Impacts of Building Dampness on Indoor Air Quality
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 [16]. 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 [18-21]. 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 [22], 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 [22]. 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 [17]. 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 14 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 [19-21, 23-26]. 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% [27]. One other study found that the concentrations of only one of the two mite allergens increased as relative humidity increased [20]. Overall; however, 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 [27-29] with increased ventilation while other studies reported significant reduction in mites or mite allergen [25, 26, 30]. The benefits of dehumidification systems have also been evaluated in a few studies. In six English houses, Custovic [31] 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 [24]. 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% [32].
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.
Table 3. Summary information from studies of how home humidity levels and humidity reduction measures affect house dust mite concentrations.
| First Author Date | Subjects | Methods | Interventions or Measures Evaluated | Major Findings |
|---|---|---|---|---|
| Arlian 1999 [19] | Controlled laboratory study, mite levels in culture chambers measured with various excursions in RH to 75% or 85% with base humidity of 0 - 35% maintained 16 to 22 hr/day | NA | Relative to mite growth with constant 75% RH, growth was reduced 97 - 98% with 35% base humidity and excursions to 75% RH on 4 - 8 hours per day. | |
| Arlian 2001 [32] | 66 US homes divided into three groups based on humidity levels | Measured and compared RH, dust mite and dust mite allergen levels in the three groups of homes | Group 1 used air conditioning and dehumidifiers to maintain RH < 51% (avg. ~ 46%) Group 2 used air conditioning, with dehumidifiers in a few homes, but RH not maintained < 51% (avg. ~ 57%). Group 3 used windows to control climate, RH > 51% (avg. ~ 60%) |
Large (98%) significant* reductions in mite levels and mite allergen levels (76%) after 17 months in Group 1 homes, but not in other homes; after 17 months, allergen levels were 10 times lower in Group 1 homes. |
| Cabrera 1995 [24] | 10 homes in Canary Islands | Measured mite allergen levels in mattress dust from closed bedrooms with and without dehumidifiers | Dehumidifier in bedroom set to maintain RH of 50% | Mean significant* reduction of 78% in two mite allergens. No significant reduction in a third mite allergen. |
| Crane 1998 [28] | 10 New Zealand homes | Intervention to reduce humidity in 10 houses; 20 control houses; measured humidity and mite levels and mite allergen levels before and after intervention | Mechanical ventilation with heat recovery; addition of a 3.4 kW space heater, addition of building insulation, draught-proofing | Relative humidity was approximately 10% to 20% lower in intervention houses but was still often near to or above 50%, there was no significant* effect of the intervention on mites or mite allergen levels. |
| Custovic 1995 [31] | 12 houses in England, 6 houses with and 6 houses without a portable dehumidifier | Monitored temperature, RH, mite counts, and mite allergen levels before and after the intervention in both intervention and control homes | Portable dehumidifiers | Humidity was not significantly* lower in the homes with dehumidifiers (57.9%) compared to homes without dehumidifiers (58.3%); mite counts did not change significantly*; mite allergen levels decreased significantly in both groups of houses, possibly because of the dust sampling performed for allergen measurements, but allergen levels were not significantly* lower in the houses with dehumidifiers. |
| Fletcher 1996 [29] | Homes of 18 mite sensitive asthmatic subjects in UK | Measured mite and mite allergen levels, temperature and RH and compared findings between intervention and control homes | Mechanical ventilation with heat recovery in 9 houses, 9 matched control houses without mechanical ventilation | Indoor RH levels in mechanically ventilated houses, compared to control houses, were significantly* lower in autumn and winter but mean RH levels remained above 50%; mite levels and mite allergen were not significantly* reduced. |
| Gross 2000 [20] | 405 houses in Germany | Cross sectional study and multivariate statistical analyses of data | NA | The concentrations of one mite allergen (Der p 1) increased significantly* with indoor RH (1.03 per 1% RH) while the concentrations of another mite allergen (Der f 1) were not correlated with humidity. |
