This survey is the first study that assessed dustborne Alternaria alternata antigen concentrations in the U.S. housing stock. Alternaria antigens were present in virtually all homes. Both regional and residential characteristics influenced Alternaria antigen concentrations. Based on our data driven prediction model, the age of the housing unit, census region, degree of urbanization, poverty, race of residents, observed mold and moisture problems in the home, use of dehumidifier, and presence of cats and dogs contributed independently to Alternaria levels. Our results suggest that antigens of Alternaria may deposit in house dust from various sources and via multiple mechanisms.
Regional factors were strongly associated with Alternaria
levels. Homes in non-metropolitan statistical areas had significantly higher Alternaria
antigen concentrations than homes in metropolitan statistical areas. Alternaria alternata
is a ubiquitous saprophyte that is found in the soil and on plants, especially on decaying vegetation.12, 30,31
antigen levels likely reflect the abundance of local vegetation. The regional variation in Alternaria
antigen concentrations was also substantial; antigen concentrations were clearly higher in Midwestern and Southern homes. Region is considered as an important determinant for fungal concentrations, both indoors and outdoors.32
Our results are consistent with previous findings that Alternaria
spores are plentiful in grain-growing areas in the Midwest.6
Alternaria antigen concentrations were significantly higher in older homes than in newer homes. Because new homes are more tightly built than older ones, fewer spores may be able to penetrate to indoor environments. Furthermore, newer homes are more likely to be better equipped to provide improved control of environmental factors (e.g. temperature, humidity) indoors. Of the demographic factors, race and poverty had the greatest influence on Alternaria levels, both white race and low income contributing consistently to higher Alternaria levels in U.S. homes.
Mold and moisture related problems in the home were strongly associated with higher Alternaria
antigen levels. Although outdoor air is considered the dominant source for indoor fungal spores, indoor sources may increase Alternaria
antigen levels considerably.15,16
Fungal growth indoors is influenced by various environmental factors. However, the most important factor controlling fungal growth is water availability.33
Water leaks, defective drainage, inadequate ventilation and moisture condensation resulting from faulty thermal insulation, and heating, cooling and ventilation (HVAC) systems have been major contributors to moisture related fungal problems in buildings.34,35
Prolonged high relative humidity has been shown to increase both dust- and airborne fungal populations in indoor environments.36-38 Alternaria
antigen concentrations were significantly higher in homes that used dehumidifiers. The presence of a dehumidifier is likely an indication of an ongoing humidity or moisture problem, although dehumidifiers may also become reservoirs for fungi. In this study, increased humidity per se was not associated with Alternaria
antigen levels (data not shown). However, this is not necessarily surprising because relative humidity was measured at only one point in time. Dehumidifier use may reflect long-term humidity levels or water availability better than a single relative humidity measurement.
Outdoor allergenic spores such as those of Alternaria
can be carried on residents' hair, skin, clothing, or shoes as well as on their pets' fur. Chew and coworkers found that the presence of a dog increased fungal populations in floor dust.39
Our results agree with their findings. The presence of dogs was associated with higher Alternaria
concentrations especially in living room floor and upholstery dusts. Dogs may spend more time in living rooms than in other rooms in the home. In our data, the presence of cats was also associated with higher Alternaria
Predictors of Alternaria
antigen concentrations may vary by location because the activities of occupants, humans and pets, can affect each location in the home differently. For example, having carpeting in bedrooms predicted lower Alternaria
antigen concentrations in beds, whereas kitchens with carpeting had significantly higher Alternaria
antigen levels than kitchens without carpeting. Carpeting in the kitchen may provide favorable environment for fungal growth because of availability of nutrients (e.g. food and beverage residues). Frequent cooking may also result in higher temperature and humidity levels in kitchens. While carpeting may reduce particle resuspension, possibly explaining the reduced levels in the beds of carpeted bedrooms, it can be a reservoir or an amplification site for fungi.19, 39
Furthermore, the presence of children predicted higher Alternaria
antigens levels in beds (p=0.015) but not in floor or upholstery dusts (data not shown). Children are more likely exposed to potential outdoor sources of the fungus than adults, because they usually spend more time outdoors than adults.40
Spores and fungal fragments could be carried into bed on children's hair and/or clothing. Although indoor smoking did not reach statistical significance in the final model for the house index, it was a strong predictor for higher Alternaria
levels in bed dust. Smoking was also significantly (p=0.03) associated with higher Alternaria
antigen levels in living rooms where smoking most likely occurs.41
Less frequent cleaning contributed to higher Alternaria antigen levels in floor and upholstery dusts, especially in living rooms. Correspondingly, levels of Alternaria antigens in beds were lower if bedding was washed more frequently. Washing temperature (cold, warm, hot) did not influence antigen levels (data not shown).
