Endotoxin is a cell wall component from Gram-negative bacteria, and inhaledendotoxin contributes significantly to the induction of airway inflammation and dysfunction. Backgroundlevels of endotoxin have not yet been extensively described. In this study, airborne endotoxin was measuredwith a standardized protocol in 5 types of background environment (169 samples) in Denmark from Octoberto May. Endotoxin levels in a greenhouse (median = 13.2 EU/m3) were significantly higher than in theother environments. The air from biofuel plants (median = 5.3 EU/m3), the air on congested streets (median= 4.4 EU/m3) and on an agricultural field (median = 2.9 EU/m3) had higher endotoxin contents than theair in industrial areas (median = 1.3 EU/m3) or in towns (median = 0.33 EU/m3). Levels in industrialareas were significantly higher than in towns. A literature study revealed background levels of endotoxinon different continents between 0.063-410 EU/m3, with median or mean values between 0.063-3.6 EU/m3.Endotoxin concentrations in towns and industrial areas were higher in April and May than in autumn andwinter, and were higher in October than in winter. These data of exposure in background environmentsand of seasonal variation are helpful for public health practitioners, epidemiologists and industrialhygienists.