Mold and Bacteria
Some mold and bacteria are present in the air of all buildings. Damp buildings may support the growth of mold and bacteria on indoor surfaces, increasing the levels of mold, bacteria, and their by-products in indoor air. Mold and bacteria can release microscopic particles into the air. The particles released include spores — the reproductive agents produced by molds and by some bacteria. In addition, fragments of molds and bacteria, often much smaller than spores, may be released into the air, potentially in much higher numbers than the spores . These particles may settle on indoor surfaces and then be resuspended by human activities or by air movement. When the particles are airborne they may be inhaled. The particles may contain allergens, chemicals that can cause inflammation in respiratory tissues, or toxic chemicals with the theoretical potential to cause other health effects such as inhibited immune system function or effects on the central nervous system [1, 17]. Microorganisms on indoor surfaces can also release microbial volatile organic compounds (microbial VOCs or MVOCs) into the air. These compounds are the source of the mold odor present in damp and moldy buildings, but their effects on health, if any, are not well understood.
House Dust Mites
The level of indoor air humidity affects the indoor levels of house dust mites and house dust mite allergens [18-23]. These mites are microscopic arthropods (a family of related animals) that live indoors and feed on skin flakes and other organic materials in dust. Bedding, upholstered furniture, and carpets are among the sites where dust mites live. The allergens, present primarily within the mite's fecal pellets or their fragments , may be inhaled when airborne. Most of the particles containing allergens are large enough to settle quickly, thus, airborne mite allergen levels are much higher when indoor surfaces have been recently disturbed . Dust mites do not drink water; they absorb water from the surrounding air. If the relative humidity of the air is maintained below approximately 50% for an extended period, mites will not survive. As humidity increases mite levels tend to increase [19, 20]. However, it is the relative humidity in the microenvironment around the mites, not the average humidity of indoor air, that affects dust mite levels and the relative humidity where mites live can be higher than the average indoor humidity. For example, the relative humidity near a carpeted floor located over a cold crawl space will be higher than the relative humidity in the air at the center of a room.
Dust mites may be absent from homes in high altitude dry environments and in the winter where it is very cold and dry outdoors . Several studies have investigated how indoor air humidity affects dust mite levels or whether mite levels can be reduced or health improved by increasing rates of outdoor air ventilation or use of portable dehumidifiers to reduce indoor humidity. Table 3 summarizes the key features and findings of 17 of these studies.
Almost all studies reviewed have reported that levels of mites or levels of dust mite allergens in dust samples increase as the indoor air humidity increases [19-23, 25-28]. One study did find that mite allergen levels in 10 bedrooms with relative humidity maintained at an average of 37% were not statistically significantly lower than allergen levels in a set of control bedrooms with an average relative humidity of 50% . One other study found that the concentrations of only one of the two mite allergens increased as relative humidity increased . A study of houses in a semi-arid high-altitude section of Colorado  showed that summer indoor humidity was higher, and dust mite allergen levels were much higher, in homes with central evaporative coolers, sometimes called swamp coolers, which increase indoor humidity. A study in Australia also reported that dust mite allergen levels were three-fold to four-fold higher in homes with evaporative coolers . Overall, the evidence of increased dust mites with increased humidity is compelling.
The evidence of related IAQ benefits from increasing outdoor air ventilation rates to reduce humidity is mixed. Some studies reported no statistically significant and substantial magnitude reductions in mite or mite allergen levels [29, 31, 32] with increased ventilation while other studies reported significant reduction in mites or mite allergen [27, 28, 33]. The benefits of dehumidification systems have also been evaluated in a few studies. In six English houses, Custovic  found that portable dehumidifiers were ineffective in reducing humidity or mite levels. Dehumidifiers in closed bedrooms of 10 Canary-Island homes reduced the concentrations of two mite allergens on average by 78% but there was no statistically significant reduction in a third mite allergen . A study performed in the U.S. found that mite and mite allergen levels fell 98% and 78% respectively when dehumidifiers together with air conditioning were effective in maintaining relative humidity below 51% (average 46%) but not when the systems were ineffective in maintaining humidity below 51% .
In summary, research has established that increased indoor humidity levels are associated with increased indoor dust mite allergen levels. However, the results of trials of using increased ventilation or dehumidifiers to reduce dust mites or dust mite allergens are mixed. It seems likely that these systems will substantially reduce mite allergens in homes and climates where humidity can be maintained below 50% with only limited term excursions to higher humidity levels.
Dampness can increase the emissions of non-microbial gaseous chemicals into the indoor air . The rate of release of formaldehyde from manufactured wood products containing urea-formaldehyde resins (glues) increases with humidity. Alcohols and degradation products from the softening agents used in many plastics can also be released when floor products containing polyvinyl chloride (PVC) are underlain by damp concrete.