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ndotoxins, part of the outer membrane of Gram negative bacteria, are a potent inducer of neutrophilic airway inflammation. A large number of studies in occupational epidemiology has shown that exposure to endotoxins increases the likelihood of organic dust toxic syndrome, chronic bronchitis, and asthma-like syndrome. In contrast, it has been shown that exposure to endotoxins in the occupational and environmental setting protects from respiratory allergies and sensitisation to allergens. With respect to asthma, the evidence is conflicting at first glance. While some studies indicated an increased risk of asthma after endotoxin exposure, others have shown that endotoxin exposure protects from asthma. However, these differences can be explained when different asthma phenotypes are taken into account. The risk of atopic asthma, mainly dominated by eosinophilic response, is decreased in those exposed to endotoxins. In contrast, the risk of non-atopic asthma, characterised by neutrophilic response, is enhanced in subjects with higher endotoxin exposure. These data are in accordance with the so-called hygiene hypothesis and have been supported by animal studies and at the cellular level. At the workplace, measures should therefore be taken in order to reduce endotoxin exposure