This study is the first to evaluate domestic endotoxin levels over a wide geographic region and across demographic groups representing urban, suburban, rural; wealthy and poor; African American (black), Hispanic, and white; apartment dwellers and people living in multifamily or single family homes; children and adults; with or without pets; with and without allergy or asthma. This allowed us to develop an understanding of the predictors of domestic endotoxin for the entire United States. shows the GM concentrations of 2,469 surface samples collected from the kitchen floor, family room floor, family room sofa, bedroom floor, and bedding. Endotoxin concentrations in samples from the kitchen and family room floors were about 4-fold higher than concentrations in the bedding, and family room sofa and bedroom floor concentrations were approximately twice those in the bedding. Endotoxin load values demonstrated that bedroom floors were substantially less contaminated than family room floors, sofas, and kitchens but more than twice as contaminated as bedding. Although family room floors and sofas had lower endotoxin concentration than kitchen floors, the amount of dust was higher, so the endotoxin loads were comparable.
Figure 1 Endotoxin concentration (left) and endotoxin load (right) in the dust samples shown as GM and 95% confidence limits (error bars). We adjusted values for survey design information and sample weighting. aEndotoxin load as EU per sample rather than EU per (more ...)
– show potential predictors of endotoxin concentrations assessed in this study for bedroom floor, family room floor, and bedding samples. lists household factors and their endotoxin concentrations (GM and p-values) compared with the referent subpopulation (the referent is the sub-population with no p-value listed). A number of household factors showed consistency as predictors of endotoxin across sampling locations. The West census region (illustrated in ) had higher endotoxin levels than the Northeast, South, or Midwest regions. When we analyzed this further using the nine U.S. census divisions, we found that the Pacific division (California, Oregon, Washington) was the highest for all sampling locations and New England (Connecticut, Massachusetts, Maine, New Hampshire, Rhode Island, Vermont) was the lowest. The Pacific division spans 2,000 km from north to south and represents both warm, dry (e.g., San Diego, CA) and cool, wet climates (e.g., Portland, OR). In we have plotted quartiles of the GM endotoxin concentrations for all households and all household sampling sites within geographic primary sampling units (PSUs) [i.e., metropolitan statistical areas (MSAs) or rural counties]. On this map, for example, the orange square over Boulder County, Colorado, represents the unadjusted GM of 52 samples collected in the cities of Boulder and Longmont (population, 225,339; PSU weight, 20.357). The red circle in western Kansas represents 81 samples collected in five adjoining counties (combined population, 23,293; PSU weight, 91.333). illustrates that the high endotoxin values for the Pacific census division were primarily in Southern California. The New England and Middle Atlantic divisions plus Delaware, Maryland, Virginia, and the District of Columbia had no PSUs in the highest quartile and had 71% in the lowest quartile.
Household predictors of endotoxin concentration in bedroom floors, family room floors, and bedding.
Field-staff–observed predictors of endotoxin concentration in bedroom floors, family room floors, and bedding.
U.S. map showing the census regions, census divisions, and quartiles of the GM endotoxin concentration for all five sampling locations within homes, aggregated by PSUs of the survey.
Another household factor relating to endotoxin was living in poverty, for which mean bedroom floor and bedding endotoxin levels were 56% (p = 0.003) and 58% (p = 0.021) higher than in nonimpoverished households, respectively. Households occupying two- or three-story homes including a basement (if present) had significantly lower bedroom floor (p = 0.002) and family room floor (p = 0.006) endotoxin. Homes on a single level or in multilevel apartment buildings had higher endotoxin. Having air conditioning, a stove exhaust fan, or an air filtration system were not significant predictors. Having electric heat as the main heating source was associated with higher bedroom (p = 0.012) and family room floor (p = 0.009) endotoxin than the other/none category. Also, whether the occupants lived in a single or multifamily dwelling or owned their home was not related to endotoxin in the homes.
Metropolitan status demonstrated higher values for MSAs with populations of > 1 million than for those with < 1 million that were significant for bed endotoxin (p = 0.035) and showed a trend for bedroom floor (p = 0.073) and kitchen floor (p = 0.080). Homes built before 1978 had higher endotoxin levels in family room floors (p = 0.040) but not in other locations.
shows the GM and p-values for a variety of endotoxin source factors in domestic environments for bedding, bedroom floor, and family room floor endotoxin. Increasing numbers of people living in the household showed a very strong relationship with increasing endotoxin concentration, as did having children residing in the home. For family room floor endotoxin, the GM was 42.7 EU/mg for households with a single resident, 58.1 for two-member households (p = 0.019), between 76.8 and 79.0 for three or four residents (p < 0.005), and 87.0 for households with > four residents (p < 0.001). We also observed this trend for bedroom floor and bedding endotoxin but it was less dramatic. Having a child or children in the home was significantly associated with higher endotoxin for bedroom floors (p < 0.001), family room floors (p = 0.028), and bedding (p< 0.001).
Endotoxin source as predictors of endotoxin concentration in bedroom floors, family room floors, and bedding.
Several other potential source factors were significantly associated with bedroom floor endotoxin. Current pets or pets in the household in the past 6 months and current or past dogs or cats were significant (all p ≤ 0.001; ). Also significant were cockroach problems in the past year (p = 0.026) and, for family room floors, cigarette smoking (p = 0.004). We found no effect on endotoxin of dehumidifier use or season in which we sampled the household.
