A substance added to flour to improve the quality of bread poses a ''considerable health risk'' to people working in bakeries and mills, researchers said yesterday.

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

The new 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 big differences in exposure levels, with the highest found in the dispensing and mixing areas of bakeries.

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