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 [35]

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

Bemt 2006 [23]

153 bedrooms in The Netherlands

measured bedroom RH and temperature and dust mite allergen levels in mattress dust

assessed associations of dust mite allergen levels with bedroom RH and type of mattress

significantly* higher levels of mite allergens in bedrooms with RH > 50% and in mattresses with synthetic, compared to cotton, upper layer (upper layer is part of the mattress, not the sheet)

Cabrera 1995 [26]

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 [31]

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 [34]

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. 

Ellingson 1995 [22]

38 homes in Colorado, half with central evaporative coolers

RH and levels of dust mite allergen monitored in May and August, in homes with and without evaporative coolers.

presence versus absence of central evaporative cooling

in May, when evaporative coolers are not used, dust mite allergen levels were low in both groups of houses; in August, when evaporative coolers are used, mean dust mite allergen levels were 60 times higher in homes with evaporative coolers and the difference was statistically significant*; average RH was 43% in homes without evaporative coolers and 59% in homes with evaporative coolers. .

Fletcher 1996 [32]

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 [27]

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 oF) to 0.0056 – 0.0063 lb moisture/lb-air (36% - 41% RH at 70 oF) 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 [36]

14 asthmatic mite-allergic subjects in study group moved to homes with mechanical ventilation; 11 mite-allergic subjects did not move

before 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 [37]

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 [29]

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

Prasad 2009 [38]

109 patients age 1 to 42 from dry desert area, 31% with evaporative coolers in their home

sensitization to allergens determined via skin prick tests

presence versus absence of evaporative cooling

42% of patients  from homes with evaporative cooling were sensitized to at least one mold allergen, compared to 19% from homes without evaporative coolers; 34 % of patients from homes with evaporative cooling were sensitized to at least dust mite allergen, compared to 18% from homes without evaporative cooling; both findings were statistically significant*

Vanlaar 2001 [30]

50 houses in a dry inland area of Australia

measured dust mite allergen in bedding and on floors

presence versus absence of evaporative cooling

in homes with evaporative coolers, dust mite allergen levels were 3.3 fold higher in beds and 3.9 fold higher on floors and the increases were statistically significant*

Warner 2000 [28]

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 [39]

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