Allergy Relief in Humid Climates

Dust mites are a problem in new homes in humid climates. What keeps them away?

March 01, 2002
March/April 2002
A version of this article appears in the March/April 2002 issue of Home Energy Magazine.
Click here to read more articles about Indoor Air Quality

        The increasing incidence of allergy and asthma, particularly among children, is a worldwide concern. About 20% of the general population suffer from allergies, and about 5% suffer from asthma.Among children the percentage of allergy sufferers is even higher. Since most of us stay indoors more than 90% of the time, the air quality in buildings is of paramount importance in maintaining respiratory health.
        Energy-efficient buildings with tight building envelopes may make for poor indoor air quality—yet allergy-resistant houses must be airtight,well insulated, and well ventilated. One cannot control and filter the ventilation air unless the house is airtight, nor can one maintain a positive pressure. Cold spots and cold floors may be present in houses that are poorly insulated. High local relative humidity (RH) hastens the formation of molds on these cold surfaces and is conducive to the breeding and proliferation of dust mites. So energy-efficient construction is a prerequisite for fighting dust mites, but it is not sufficient in and of itself. 

Allergens and Irritants

        The major allergens found in homes today are environmental tobacco smoke and dust mite allergens, the focus of this article. Other house allergen sources and irritants include pollen, pet dander, fungi and molds, cockroaches, endotoxins (from bacteria), volatile organic compounds, fine mineral dust, and radon. (For more general information on fighting allergens, see gCreating Airtight and Healthy Homes,hHE Nov/Dec f01, p. 18.)
        Environmental tobacco smoke. About 25% of North American adults smoke today. Sidestream and exhaled smoke (environmental tobacco smoke, or ETS) is a major source of breathable particles indoors.and of various carcinogens, other toxic compounds, and odors as well. Some air-filtering and -purifying devices can clear up ETS somewhat. High levels of localized ventilation may also help.However, the most practical solution seems to be banishing the smokers to the outside of the house. Building science is probably of limited help in alleviating ETS.
        Dust mites. In humid climates, dust mites are a leading cause of asthma. In the hot, humid central Florida climate, approximately 60% of patients complaining of respiratory problems tested positive to a dust mite skin prick test.rates 2 to 3 times greater than the rates for any other allergen. In a study of North Carolina schoolchildren, 30% tested positive for house dust mite allergy.
        Dust mites feed on human skin flakes and are about 300 micrometers (mm) in size.There are two types of dust mite allergens.Type 1 allergens, the most common form, come from dust mite fecal particles.Type II allergens come from dust mite body parts.The fecal particles are 5.10 mm in size and heavier than air.When airborne, they settle in about five minutes. Dust mites live and breed on carpets (see gWhich Carpets Do Dust Mites Like Best?h p. 32), sofas, beds, pillows, and other soft and fluffy furnishings.A person lying down on a bed or walking across a carpet can release puffs of dust containing dust mite fecal particles.These particles are easily breathed in before they can be filtered out using a room or whole-house air filter. Dust mite allergen concentrations of 2 micrograms (ƒÊg) per gram of dust or higher can become a health problem for house occupants.
        Current recommended actions to reduce allergen exposure in the home include encasing mattresses, pillows, and box springs;washing bedding weekly in water hotter than 130oF; and the regular use of vacuum cleaners that have allergen trapping features or that are ducted to the outside (central vacuums).Another current recommendation is to remove carpets from houses in humid climates.
        Laboratory studies of dust mites indicate that their bodies will dehydrate in an environment with RH below 58% at a temperature of 77oF, or RH below 52% at 59oF.While dehydration does not necessarily lead to death, it reduces fecal and allergen production.Along with thorough vacuuming, dehumidification can help to control dust mites and dust mite allergens.