In cross-sectional studies, the temporal sequence of cause and effect cannot necessarily be determined. It is possible that the results from a dust sampling conducted at a single point in time may not represent exposure throughout the entire year because fungal exposure is prone to temporal, particularly seasonal, and spatial variations.37,39
However, settled dust samples are often used as surrogate measures for long-term exposure because they are considered less influenced by temporal and spatial variability than air samples.42,43
Sampling in the study was conducted throughout summer, fall, and winter months in each geographic region in order to capture the seasonal variability in the data. We acknowledge that one limitation of the study is that we cannot determine the variablity in Alternaria
antigen exposure in different geographic regions in the U.S. because of limited number of homes in each region. Although our approach does not allow for assessments of seasonal variability in antigen levels in individual homes, this is the most cost-effective method of sampling for a large-scale national survey that requires in-person home visits. For other common indoor allergens, measurement of allergen concentration in reservoir dust has generally been used as the standard index of exposure. The presence of missing values is another limitation of the study. Insufficient amount of dust to assay Alternaria alternata
antigens contributed most to the missing values. We maximized the number of samples in our analyses by using imputed values for the samples that had concentrations less than the detection limit. To evaluate potential bias, we conducted our analyses excluding the imputed values. The final prediction model remained the same in both analyses.
Although monoclonal antibody-based assays are more sensitive and specific for a single allergenic protein, for example for Alt a 1, an important advantage of a polyclonal assay is that it captures the allergen variability that is characteristic to fungal allergen exposure. Allergenic fungi, including Alternaria alternata
, express great variability in allergen profiles depending on the environmental conditions under which they grow.24, 29
For example, Barnes et al.24
observed substantial discrepancies in GP70 and Alt a1 levels over time when these two glycoproteins were measured in air samples. The life cycle of fungi can also affect the different patterns of allergen release; germination has been shown to increase allergen release from Alternaria
We acknowledge that polyclonal antibody-based assays have their disadvantages; polyclonal antibodies are directed only toward antigens recognized by source species, they can be variable in composition and potency, and cross-reactivity is possible.16
Because there is a finite supply of the polyclonal antibody that we used, it may not be possible to reproduce the findings of this study in the future. However, some previous studies that have used monoclonal antibody-based assays have had difficulties to detect Alternaria
allergens in environmental samples,27
even among populations where Alternaria
sensitivity and exposure to Alternaria
spores are known to be common.27,45,46
species -sensitive subjects elicit positive skin test reactions to other Alternaria
not only to Alt a 1.
The major strength of this study is that the survey sample is nationally representative. The weighted characteristics of the survey sample, including distributions of housing characteristics, socioeconomic, and demographic factors, did not differ significantly from nationwide characteristics obtained from other national surveys,28
which strengthens the external validity of our findings. To improve the internal validity, quality assurance was integrated into all components of the study; sampling procedures, data collection and analysis followed detailed, well-defined protocols.28
The NSLAH is the first study to estimate levels of Alternaria
antigens in the U.S. housing stock. The NSLAH data provide valuable new information on exposure to Alternaria
antigens in indoor environments, although further validation of fungal immunoassays is warranted in order to determine clinically relevant exposure levels in the future. Immunoassays that are used to assess fungal allergen exposures have not yet achieved the same reliability as have similar assays for other allergens.21
In conclusion, Alternaria antigens are commonly detected in U.S. homes. Antigen levels are influenced not only by regional and housing characteristics but also by residents' behavior. The age of the housing unit, census region, urbanization, poverty, family race, signs of mold and moisture problems, use of dehumidifier, and presence of cats and dogs can affect Alternaria antigen levels significantly. In addition, the frequency of cleaning activities in the home contributes to Alternaria antigen concentrations. Smoking indoors may also increase Alternaria antigen levels in home environments. Preventing mold and moisture related problems, avoiding indoor smoking, and regular household cleaning could potentially lower Alternaria antigen concentrations in homes.