Weighted estimates for endotoxin recovered from the five sampling locations were expressed as both endotoxin concentration in the sieved dust (EU/mg) and endotoxin load (EU/m2 of vacuumed area). For samples recovered from the kitchen floor and family room sofa, the area sampled was variable and so total load (total EU in the sample) was specified. The geometric mean endotoxin values, geometric standard errors, 95% confidence intervals (CIs), and 5th and 95th percentiles are reported in (unweighted values are provided in the online supplement). These data show that concentrations were highest for kitchen floors and living room floors and lowest for the bedding, which includes the mattress and pillow. Although there was a high degree of variability for kitchen floors, family room floors, and sofas, the variability was considerably less for bedding and bedroom floors. Still, the 5th to 95th percentile values displayed a 70-fold range for bedding and a 50-fold range for bedroom floor endotoxin concentrations. The geometric mean concentration of endotoxin in the kitchen floor dust was 2.3-fold higher than bedroom floor dust and 4.3-fold higher than bedding dust. However, because the degree of contact with bedroom floor/bedding dust and the contact time are high, especially for children, exposure to dust from these sites is of great importance.
GEOMETRIC MEAN AND DISTRIBUTION OF ENDOTOXIN CONCENTRATION AND LOADING FOR THE FIVE SURFACES SAMPLED
To enhance our understanding of the relationship of endotoxin concentration to endotoxin load, we plotted one against the other separately for the sampling locations (Figure E1 in the online supplement). There was a linear relationship between the logarithms of the two variables, with correlation coefficients of 0.73 for the bedroom floor samples and 0.79 for the bedding samples. The coefficient for family room floor samples was also 0.73. These data showed that either concentration or load could be used as measures of household endotoxin. Thus, subsequent analyses focused on concentration because the mass of dust collected is known with greater accuracy than the area sampled, particularly for sofa and kitchen floor.
It was of interest to explore the relationship of endotoxin concentrations to those of household allergens measured from the same samples to assess the potential for confounding. Correlation coefficients between endotoxin and eight animal, arthropod, and fungal allergens in the five household sites were low, ranging from −0.03 to 0.43 (see ). Only 5 of the 40 coefficients exceeded 0.30, and four of those were for Alternaria alternata, a common species of fungus. We also determined overall Pearson correlation coefficients for endotoxin pairs within the same households (). Endotoxin concentrations were not highly correlated between rooms of the same households, and coefficients ranged from 0.44 to 0.12.
WEIGHTED PEARSON CORRELATION AT EACH HOUSEHOLD SITE FOR LOG-TRANSFORMED ENDOTOXIN AND LOG-TRANSFORMED ALLERGEN CONCENTRATIONS
OVERALL WEIGHTED PEARSON CORRELATION BETWEEN PAIRS OF HOUSEHOLD SITES FOR LOG ENDOTOXIN CONCENTRATION
This study yielded a prevalence rate of 11.3% for diagnosed asthma among persons living in noninstitutional U.S. housing that is permanently occupied and allows resident children. This compared favorably with the 11.1% prevalence of diagnosed asthma determined from the 2002 National Health Interview Survey (39
). This study also compared favorably for diagnosed asthma when separated by sex and age categories (see
the online supplement). Logistic regression analyses were performed without adjustment for health outcomes using log10
endotoxin as a continuous variable to determine if there is a relationship between household endotoxin and health outcomes (). Although there was no relationship of endotoxin exposure with hay fever, the relationships between endotoxin and other health outcomes were highly significant for bedroom floor endotoxin. Endotoxin from other sampling locations yielded p values significant or suggestive of an effect for asthma outcomes (p = 0.02–0.10).
