Farmers, hairdressers, welders, teachers, clergymen, bankers... workers in certain jobs appear to be at greater risk of degenerative brain diseases, such as Alzheimer's and Parkinson's, according to a study of 2.6 million death certificates in the US.

Of the deaths, just over 4 per cent were attributed, at least in part, to a degenerative brain disease.

Many of the associations can be explained by on-the-job exposures to the chemicals that farmers, welders and hairdressers routinely use or inhale.

But other findings, however, such as the higher risks among teachers, clergy and bank tellers, are not easily explained.

Death records obviously don't reveal everything about a person's employment but they do offer pointers for further research.

In their analysis, published in the American Journal of Industrial Medicine, Robert Park of the National Institute for Occupational Safety and Health, Cincinnati, Ohio and his team found that the bank tellers, clergy, aircraft mechanics and hairdressers had highest odds of dying from Alzheimer's disease.

For Parkinson's disease, the highest risks were among biological scientists, teachers, clergy members and other religious workers.

The risk of death from presenile dementia - dementia that arises before the age of 65 - was greatest among dentists, graders and sorters in industries other than agriculture and, again, clergy.

Veterinarians, hairdressers and graders and sorters had the highest risks of dying from motor neurone disease.

Degenerative brain diseases are marked by progressive, irreversible damage to cells of the central nervous system.

It's thought that genes are a big factor but growing evidence also points to environmental factors, including some on-the-job exposures, and this research supports this.

For example, farmers exposed to pesticides have been shown in some studies to have a higher-than-average risk of Parkinson's disease, as have welders exposed to fumes containing the mineral manganese.

Both of these occupations were associated with Parkinson's in the current study as well.

Hairdressers were at increased risk of death from Alzheimer's disease, presenile dementia and motor neuron disease.

These findings suggest a role for hair dyes, solvents or other chemicals used in salons.

Other job-disease relationships, including the higher risks for several neurodegenerative conditions among teachers and clergy, "are difficult to interpret," according to Park.

One possibility, he and his colleagues note in the report, is that people in professional jobs have lower risks of common, lifestyle-related diseases like heart disease, which makes them more likely than others to die of a neurodegenerative disorder.