The BBC's Harvey Cook: "Highest levels of exposure were found in disposing and mixing areas"

The findings, published in the journal Occupational and Environmental Medicine, could explain why the bakery industry has one of the highest reported rates of occupational asthma in the UK.

Alpha amylase, derived from the fungus Aspergillus oryzae, is routinely added to flour to speed up the baking process and enhance the quality of loaves. But it is has also been identified as a cause of asthma.

Alpha amylase sensitivity

The study by doctors in the UK and the Netherlands showed the extent to which workers were exposed to alpha amylase.

Nearly 500 flour dust samples were taken from employees at three large bakeries, three flour mills and one packing station in Britain.

The findings showed major differences in exposure levels with the highest found in the dispensing and mixing areas of bakeries.

Workers exposed to high levels were almost 10 times more likely to have become sensitised to alpha amylase than those exposed to low levels.

Overall, one in 20 employees exposed to the substance became sensitised and susceptible to asthma.

Symptoms

Symptoms included chest tightness, wheezing, and itchy eyes, noses and skin rashes.

The researchers, led by Dr Mark Nieuwenhuijsen, from the Imperial College of Science, Technology and Medicine, London, concluded: "This study suggests that exposure to a-amylase is a considerable health risk in British bakeries and flour mills.