Background Asthma prevalence and acute exacerbations have been associated with endotoxin exposure. However, there are limited data on relations between acute asthma outcomes in children and personal exposure to endotoxin or whether this relation is modified by personal air pollution exposures.

Methods We made repeated measurements of the fractional concentration of exhaled NO (Fe_NO), forced expiratory volume in 1 s (FEV₁) and personal endotoxin exposures in patients with persistent asthma aged 9–18 years, each of whom was followed for 10 consecutive days in Riverside and Whittier, California. Endotoxin was measured in PM_2.5, and simultaneously we measured personal exposure to air pollutants: NO₂ and PM_2.5 mass, elemental carbon and organic carbon. Endotoxin exposure–response relations and interactions between endotoxin and air pollutants were analysed with mixed models controlling for personal temperature, humidity and the 10-day period.

Results Neither percent-predicted FEV₁ nor Fe_NO was associated with personal endotoxin overall; however, endotoxin was associated with FEV₁ among patients with average percent-predicted FEV₁<80%. When NO₂ was above its median, Fe_NO increased by 2.2% (95% CI −0.8% to 5.2%) for an interquartile increase in personal endotoxin, whereas Fe_NO was lower by −1.8% (95% CI −4% to 0.5%) when NO₂ was≤its median. However, this is out of 12 interaction tests between personal endotoxin and a binary air pollutant for each outcome (FEV₁ and Fe_NO), and there were no interactions with any continuous-scaled pollutant.

Conclusions Personal endotoxin exposure was not associated with acute daily changes in Fe_NO or FEV₁ in a cohort panel of schoolchildren with asthma, except for decreased FEV₁ among patients with more severe asthma (percent-predicted FEV₁<80%). There was limited evidence of effect modification of endotoxin by personal exposure to air pollution.