regarded as minimal risk estimates, support this classification and emphasise the need for a precautionary approach in managing occupational (and non-occupational) exposures to 50 Hz magnetic fields.

  Dr Bruce Hocking Specialist in occupational and environmental medicine. 9 Tyrone St Camberwell Victoria 3124 Australia.

  1. Karipidis K, Benke G, Sim M, Fritschi L, Yost M, Armstrong B, Hughes AM, Grulich A, Vajdic CM, Kaldor J, Kricker A. Occupational exposure to power frequency magnetic fields and risk of non-Hodgkin lymphoma. Occup Environ Med. 2007 Jan; 64 (1):25-9.

  2. Skotte JH, Hjollund HI. Exposure of welders and other metal workers to ELF magnetic fields. Bioelectromagnetics. 1997; 18 (7):470-7.

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  Conflict of Interest:
  None declared.

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