The Critical Factor for Healthy Indoor Air Quality
A conventional air-conditioning system is not designed to control humidity below 50 percent on a continuous basis. Air conditioners are actually air coolers. Warm air is passed across cooled refrigeration coils that absorb heat from the air and pump it outside. This leaves the air cooler. When moisture condenses on these cooling coils, extra heat is released. It is not surprising that new high-efficiency air conditioners try to reduce the amount of moisture removal. This is accomplished by doing less cooling to a larger volume of air to avoid condensation on the coils. Air-conditioning is controlled by a thermostat that reacts to a demand for cooling. If it is 72 degrees outside and raining, there is not a need for cooling, but there is a significant need for dehumidification.
The study was conducted in Dayton, Ohio. In that climate, there is only a summer demand for cooling. Indoor relative humidities below 50 percent are common throughout much of the rest of the year. The second chart shows the live dust mite counts for the same group of homes. A count of 100 mites per gram of dust is the accepted threshold where a significant reaction to the allergen occurs. Even the short period of time the air-conditioned house is above 100 live mites per gram of dust is sufficient to "dose" the house for a year because the mite allergen remains potent for nine months.
Dust mites and mold are the two most common allergens present in higher humidity climates.
Dust mites live in upholstered furniture, carpets, and mattresses. They feed on shed human and animal skin scale. In order to stay hydrated, dust mites must absorb their moisture needs from the air. If the relative humidity is below 50 percent, dust mites desiccate (dry out). Their appetites increase as the relative humidity climbs. This is significant because their fecal material is the source of the dominant dust mite allergen. Ninety percent of the population that experiences any allergy-based sensitivities react to the dust mite allergen.
Once produced, the allergens settle into fabrics and carpet. The dust mite allergen remains potent for approximately nine months. Individuals come into contact with the allergen after some activity like vacuuming, dusting, or making beds has "stirred-up" the area and caused the allergen to become airborne. Due to its size and shape, the allergen remains airborne for only a short period of time. For this reason, air duct cleaning and air filtration devices are ineffective methods of combating the dust mite allergen. Vigorous vacuum cleaning and use of acaricides can offer some short-term reduction in allergen levels. According to studies conducted at Wright State University, dust mite infestations will be eliminated if the relative humidity is consistently maintained below 50 percent.
Molds are fungi. They only require high humidity (70 percent or higher) and an organic-based material on which to feed. It is no surprise that they are plentiful in humid climates. Cooler surfaces create a perfect breeding area for mold because the relative humidity next to them will be near saturation or the dew point. The surface may even be wet with condensation.
The most common molds are the species Afternaria, Epicoccum, Penicillium, and Cladosporium. Individuals may react to the mold's spores or to mycotoxins (a toxic gas) released by the mold. Unlike the dust mite allergen, mold spores remain airborne for significant periods of time and are always present, to some degree, in outdoor air. Most mold spores measure 5 to 10 microns in size and can be filtered out of the air with the proper efficiency filter media. Mold growth can be eliminated in the home through humidity control that maintains reduced relative humidity levels and prevents cooler surfaces from becoming damp.
Carbon
Monoxide
Dust Mites
Filtration
Air Leaks
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