Motor neurone disease

In the first New Zealand population-based case-control study on risk factors for Motor Neurone Disease (MND) we found that an increased risk of developing MND is associated with a range of occupational exposures. These include exposure to pesticides (in particular insecticides and fungicides), fumigants (methyl bromide), petrol/diesel fuel, and unspecified solvents. People who are employed in a job with a high potential for electric shocks – such as electricians – are also at risk of developing MND(1) 

The study showed that occupational exposure to pesticides(2) as a broad category was associated with a 70% increased MND risk for both males and females, and that the risk increased with the duration of exposures. Of those reporting exposure to pesticides, the risk of developing MND was more than doubled for those who applied pesticides themselves (such as pesticides applicators).  

Among the different pesticides groups, increased MND risks were found for exposure to insecticides and fungicides. Exposure to insecticides was associated with a three-times greater MND risk, and these associations were observed for several insecticide classes (organochlorines, organophosphates, and pyrethroids).  

Exposure to fungicides, especially inorganic (copper) fungicides, also doubled the risk of MND. Fumigants which are predominantly used as insecticides were also associated with an increased risk and also observed in both males and females, and again we observed that the risk increased with the duration of employment. 

Among the fumigants groups, exposure to methyl bromide was associated with a five-fold risk of developing MND. However, in this study, methyl bromide predominantly occurred in horticulture where it has been used to sterilise the soil.  

Among other occupational exposures, males who were exposed to petrol/diesel fuel and unspecified solvents, and females who were exposed to textile fibres, disinfectants and cleaning products were associated with MND risk.  

The study also found a two-fold increased risk for high potential exposure to electric shocks at work in both males and females, although for females, employment in a job with high potential for electric shocks was rare. This increased MND risk was found for both short employment duration (<3 years) and longest employment duration (>24 years) in a job with high potential electric shocks. 

The study size is relatively large compared with similar studies overseas which also accessed lifetime occupational exposures histories. A total of 321 MND patients and 605 population controls participated in the study, and more than 6300 jobs with 11,000 occupational exposure data were collected from a wide range of exposure categories. All results were adjusted for age, sex, ethnicity, socioeconomic status, education, smoking, alcohol consumption, physical activities, head/spine injury, and mutually other occupational exposures.  

Our study has confirmed that several common occupational exposures, such as insecticides, methyl bromide and electric shocks, are associated with an increased risk of MND. The finding of positive duration-response associations, and the finding of increased risks for both males and females, strongly suggest these are not chance occurrence. 

Among the identified occupational risk factors, the highest proportion of MND in the study population could be attributed to employment in agriculture and application of pesticides, in particular insecticides, and occupations with high potential for electric shocks at work.  

These results confirm previous overseas findings and support policies to prevent electric shocks at workplaces and reduce exposures to pesticides in the work environment by providing appropriate personal protective equipment (PPE), and where feasible, replacing harmful pesticides with less harmful ones.  

Attention should also be given to hazard-free methods of pesticides application. The use of these principles could help reduce the risk of developing MND.

References: 

  1. Chen GX, 't Mannetje AM, Douwes J, et al. Associations of Occupational Exposures to Electric Shocks and Extremely Low-Frequency Magnetic Fields With Motor Neurone Disease. Am J Epidemiol 2021;190(3):393-402. doi: 10.1093/aje/kwaa214 [published Online First: 2020/10/10]
  2. Chen GX, Douwes J, van den Berg L, et al. Occupational exposures to pesticides and other chemicals: a New Zealand motor neuron disease case–control study. Occupational and Environmental Medicine 2022:oemed-2021-108056. doi: 10.1136/oemed-2021-108056

Grace Chen is a research assistant with Massey University’s Centre for Hauora and Health (formerly the Centre for Public Health Research). She is completing a PhD on the link between workplace exposures and MND.