Environmental control of indoor biologic agents

Abstract

Relative humidity The indoor environment contains unique pollutants, such as environmental tobacco smoke and indoor allergens. In addition, air pollutants may reach higher levels indoors than outdoors, and in some instances may exceed the national standards for outdoor exposure. Nonindustrial indoor settings, such as offices, residences, and vehicles, also have air quality problems. Although the role of occupational air pollutants (organic and inorganic) in causing complaints in the workplace has been clearly documented, the relationship between poor air quality and health complaints is often controversial. The main causes of poor indoor air quality are an inadequate ventilation or fresh air intake, biologic contamination of the indoor environment, and an accumulation of noxious compounds. A list of important indoor air pollutants is shown in Table I. TABLE IMajor indoor air pollutantsRespirable particlesNO, NO2COCO2Biologic agents (indoor allergens)Domestic mitesFurry pets (cats, dogs, rabbits, gerbils, mice, rats, hamsters, guinea pigs, and others)BirdsInsects (cockroaches, fleas)Molds (outdoor and indoor molds) and bacteria in air-conditioning unitsFormaldehydeRadon and radon daughtersVolatile organic compoundsSemivolatile organic compoundsAsbestos Open table in a new tab Because indoor allergen concentrations are higher in homes than in public places, a greater effort should be made to control allergens at home. A reason for this difference may be the more aggressive chemical treatment of carpets in public places. However, in some instances high mite, cat, and dog allergen levels have been detected in schools and day care centers. These findings have important implications for families who are practicing allergen avoidance at home because the effort and time spent controlling allergen exposure could be jeopardized by exposure in the school environment.[1]Warner JA Environmental allergen exposure in homes and schools.Clin Exp Allergy. 1992; 22: 1044-1045Crossref PubMed Scopus (22) Google Scholar Studies conducted in schools have demonstrated that carpets contain high levels of a variety of allergens including pollen, cat, dog, food, mite, and mold allergens.[2]Dybendal T Hetland T Vik H Apold J Elsayed S Dust from carpeted and smooth floors. I. Comparative measurements of antigenic and allergenic proteins in dust vacuumed from carpeted and non-carpeted classrooms in Norwegian schools.Clin Exp Allergy. 1989; 19: 217-224Crossref PubMed Scopus (31) Google Scholar Carpets in schools and homes are an important reservoir of allergens. The control measures discussed in this review can be applied, with minor modifications, to both homes and public places. Before strategies of environmental control in ″dirty environments” are designed, several factors, such as dampness and ventilation, should be considered. Studies in North America indicate that dampness in homes is widespread, in the range of 40% to 60%.[3]Brunekreef B Dockery DW Speizer FE Ware JH Spengler JD Ferris BG Home dampness and respiratory morbidity in children.Am Rev Respir Dis. 1989; 140: 1363-1367Crossref PubMed Scopus (292) Google Scholar The association of dampness in the home with respiratory symptoms is substantial, with odds ratios of 1.5 to 2.5. Moisture within homes and the concomitant growth of mold, bacteria, and mites are substantial contributors to infectious and allergic respiratory diseases. Dust mite allergen and endotoxin levels are directly related to indoor humidity. The most effective ways to control microbial growth are shown in Table II. TABLE IIEffective ways to control microbial growth1. Install exhaust fans vented to outdoors, powerful enough to remove moist air from kitchens and bathrooms.2. Vent dryers to the outdoors.3. Ventilate the attic and basement crawl space to reduce excess moisture. A polyethylene ground cover can be placed on top of the soil in the crawl space.4. Clean the basement floor drain frequently with a disinfectant to prevent the buildup of microorganisms.5. Provide extra ventilation if green lumber or wet wood is used for construction.6. Put a vapor retarder under the concrete slab floor and add waterproofing materials to the outside of the foundation if you are building a new house or an addition.7. Use storm windows or double-pane windows to control condensation.8. Repair leaks and eliminate standing water immediately.9. Use dehumidifiers10. Maintain a clean house11. Avoid wall-to-wall carpeting (!) Open table in a new tab Increasing fresh air ventilation through a house is a useful corrective measure. Ventilation is defined as the unaided movement of air into and out of the house. Ventilation rates are determined by differences between indoor and outdoor air temperatures, by differences in temperature within the house, and by wind velocity and direction. As a