Those who are most at risk include farmers, painters and plastics workers, according to a Spanish study published in The Lancet medical journal.

Cleaners are another at risk group

Investigators led by Dr Manolis Kogevinas, from the Respiratory and Environmental Health Research Unit in Barcelona, analysed data on 15,637 randomly selected people aged 20 to 44.

The study covered 26 areas of 12 industrial countries around the world, including Britain and the United States.

Participants answered questions about asthma-related symptoms and job history.

The researchers found that compared with clerical workers, farmers were more than 2.5 times more likely to have asthma.

The risk was more than double for painters and plastic workers, and just under this level for cleaners and paint-sprayers.

Common disorder

Asthma is the most common occupational respiratory disorder in industrialised countries.

It is a chronic, inflammatory lung disease characterised by recurrent breathing problems. People with the disease suffer "attacks", or acute episodes, when the air passages in their lungs narrow and breathing becomes difficult.

Attacks are caused by the airways over-reacting to certain environmental factors. They then become inflamed and clogged.

About 250 specific substances found in the workplace are associated with asthma, including dyes, grain dusts, mites and animal material.

Cleaners, for example, are commonly exposed to irritants such as chlorine and acids, detergents and dust.

Housewives also had a small but statistically significant increased risk for asthma. About 5% of asthma among women was thought to be due to dust and other airborne material in the home.

"Occupational asthma should be an important public health issue in industrialised areas," the researchers concluded.

Fiona Costello, helpline teamleader for the National Asthma Campaign, said the study would help get the message across to people with asthma that their working conditions might play a role in their illness.