Epidemiological evidence suggests that exposure to nitrogen dioxide (NO2) through the use of unvented gas cookers in homes is associated with respiratory symptoms. Toxicological evidence, mainly in animal models, suggests that NO2 may increase the susceptibility to infection by viruses and bacteria. This review examines the evidence and proposes mechanisms whereby such exposure may occur, in particular how NO2 may aggravate respiratory symptoms in the presence of coexistent infection.