result, natural ventilation rates will vary widely depending on the weather, the location and orientation of the home, and the positions of windows and doors. Natural ventilation improves indoor air quality, but it has several limitations. It is an inefficient means to dilute contaminants in a home, because air is not uniformly distributed and the flow cannot be controlled. Increased ventilation could also increase pollen and mold spore levels, if outdoor counts are elevated. An alternative to natural ventilation is mechanical ventilation systems, which use electric fans and blowers to pull fresh air inside, to circulate the air through the building or apartment, and to exhaust stale indoor air to the outside. Bathroom and kitchen exhaust fans, air-to-air heat exchangers, and heat pump ventilators are types of mechanical ventilation used to maintain indoor air quality. Ceiling fans simply recirculate indoor air, which reduces dampness caused by condensation but has little effect on the concentration of contaminants. Inadequate ventilation is probably one of the main causes of poor indoor air quality. However, a study has challenged the idea that increasing fresh air circulation could significantly benefit the management of poor indoor air quality.[4]Menzies R Tamblyn R Farant JP Hanley J Nunes F Effect of varying levels of outdoor-air supply on symptoms of sick building syndrome.N Engl J Med. 1993; 328: 321-327Crossref Scopus (124) Google Scholar The efficiency of ventilation is influenced by fresh air availability, fresh air movement and distribution (dilution), the presence of mass and energy pollutants, occupancy factors and filtration and control methods. Most mechanical ventilation systems in large buildings primarily use recirculated air during hot or cold weather. Setting mechanical air handling systems to deliver outside air and opening windows are inexpensive remedial actions in homes but are frequently not achievable in the workplace. A reduction in the air exchange rate results from several ″home improvement” methods. Installation of weather stripping reduces air exchange rates by 1%, caulking of window seals by 3%, the addition of storm windows by 6%, increasing the insulation by 10%, and sealing of air ducts by 23%. The recommended rate exchange of fresh air in homes is 0.33 air exchanges per hour. The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE Standard 62-1989) has established minimum ventilation rates acceptable for residential areas. These include a minimum of 15 cubic feet per minute (cf-min) per person in the living area; 100 cf/min intermittent or 25 cf/min continuous in the kitchen; 50 intermittent or 20 cf/min continuous in the bathroom, and 100 cf/min per car in the garage. The first requirement for achieving environmental control of indoor air is to identify, remove, isolate, and/or replace the source of the pollutants. However, certain sources cannot be easily controlled, particularly when the source is in or on human beings. For example, contagious diseases such as influenza, viral upper respiratory tract infections, chicken pox, and tuberculosis are controlled only by reducing exposure to infected individuals. Source control of indoor air pollution may not be easily achieved, and corrective measures may be costly. For example, removal of a domestic pet from the home of an individual who is allergic to cats has emotional barriers, which make implementation difficult. Removal of carpeting and furniture containing house dust mite or cat allergens is costly. Major expenses may be incurred with the installation or modification of ventilation systems or air-conditioning units. Building owners, managers, or home owners must be convinced that the modification and expenditure is essential and that a substantial health benefit will follow. Medical advice should be obtained to determine the relative contribution of the indoor environment on the affected subject's medical problem. Other factors such as outdoor allergic sensitivities, occupational exposure, cigarette smoking, and medical problems must be identified. Mold and bacterial contamination can be prevented or diminished by limiting the access of outdoor aerosols to the indoor environment. Adequate filtered air for dilution of indoor contaminants should be provided. Ideally, a constantly renewed source of filtered outdoor air should be maintained. Indoor humidity should be maintained below 60%. The intrusion of moisture should be prevented, and the use of water spray devices should be limited. Existing contamination can be reduced by air cleaning or surface decontamination. Electrostatic precipitators and high-efficiency particulate air filters are commonly used filters in houses. The electrostatic precipitators do remove the majority of large particles, but removal of smaller particles is limited. These filters remove intact bacteria and