Abstract

Background
Despite having a work related deterioration in peak expiratory flow (PEF), many workers with occupational asthma show a low degree of within day diurnal variability atypical of non-occupational asthma. It was hypothesised that these workers would have a neutrophilic rather than an eosinophilic airway inflammatory response.

Methods
Thirty eight consecutive workers with occupational asthma induced by low molecular weight agents underwent sputum induction and assessment of airway physiology while still exposed at work.

Results
Only 14 (36.8%) of the 38 workers had sputum eosinophilia (>2.2%). Both eosinophilic and non-eosinophilic groups had sputum neutrophilia (mean (SD) 59.5 (19.6)% and 55.1 (18.8)%, respectively). The diurnal variation and magnitude of fall in PEF during work periods was not significantly different between workers with and without sputum eosinophilia. Those with eosinophilia had a lower forced expiratory volume in 1 second (FEV1; 61.4% v 83% predicted, mean difference 21.6, 95% confidence interval (CI) 9.2 to 34.1, p=0.001) and greater methacholine reactivity (geometric mean PD20 253 µg v 1401 µg, p=0.007). They also had greater bronchodilator reversibility (397 ml v 161 ml, mean difference 236, 95% CI of difference 84 to 389, p=0.003) which was unrelated to differences in baseline FEV1. The presence of sputum eosinophilia did not relate to the causative agent, duration of exposure, atopy, or lack of treatment.

Conclusions
Asthma caused by low molecular weight agents can be separated into eosinophilic and non-eosinophilic pathophysiological variants with the latter predominating. Both groups had evidence of sputum neutrophilia. Sputum eosinophilia was associated with more severe disease and greater bronchodilator reversibility but no difference in PEF response to work exposure.

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