Exposure to multiple indoor allergens in US homes is common; over half of the surveyed homes had detectable levels of all studied allergens (dust mite, dog, cat, cockroach, mouse, and Alternaria) and most homes had 2-3 allergens in elevated levels. Occupants’ race, income level, housing type, absence of children, and presence of smokers, pets, cockroaches, rodents and mold/moisture related problems contributed independently to high allergen burden in the home. Households with asthmatics were more likely to have a greater number of elevated allergens than households without asthmatics. Among atopic individuals, high allergen burden in the home significantly increased the odds of having asthma symptoms.
The NSLAH was the first study to characterize how indoor allergen exposures vary in homes throughout the US. Of the measured allergens, Alternaria,
cat, and dog allergens were the most commonly detected and they were also the ones that were most often found in elevated levels. Individual allergen levels were strongly associated with regional, ethnic, and socioeconomic factors, but each allergen appeared to have a distinct set of predictors.15-19
Consistent with previous studies,9, 28, 29
allergen levels in homes varied by location in the home.
Beyond individual allergen exposures, it is important to characterize which factors contribute to total allergen burden in homes. The NSLAH data suggested that one of the strongest predictors of high allergen burden was race; burden of exposure was significantly higher among Whites than non-White residents. White households were more likely to have elevated levels of Alternaria, cat, dog, and dust mite (Der f 1) allergens than non-White households, whereas the presence of cockroach and mouse allergens in elevated levels was significantly more prevalent in non-White households.
Family income influenced allergen burden significantly; households with lower income (<$40,000) were more likely to have high allergen burden than households with higher income levels (23.3% vs. 11.9%, p < 0.01 for difference). The prevalence of elevated levels of cockroach, mouse and dust mite (Der p 1) allergens was significantly higher in homes with lower income. Although elevated cat and dog allergen levels were strongly associated with higher income levels, the majority of the homes with high allergen burden and lower income (<$40,000) had elevated cat and dog allergen levels (73.0% and 74.5%, respectively). Living in a single family home was a strong predictor of high allergen burden. Except for cockroach and mouse allergens, elevated allergen levels were more prevalent in single family homes than in multi-family homes; differences were statistically significant for Alternaria
, dog, and dust mite (Der p 1) allergens. Consistent with published literature, the presence of elevated cockroach allergen was significantly higher in multi-family homes.10, 30
Previous studies suggest that allergen levels are not only associated with socioeconomic factors and ethnicity, but are also influenced by environmental factors that tend to differ between housing types (e.g., temperature and humidity levels, differences in likelihood of having pets).9, 10, 30
Smoking is typically associated with socio-economic factors,31
but the presence of smokers remained an independent predictor of high allergen burden in our multiple regression models. In particular, elevated levels of cockroach and dog allergen levels were more frequently detected in homes of smokers than in homes of non-smokers. Interestingly, homes with children were less likely to have high allergen burden. It is possible that cleaning frequency is higher in homes where children reside. Indeed, presence of smokers and less frequent cleaning have been found to contribute elevated dust weight levels in homes.32
As expected, the presence of pets, cockroaches and rodents in the home predicted high allergen burden. The presence of pets was the strongest predictor of allergen burden; pets in the home, particularly cats and dogs, increased the odds of having high allergen burden by 3-fold. Other recent studies have also shown that signs of roach and rodent activities in the home tend to predict elevated levels of cockroach and mouse allergens.33, 34
The reported presence of roaches and rodents contributed significantly to elevated cockroach and mouse allergen levels, whereas the presence of household pests tended to be inversely associated with elevated levels of cat, dog, and dust mite (Der f 1) allergens, which is consistent with previous findings.9, 10
Mold and moisture related problems were not only associated with elevated fungal (Alternaria)
levels but also with elevated levels of several other allergens, including dust mite (Der p 1), cockroach, cat, and mouse allergens, in agreement with published work.35
Although regional factors can influence allergen-specific levels significantly,8
census region and level of urbanization (MSA) did not remain significant predictors of high allergen burden in US homes. The relative importance of different allergens may vary regionally. For example, Alternaria,
cat, and dust mite levels differed significantly by census region; and concentrations of Alternaria
and cockroach allergen were strongly associated with level of urbanization.15-17,19
Nevertheless, geographic and climatic factors seemed to play a less significant role in overall exposure burden. Racial and socioeconomic factors are strongly associated with the exposure burden and types of allergens that are present in elevated levels. However, high exposure burden to multiple allergens is not limited to populations that have been found to be at disproportionately high risk for adverse asthma outcomes (e.g., children in inner-city neighborhoods of low socio-economic status and high minority representation).36
Our analyses showed that allergens tend to cluster together in high levels. We found strongest positive associations between the following allergens: mouse and cockroach, Alternaria and dust mite (Der p 1), Alternaria and mouse, and cat and dog allergens. On the contrary, elevated concentrations of cockroach allergen were inversely associated with elevated levels of cat, dog, and dust mite (Der f 1) allergens. It is likely that both socio-demographic and environmental factors contribute to the observed patterns.