During household visits, our field staff conducted a walk-through survey noting specific factors relating to characteristics of the home. lists staff-observed factors and their relationship with endotoxin concentrations. For both bedroom floors and family room floors, evidence of smoking (p = 0.012; p < 0.001), cockroach stains (p = 0.041; p = 0.009), and food debris (p = 0.044; p < 0.001) were significant predictors of endotoxin. Observed mold or mildew in the room was associated with higher bedroom endotoxin but was rarely observed (21 of 581). Carpeted floor, room air conditioner, and room air cleaning device were not significant predictors. Extreme room temperatures on the day of the survey [i.e., < 18°C (65°F) or > 29°C (84°F)] were associated with higher endotoxin concentration for bedroom floors (p = 0.008) and family room floors (p = 0.033). Relative humidity in the room on the survey day was not a factor for family room floor or bedding endotoxin. However, for bedroom floor endotoxin, relative humidity < 40% was associated with higher endotoxin than the other four humidity ranges from 40% to > 69%. Field staff recorded whether or not the bed in the sampled bedroom was equipped with an impermeable cover for the mattress, box spring, or pillow. Interestingly, all three covers were significantly associated with higher bedroom floor endotoxin concentration (). Having a stuffed animal (e.g., teddy bear) in the bed also increased bedding endotoxin (p= 0.024).
lists data for significant predictors of kitchen floor endotoxin, which show that the kitchen floor had a distinct profile of endotoxin predictors. As with the other household sampling locations, kitchen endotoxin levels were significantly lower for the Northeast census region and the New England census division. Kitchen endotoxin was higher for those living in poverty (130 vs. 75 EU/mg; p = 0.001), with lower household income (p = 0.001), and with lower educational attainment (p = 0.021). Problems with cockroaches, live or dead cockroaches in the kitchen, and cockroach stains were all strong predictors of endotoxin levels (p < 0.001). Households reporting problems with cockroaches in the past 12 months had 2-fold higher endotoxin than did those without cockroaches. Within the subpopulation of those with cockroach problems, households where the residents sighted > 50 cockroaches per day (n = 7) had a mean kitchen floor endotoxin level of 838 EU/mg, 10-fold higher than the overall mean of 80.5 EU/mg. In addition, evidence of rodents (p = 0.002), cigarette smoking (p < 0.001), and mold or mildew (p = 0.02) were highly significant predictors of increased kitchen endotoxin concentration. In contrast to other locations in the homes, people of black race had significantly higher endotoxin in kitchen floor dust samples than did whites or other races (p= 0.005).
Predictors of endotoxin concentration in kitchen floors.
Next we sought to identify the optimal set of candidate predictors of household endotoxin using rANOVA with household as subject and the five sampling locations as repeated measures. To streamline the analysis, 37 candidate predictor variables were partitioned into five logical sets (S1–S5) shown in . After all permutations were explored, the model shown in yielded high predictive values with strong statistical significance. Coefficients for sampling locations mirror the data shown in , with bedding lowest and kitchen floor highest in endotoxin concentration. With the New England census division as the referent, Mountain, West North Central, and Pacific were 73–91% higher (p < 0.001) in household endotoxin concentration. Higher endotoxin concentration was associated with lower educational attainment (p = 0.014), children in the home (p = 0.035), currently having a dog in the household (p < 0.0001), problems with cockroaches in the past 12 months (p = 0.0022), field-staff–observed food debris (p = 0.029), cockroach stains (p < 0.0001), and evidence of smoking (p = 0.0087). When we ran the analysis for bedroom bedding and included floor endotoxin alone and S5 variables, the only additional variable from S5 that emerged was having an encapsulating mattress case on the sampled bed (p = 0.048). The rANOVA analysis for endotoxin load () revealed that sampling location, census division, education, dog in home, problems with cockroaches, food debris, and cigarette smoking were significant predictors (p < 0.0001 for all). Additional predictors for endotoxin load were cat in home (p = 0.0035), mold/mildew observed (p = 0.0012), and lower relative humidity (p < 0.0001). The rank ordering of endotoxin load by census division was somewhat different than for endotoxin concentration, although Mountain, West North Central, and Pacific were the highest for both measures of endotoxin and New England was the lowest or second lowest.
Variables entered into the repeated measures ANOVA.
Major predictors of endotoxin concentration and endotoxin load from rANOVAs.
The finding of a geographic trend for higher endotoxin and data suggesting an effect of poor indoor temperature control, low humidity, and type of heating led us to consider if the local temperature range or amount of precipitation during the study year were related to endotoxin concentration in homes. We reasoned that measurement of temperature and humidity on a single day could produce misclassification and be a poor measure of typical local climate or usual indoor conditions. Using spatial coordinates for each of the study households, we queried the Prism data explorer for annual precipitation and maximum–minimum temperatures for the year in which we sampled the home. Linear regression analysis of these factors with endotoxin concentration in each sampling location revealed no relationship of these factors for bedroom or family room floor endotoxin (). However, precipitation during the study year was a significant predictor of bedding endotoxin (p = 0.033). Temperature maxima and minima were related to kitchen floor endotoxin (p = 0.001 and p = 0.013, respectively) but showed no relation with endotoxin for other sampling locations.
Consideration of potential role of local meteorologic data (p-values) during the study year on endotoxin concentration indoors.