The manufacture of citric acid by fermentation of molasses with Aspergillus niger has previously been described as a cause of occupational asthma in a factory. A longitudinal survey of the workforce of this factory has been carried out from 1984 to 1991. Over this period 160 of the original 278 workers left the workforce, together with 39 of 76 new recruits. Partial enclosure of the process and exhaust ventilation, installed in 1984, was effective in preventing any new cases of occupational asthma over the eight year period, and no new skin sensitisation was detected. Spore counts of A niger averaged about 100 times those in the outside air. Health in the 1984 survey had a striking influence on subsequent retiral; only 11 of the 79 with respiratory symptoms remained in 1991, compared with 90 of the 182 with no symptoms. In conclusion A niger is a weak antigen and simple hygiene measures protect the workforce. Exclusion of recruits with positive skin tests is not necessary if such measures are taken. The survey provided evidence of the selection factors operating within a workforce over this period contributing to retiral of the less healthy.
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