Studies

Asthma: Exposure Assessment and Prevention

• Introduction
• National allergen survey
• Allergen mitigation strategies
• Clinical asthma research program

Introduction

The asthma research program involves both exposure assessment and prevention components, and focuses on the relationship between exposure to indoor allergens/endotoxins and asthma/allergy prevalence/morbidity (Figure 1).

Figure 1: Effects of genetic predisposition and environmental factors on asthma

National allergen survey

The National Survey of Lead and Allergens in Housing, a descriptive study of allergen types and levels in floor and bedding dust in the nation's housing, is the first study to provide estimates of allergen exposure in the U.S. population. The survey includes 2456 individuals living in 831 homes located in 75 different areas throughout the country (Vojta et al., Env. Health Perspect., 2002 (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12003758&dopt=Abstract) ; Jacobs et al., Env. Health Perspect., 2002 (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12361941&dopt=Abstract) ). The major endpoints are measurements of dust mite, cockroach, cat, dog, mouse and fungal allergen levels, as well as endotoxin in dust samples collected from an array of sites in the surveyed homes.

Results from analysis of dust mite allergens suggest that approximately 24% of U.S. homes have beds that contain >10 micrograms of dust mite allergen per gram of dust, a level previously associated with symptomatic asthma, and approximately 46% of U.S. homes have beds that contain >2 micrograms of dust mite allergen per gram of dust, a level previously associated with mite allergen sensitization, (Arbes et al., J. Allergy Clin. Immunol., 2003 (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12897740&dopt=Abstract) ). The likelihood of having a high dust mite allergen level in the bed was greater for older homes, non-western region homes, lower income households and bedrooms with higher humidity.

Detectable cockroach allergen is present in approximately 6% of U.S. beds (Cohn et al., Env. Health Perspect., 2006). (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=15208600&query_hl=4&itool=pubmed_docsum) The likelihood of having a high bed cockroach allergen level was greater for lower income households, homes with evidence of moisture and homes with evidence of food debris. 

Dog and cat allergens were universally present in U.S. homes (Arbes et al., J. Allergy Clin. Immunol., 2004 (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=15241352&query_hl=102&itool=pubmed_docsum) ). Homes with an indoor dog or cat had much higher allergen concentrations than homes without an indoor dog or cat, but even in homes without indoor pets, allergen levels exceeded the clinically relevant sensitization threshold of 1-2 micrograms per gram of dust. Independent predictors of elevated allergen concentrations in homes without pets were all demographic variables that linked to a higher prevalence of pet ownership. 

Researchers detected Alternaria allergens in >95% of homes (Salo et al., J. Allergy Clinical Immunol., 2005 (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16159634&query_hl=105&itool=pubmed_docsum) ). The age of the housing unit, geographic region, urbanization, poverty, family race, observed mold and moisture problems, use of dehumidifier, and presence of cats and dogs were independent predictors of homes with high Alternaria allergen concentrations. 

Eighty-two percent of U.S. households contain mouse allergen (Cohn et al., J. Allergy Clin. Immunol., 2004 (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=15208600&query_hl=108&itool=pubmed_docsum) ). In about one in five homes, the amount of mouse allergen on the kitchen floor is high enough to be associated with allergies and asthma.  Residents of high rise apartments and mobile homes are at greatest risk, but the allergen is also found in a large number of other types of homes.

All homes contain endotoxin with the highest levels present on kitchen and family room floors (Thorne et al., Am. J. Resp. Crit. Care Med., 2005 (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16141442&query_hl=110&itool=pubmed_docsum) ). There was a significant relationship between increasing endotoxin levels and doctor-diagnosed asthma, asthma symptoms in the past year, current use of asthma medications and wheezing among residents of the homes. Thus, household endotoxin exposure is a risk factor for asthma.

This nationally representative survey has:

• Provided current estimates of indoor allergen exposure in the U.S. population
• Established a database that can be used to stimulate future studies which attempt to correlate allergen exposure to disease outcome
• Provided a baseline that can be used as a reference point for future allergen surveys
• Facilitated evaluation of regional, ethnic, socioeconomic, and housing characteristic differences in the allergen burden

Allergen mitigation strategies

Early childhood exposure to indoor allergens appears to be an important risk factor for allergic sensitization and the development of asthma. Asthma prevalence and morbidity rates are highest among individuals of low socioeconomic status living in inner-city areas. Therefore, the group conducted studies to test the feasibility and effectiveness of various interventions to reduce indoor allergen levels in inner-city homes.

Since most of the available data suggests that sensitization to dust mite and cockroach allergens are important asthma risk factors and since the best methods to reduce these allergens have not been established, the group focused its initial efforts on the development of effective dust mite and cockroach allergen mitigation strategies.

For dust mite mitigation, research indicates that placement of impermeable mattress/pillow covers together with either home or professional laundry of bedding leads to a significant reduction of mite allergens in the bed (Vojta et al., Env. Health Perspect., 2001 (http://ehpnet1.niehs.nih.gov/docs/2001/109p815-819vojta/abstract.html) ). Both intensive vacuuming and steam cleaning result in a significant reduction in mite allergens in carpet; however, the steam cleaning effect persists for at least eight weeks whereas the intensive vacuuming effect is transient.

For cockroach mitigation, studies suggest that intensive cockroach extermination and cleaning significantly reduces cockroach allergen levels in cockroach infested inner-city homes (Arbes et al., J. Allergy Clin. Immunol., 2003 (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12897740&dopt=Abstract) ). Applying a novel spatial analysis method to determine cockroach and cockroach allergen distribution in homes leads to better insecticide placement. Improved cleaning leads to improved allergen mitigation. More recently, the group has shown that cockroach extermination alone (without cleaning) can effectively reduce cockroach allergen levels in low-income homes and maintain low levels of cockroach allergen for up to one year (Arbes et al., J. Allergy Clin. Immunol., 2003 (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12897740&dopt=Abstract) ; Arbes et al., J. Allergy Clin. Immunol., 2004 (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=14713915&query_hl=112&itool=pubmed_docsum) ).

Clinical asthma research program

Additional information about the group's clinical asthma research program appears at Asthma Research at NIEHS (http://www.niehs.nih.gov/health/topics/conditions/asthma/allergens.cfm).

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