A Study of New and Existing Homes

        Since many consumers living in humid climates prefer carpeting in their new houses,we performed a study to determine the best way to eliminate dust mite allergens in new houses with wall-to-wall carpets and in humid climates.We studied five pairs of homes, all with central cooling and heating systems (see Table 1).
        The homes were in four cities in the southeastern United States (Orlando, Florida; Jacksonville, Florida; New Orleans, Louisiana; and Hunstville, Alabama). Each home pair consisted of a new home with a central dehumidifier and an older existing home without a central dehumidifier. The existing homes were located in the same city as the new homes and were used for comparison purposes.
        Each home was monitored on average for 12 months (minimum 8 months, maximum 16 months). Interior temperatures and RH were measured at the carpet level by small battery-powered data loggers, which recorded the instantaneous data every hour. Dust samples were collected in each home at a minimum of four locations.master bedroom mattress, master bedroom carpet in front of the bed, family room sofa, and the family room carpet in front of the sofa.
        Dust mite allergens in the air are hard to measure as they quickly settle down. Since mites live near surfaces,we sampled surface dust monthly by vacuuming an area of approximately 11 ft2 for one minute. Dust samples were collected with a portable vacuum cleaner whose inlet was modified to accept a standard paper coffee filter, which collected the dust. Students collected dust samples in all the Orlando area homes. Homeowners, who were given a similar set of dust collection equipment and instructed in its use, collected dust samples in the rest of the homes.                 Existing homes. The summertime average temperatures for the five existing homes were below or near 77oF. The average RH for the existing homes during the summer months (second and third quarters of every year) was below or near 58%. During the winter months (first and fourth quarters), the central air conditioners, which dehumidify indoor air,were not used much. Due to occupant activities and infiltration of outside humid air, the average carpet level RH for all the existing homes during the winter quarters was higher than it was during the summer quarters, while the average carpet level temperatures went down during the winter.This promotes dust mite growth in the winter months in the southeastern United States.
        As expected, the allergen levels of the existing homes were high, with one 3 (see Figure 1). For the purpose of our study,we defined thorough vacuuming as vacuuming at least twice a week with an allergentrapping dust bag, or once a week with a central vacuum cleaner exhausted to the outside of the conditioned living area. Existing home 1 had a central vacuum cleaner, which was used regularly. Despite not having a central dehumidifier, this house had low allergens for the first three quarters of the year. It was only in the last quarter of sampling, after a period of prolonged high humidity, that the maximum allergen level in the living room carpet exceeded the threshold value of 2 ƒÊg per gram of dust.
        Existing home 2 had an allergentrapping vacuum cleaner, but it was not used often. Consequently, the house had high allergen levels. Existing houses 4 and 5 had standard vacuum cleaners, and, despite frequent vacuuming, had significant allergen levels. In existing home 3, a housekeeper vacuumed two or three times a week with an allergentrapping vacuum cleaner.This house never had measurable allergens.
        New homes. The yearly average measured RH values for the new homes were lower than 58%, although some days the RH did exceed 58%.The central dehumidification systems performed well in controlling the winter indoor humidity.The average temperatures were comparable to those in the existing homes.
        New home 1 received a regular cleaning with a standard vacuum and, despite RH control, had a significant number of dust mite allergens (see Figure 2).We discovered studying data from new home 1 that new homes could be infested with dust mites in a few weeks. This is also reported in the literature. The fact that this new house had high allergen levels is also consistent with the results of other studies that found that RH control alone was ineffective in controlling dust mite allergens.
        New homes 2, 4, and 5 all had wallto- wall carpets and they all received thorough vacuum cleaning.The highest measured type 1 allergen level at any location in these three homes was only 1.04 ìg per gram of dust.Although new home 3 did not have wall-to-wall carpeting, just area rugs, it had allergen levels occasionally exceeding the threshold value.The homeowners used a standard vacuum cleaner.

How to Maintain an Allergen-Resistant House

        Washing bed sheets and clothing in hot water (at least 130° F) and covering mattresses, box springs, and pillows with impermeable covers can reduce exposure to dust mite allergens, as explained above. So can removing carpets from homes in humid climates. But our study showed that, even in humid climates, thorough vacuuming and a central dehumidification system installed in conjunction with central cooling and heating can maintain dust mite allergens below clinically significant levels in new homes with wall-to-wall carpeting.
        Whole-house dehumidification that runs independently of the central heating and cooling is recommended to control mold, mildew, endotoxins, and dust mites. Independent whole-house dehumidification is especially important in the months when the weather is humid, the temperature is mild, and the cooling or heating system does not operate frequently. Room dehumidifiers are unlikely to control humidity throughout the house, although they might benefit single rooms.
        Our research showed that it is difficult to maintain RH below 58% at the floor level in humid climates at all times, even if the average air RH level is maintained at or below that level with whole-house central dehumidifiers.However,we still consider this to be a very promising strategy, especially when combined with thorough vacuum cleaning.

Looking to the Future

        The new houses studied (average floor area is 3,180 ft2) are larger than typical new U.S. homes (average floor area is 2,225 ft2). Census data indicates that in 1997, 15% of new home construction consisted of houses with 3,000 ft2 or more of conditioned area.The number of occupants of new houses in our study did not exceed four per house. Future studies should look at smaller new and existing homes with higher occupant densities, lower-cost central dehumidification systems, and allergen trapping-vacuum cleaners to determine the range of housing types that can benefit from this promising dust mite control strategy. Future studies should also quantify the benefit of thorough vacuum cleaning alone in controlling dust mite allergens.

Subrato Chandra is a program director and David Beal is a research analyst at the Florida Solar Energy Center of the University of Central Florida.

This research was sponsored by the U.S. Department of Energy, Office of Building Technology, State and Community Programs.

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