Endotoxin Exposure Is a Risk Factor for Asthma: The National Survey of Endotoxin in United States Housing
Authors: Chinn Ivan N. , Williams Larry W. 
Published: November 2007

Thorne PS, Kulhankova K, Yin M, et al. Am J Respir Crit Care Med. 2005;172:1371–1377

PURPOSE OF THE STUDY. Studies have shown that endotoxin exposure associated with living in farm environments correlates with protection from atopy and asthma. Other studies offer conflicting data. The authors considered the possibility that regional sampling within the United States might explain such discrepancies.

STUDY POPULATION. Household dust samples were assayed from 831 housing units that were selected as demographically representative of the 96 million homes in the United States.

METHODS. Data gathered on all household occupants included diagnosed hay fever, diagnosed asthma, asthma symptoms in the past year, current asthma-medication use, and wheezing (ever, in the past month, and in the past year). Field-workers distributed questionnaires and collected dust samples, which were processed for endotoxin. Adjusted odds ratios (ORs) were calculated, controlling for census region, season, frequency of indoor cigarette smoking, education, poverty, race, ethnicity, presence of a ≤6-year-old child, and exposure to dust mite, cat, and dog allergens.

RESULTS. The highest endotoxin concentrations were in dust from kitchen and living room floors (geometric means: 80.5 and 63.9 EU/mg, respectively). The lowest concentration was found in the bedding (18.7 EU/mg). In multivariate analysis, endotoxin exposure was significantly associated with multiple asthma outcomes, with the strongest for bedroom-floor and bedding endotoxin concentrations. No protective effects of endotoxin were seen. Instead, bedroom-floor–endotoxin concentrations above the first quartile (&gt;16.6 EU/mg) had adjusted ORs of 1.95 to 2.78 for asthma symptoms in the past year, wheezing in the past month, and wheezing at any time compared with those in the first quartile. Similarly, bedding concentrations above the second quartile (&gt;19.6 EU/mg) demonstrated adjusted ORs of 2.01 to 2.05 for wheezing at any time, in the past year, and in the past month compared with those in the first 2 quartiles. The authors noted that the endotoxin levels in the samples were higher than in similar data from Europe. Subanalysis showed no significant adjusted ORs for children, indicating that adults were responsible for the endotoxin effects found. Allergic subjects had no increase in asthma symptoms compared with nonallergic subjects with higher endotoxin exposure.

CONCLUSIONS. In a broad sample of US housing, a significant correlation between household endotoxin and asthma outcomes was demonstrated, driven largely by adults. No protective effect of endotoxin exposure was seen. Current endotoxin exposure had little impact on allergy status. Instead, household endotoxin exposure seemed to be a significant risk factor for increased asthma prevalence.

REVIEWER COMMENTS. These data from the National Survey of Lead and Allergens in Housing demonstrate a positive correlation between asthma symptoms in occupants and increasing endotoxin exposure. The implications and sources of higher endotoxin exposures in the United States, noted by the authors, must be considered in this and other studies. Because this study was cross-sectional rather than longitudinal, it could not assess the impact of early endotoxin exposure on the development of asthma and atopy. Thus, the study suggests that endotoxin is a risk factor for asthma and wheezing, but it neither supports nor contradicts the hygiene hypothesis of the development of atopy.