| Harving 1993 [21] | Homes of 96 asthmatics in Denmark | Cross sectional survey, with humidity and mite concentration measurements and statistical analyses | NA | A significant* positive correlation was found between higher humidity and higher mite concentration in mattress dust; a significant* correlation was found between higher air exchange rate and reduced mite concentration in mattress dust. |
| Harving 1994 [25] | 53 asthmatic subjects in Denmark | 30 subjects moved to homes with mechanical ventilation, mite levels measured before and after subjects moved and also in homes of subjects who did not move; air exchange rates and humidity also measured | Move to home with mechanical ventilation and heat recovery | Large significant* increase in air exchange rate from 0.4 to 1.52 ach in study group with no change in control group; statistically significant but modest reduction in absolute humidity from 0.0066 lb moisture /lb air (42% RH at 70 °F) to 0.0056 - 0.0063 lb moisture/lb air (36% - 41% RH at 70 °F) in study group with no change in control group; large and significant* decrease in mite levels from 110 to 20-35 mites per gram of mattress dust; mites levels increased non-significantly* from 105 to 210 mites per gram of dust in the control group. |
| Harving 1994 [33] | 14 asthmatic mite-allergic subjects in study group moved to homes with mechanical ventilation; 11 mite-allergic subjects did not move | Before and after measurements of lung function, medicine use, bronchial hyper-responsiveness | Move to home with mechanical ventilation and heat recovery | Significant* improvements in forced expiratory volume, peak expiratory flows, medicine score, symptom score, serum IgE in study group relative to control group; insignificant* improvement in histamine provocation concentration and blood eosinophils in study group. |
| Korsgaard 1983 [34] | 46 mite-allergic asthma patients in Denmark | Intervention performed in 23 study-group houses but not in 23 control-group houses; humidity, dust mite levels and various health outcomes were measured in both sets of houses | Increased opening of windows to reduce humidity plus replacement of bedding, removal of carpets, increased cleaning | Indoor humidity measured in the evening was reduced modestly and significantly* in study-group homes relative to control homes from above 50% RH to slightly below 50% RH, but morning RH was not significantly* reduced. Mite levels were significantly* reduced on bedroom floors but not in bedding. Asthma symptom scores improved markedly and significantly* in the study group with smaller insignificant improvements in the control group. Asthma medication use improved in both groups but more in the study group. Peak expiratory flow was not improved. |
| Niven 1999 [27] | 10 houses in UK retrofitted compared to 10 control houses, allergens and humidity measured | Measured and compared RH and dust mite allergen | Mechanical ventilation with heat recovery with integral dehumidification | Allergen levels fell in both groups of houses with no significant* advantage evident from the intervention, despite the fact that winter-average humidity was 37% in intervention house bedrooms and 50% in control house bedrooms. |
| Warner 2000 [26] | Asthmatics sensitive to house dust mites in 40 homes in United Kingdom | Measured absolute humidity, mite levels, symptoms, bronchial hyper-responsiveness, and lung function before and after interventions | Group 1: mechanical ventilation and high efficiency vacuum Group 2: mechanical ventilation Group 3: high efficiency vacuum cleaner Group 4: no intervention |
Significant* reduction in humidity, mite levels, and mite allergen concentrations in homes with mechanical ventilation; no significant* improvement in symptoms or lung function; trend for improvement in histamine needed to provoke 20% reduction in forced expiratory volume in mechanically ventilated homes but trend was not significant*. |
| Wickman 1994 [35] | 70 houses in Stockholm | Cross sectional survey with building characterization, humidity measurement, measurement of mite allergen in mattress dust; data were analyzed statistically | Mechanical supply and exhaust ventilation | Significantly* smaller percentage of houses with mechanical supply and exhaust ventilation had an indoor absolute humidity above 0.007 lb water per lb air when compared to control houses; not having mechanical supply and exhaust ventilation was a significant* risk factor for dust mite allergen levels in mattress dust above the median measured value. |
*significant = statistically significant, i.e., the reported findings have a 5% or smaller probability of being chance findings
Non-microbial Chemicals
Dampness can increase the emissions of non-microbial gaseous chemicals into the indoor air [1]. 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.