VALUES AND β COEFFICIENTS FOR UNADJUSTED LOGISTIC REGRESSION MODELS FOR PRESENCE OF HEALTH OUTCOMES AMONG ANYONE IN THE HOUSEHOLD USING LOG ENDOTOXIN CONCENTRATION AS A CONTINUOUS VARIABLE
To further explore the role of bedroom and family room endotoxin in asthma, we developed logistic regression models using dichotomized endotoxin levels (). Significantly elevated crude odds ratios (ORs) were identified with exposure to bedroom floor endotoxin for the following outcomes: asthma symptoms in the past year (OR, 2.82), current asthma medication use (OR, 2.42), wheezing ever (OR, 2.13), wheezing in the past month (OR, 1.98), and wheezing in the past year (OR, 2.23). After adjustment, ORs were still significantly elevated for most of these outcomes (). Similar results (but with lower ORs) were observed when the prevalence of disease outcomes was evaluated for association with bedding endotoxin concentration (). In both the unadjusted and fully adjusted models for bedding, increased endotoxin concentration was most strongly associated with wheezing ever, in the past month, and in the past year. ORs for the effect of family room floor endotoxin on health outcomes were not significant, although diagnosed asthma, symptomatic asthma, and taking asthma medication had adjusted ORs of 2.0 or more. It is noteworthy that bedroom floor, family room floor, or bedding endotoxin were not protective factors for self-reported doctor-diagnosed hay fever, an indicator of atopy.
TABLE 5. ODDS RATIOS AND 95% CONFIDENCE INTERVALS FROM LOGISTIC REGRESSION MODELS FOR THE PRESENCE OF HEALTH OUTCOMES USING ENDOTOXIN EXPOSURE LEVELS ABOVE AND BELOW THE FIRST QUARTILE FOR BEDROOM FLOOR AND FAMILY ROOM FLOOR ENDOTOXIN AND THE SECOND QUARTILE FOR (more ...)
To understand the effect of aggregation to the household level, we also performed individual level analyses for bedroom floor endotoxin separately for adults (
18 yr, 65% of subjects) and children (< 18 yr). This allowed for adjustment for potential confounders at the individual level. This analysis yielded findings consistent with the household level analysis for adults with significantly elevated adjusted ORs for symptomatic asthma and medication use. None of the ORs for children were significant and point estimates were generally close to 1.0, demonstrating that the effect of endotoxin was in adults. These results are provided in the online supplement.
Relationships between endotoxin concentration and disease outcomes were assessed using nonparametric regression analysis while controlling for the covariates mentioned above. These smoothed plots are shown in for bedroom floor endotoxin and four health outcomes. There was a marked increase in prevalence with increasing endotoxin for each health outcome. None of the outcome variables demonstrated a threshold below which there was no increase in prevalence with increasing endotoxin. Thus, no level of endotoxin observed in this study could be clearly identified as a no-effect level.
Figure 1. Smoothed plots showing adjusted prevalence of various health outcomes by endotoxin concentration for bedroom floor. Top left: adjusted prevalence of doctor-diagnosed asthma; bottom left: adjusted prevalence of asthma medication use in the past 12 mo; (more ...)
To further evaluate the role of bedroom floor and bedding endotoxin on prevalence of asthma symptoms in the past year, we modeled the joint effect above and below 19.6 EU/mg for 373 homes that had bedroom floor and bedding endotoxin measurements and data on asthma symptoms. The OR for households with both bedroom floor and bedding endotoxin over 19.6 EU/mg compared with those with both values below this mark was 3.18 (95% CI, 1.31–7.72; n = 373). Adjustment for census region, season, smoking inside, education, poverty, ethnicity, and race resulted in an OR of 2.83 (95% CI, 1.01–7.87; n = 338).
Several recent studies have suggested that the role of endotoxin in asthma may differ for those with allergic asthma and those with nonallergic asthma. We compared diagnosed asthma, asthma symptoms, and wheeze in the past year, stratified by allergy status at the household level (self-reported doctor-diagnosed allergy). The adjusted OR for wheezing was higher for households with allergic residents that had higher bedding endotoxin (OR, 2.16; 95% CI, 1.12–4.15; referent is allergic with low endotoxin) as compared with nonallergic wheezing subjects with higher endotoxin exposure (OR, 0.80; 95% CI, 0.33–1.92; referent is nonallergic with low endotoxin). However, this analysis demonstrated no significant interaction between health outcomes and allergy status (interaction p value = 0.11; see Table E5 in the online supplement). This suggests that, in this cohort, atopic wheeze increased with higher bedding endotoxin exposure. For bedroom floor endotoxin, asthma symptoms and wheezing both yielded elevated adjusted ORs (OR > 2.20) in allergic subjects, again with no significant interaction terms. No other health outcomes or sampling sites showed differences with stratification by allergy status.