We found that endotoxin levels increased with increasing allergen burden. Our results suggest that residents who have high exposure burden to allergens are also apt to be exposed to elevated levels of endotoxin in their home environment. Elevated endotoxin levels were particularly associated with elevated Alternaria,
cockroach and mouse allergen levels. Thorne and coworkers have previously shown that household endotoxin exposure is associated with asthma-related outcomes in this population.13
Many studies have shown that exposure to allergens contributes to exacerbation of allergic asthma and persistence of symptoms.4, 6, 37
Our results reinforce the important role of indoor allergen exposures in asthma exacerbations. Among atopic individuals, high allergen burden in the home was significantly associated with current asthma. Atopy per se, however, was not associated with high allergen burden. We did not have detailed information on subjects’ asthma severity, but high allergen burden was associated with asthma symptoms among atopics irrespective of medication use. Although allergen exposure and sensitization have been predominantly linked to asthma morbidity among children, it has been shown that this relationship also persists in older populations.38, 39
We did not find strong evidence that the observed effect differed significantly by age, although the point estimate was higher among children than adults (data not shown). The association between asthma symptoms and high burden of allergens remained consistent after adjusting for potential confounders, including exposure to endotoxin.
Temporal relationships may be difficult to determine in cross-sectional studies. To reduce bias due to temporal changes over time, we focused primarily on asthma symptoms in the past year. We were not able to ascertain allergen-specific sensitization among the participants but assessed atopy based on self-reported physician-diagnosed allergies. We used the reservoir concentration as a surrogate measure of recent exposure, which may not necessarily reflect personal exposure levels. For example, activity levels of subjects, occupant density, air exchange rates, and air movement within the indoor environment can affect individual exposure levels. While airborne concentrations are considered more relevant measures of exposure, single time point air sampling for aeroallergens and endotoxin is usually a weak exposure measure because of large within-subject variance due to temporal, spatial, and activity-related effects. In general, using allergen concentrations in dust as a proxy of exposure is an accepted and widely used method to assess indoor allergen exposures, particularly in large-scale epidemiological studies. To characterize the exposures in detail, we assessed exposure levels across multiple sampling locations. Although we were not able to assess seasonal variability in allergen levels in individual homes, sampling in the survey was conducted throughout summer, fall, and winter months in each geographic region to capture seasonal variation in the data. We acknowledge that the literature-derived cut points used to assess the allergen burden are somewhat arbitrary, and need to be interpreted with caution. The thresholds may be influenced by host factors and are not well established for all allergens. For example, clinically relevant exposure levels for Alternaria
remain unknown because immunoassays to assess fungal exposures (antigens and allergens) have not yet attained the same reliability as have similar assays for other allergens. The possibility of fungal cross-reactivity cannot be excluded in the current study, but exposure to Alternaria
was estimated with the best available assay that minimizes cross-reactivity between genera.20
Despite the limitations, this study provides valuable information about allergen burden and variability across the US housing stock.
One of the greatest strengths of this study is that the weighted characteristics of the survey sample, including distributions of housing characteristics, socioeconomic, and demographic factors, were very similar to characteristics obtained from other national surveys.14
Moreover, the prevalence of asthma in this population was comparable to other national prevalence estimates.2, 40
This study demonstrated that exposure to multiple indoor allergens in US homes is common. Residential allergen burden and the variability of allergens that are present in elevated levels were strongly affected by sociodemographic factors and the presence of apparent sources of allergens in the home. Allergen burden was also strongly associated with endotoxin levels in the home. Current asthma was positively associated high allergen burden among atopic individuals, suggesting that atopic asthmatics may achieve better asthma control by reducing allergen burden at home. Our results highlight the importance of exposure reduction as a fundamental part of asthma management.