most mold spores but not the metabolic products of these organisms. These filters require regular maintenance and overload rapidly. High-efficiency particulate air filters remove microbial contamination and have a steady state that depends on particulate collection efficiency, the amount of air passing through the filter, and the strength of the pollutant source. Hard surfaces should be cleaned with a 5% to 10% solution of sodium hypochlorite (bleach), which kills microorganisms, but this will not prevent recontamination. Humidifiers and vaporizers should be scoured regularly, and fabrics should be cleaned in hot water. Carpets cannot be effectively cleaned unless they are removed and commercially processed. The following measures have been recommended[5]Prahl P Reduction of indoor mold spores.Allergy. 1992; 47: 362-365Crossref PubMed Scopus (27) Google Scholar to reduce indoor mold contamination: (1) increase ventilation, (2) heat all rooms during the winter, (3) avoid line drying clothes indoors, (4) repair insulation defects, (5) increase cleaning, (6) increase bathroom ventilation, and (7) reduce the number of indoor plants and keep them healthy. The effect of cleaning air ducts on clinical symptoms has not been studied in a controlled manner. Commercial firms offer duct cleaning treatments that are effective, at least immediately, in reducing mold and bacteria counts. Table III shows the predominant genera of mold and bacteria in 16 homes in central Florida. Table IV shows the results achieved in a home after duct cleaning. Similar results were obtained in other homes. TABLE IIIFrequency of isolation of mold and bacteria in air ducts of 17 central Florida homes*Benera% BldgMean CFU/m3RangeBacillus94.11100450-4700Staphylococcus88.21150150-2700Corynebacterium64.71250200-2600Penicillium35.21000150-4250Mucor41.110050-1250Cladosporium29.21500100-3750Aspergillus29.2300100-700Curvularia23.5150100-200Rhizopus17.630050-800Drechslera17.610050-150Gram negative11.7350150-300rodsNonsporulating11.710050-100Nocardia5.8600100-600Graphium5.812501250Paecilomyces5.8300300Syncephalastrum5.8750900Bldg, Building.*Courtesy of Pure Air Control Services. Open table in a new tab *Courtesy of Pure Air Control Services. TABLE IVMold and bacteria concentrations in CFU/m3 before and after duct-garde treatment*GeneraBeforeAfter% ReductionAspergillus niger2000100Aspergillus1500100Rhizopus4000100Penicillium750100Bacillus no. 18505094Bacillus no. 232510069Staphylococcus11255095Total312520093.6*Courtesy of Pure Air Control Services. Open table in a new tab *Courtesy of Pure Air Control Services. Some knowledge of house dust mite biology is required to understand the different methods that are currently available to control house dust mites.6Colloff MJ Ayres J Carswell F et al.The control of allergens of dust mites and domestic pets: a position paper.Clin Exp Allergy. 1992; 2: 1-28Crossref Google Scholar, 7Arlian LG Biology and ecology of house dust mites Dermatophagoides spp. and Euroglyphus spp.Immunol Allergy Clin N Am. 1989; 9: 339-356Google Scholar, 8Colloff M Practical and theoretical aspects of the ecology of house dust mites (Acari: Pyroglyphidae) in relation to the study of mite mediated allergy.Rev Med Vet Entomol. 1991; 79: 611-630Google Scholar The water loss of the mite body regulates colonization and population growth. House dust mites are able to extract water vapor from unsaturated air by means of a hygroscopic salt solution in the supracoxal gland. If the humidity falls below a critical level, the salts crystalize, obstruct the entrance of the gland, and slow down the rate of dehydration. The process of water uptake also depends on the temperature. Dermatophagoides farinae maintain water balance and survive at relative humidities of about 45% at 25° C, and 65% at 30° C. The ability to survive at relative humidities below saturation accounts for their successful colonization of human dwellings. Although house dust mites are considered perennial allergens, mite populations are affected by seasonal changes. Peak house dust mite population densities occur in the summer in temperate climates, and the lowest levels occur during the late winter months. Several studies in the United States have demonstrated a seasonal rise in mite numbers, which correlated with increases in humidity.9Arlian LG Woodford PJ Bernstein IL Gallagher JS Seasonal population structures of house dust mites, Dermatophagoides spp. (Acari: Pyroglyphidae).J Med Entomol. 1983; 20: 99-102PubMed Google Scholar, 10Lang JD Mulla MS Seasonal dynamics of house dust mites, Dermatophagoides spp. in homes in southern California.Environ Entomol. 1978; 7: 281-285Google Scholar, 11Murray AB Ferguson AC Morrison B The seasonal variation of allergic respiratory symptoms induced by house dust mites.Ann Allergy. 1980; 45: 347-350PubMed Google Scholar, 12Murray AB Zuk P The seasonal variation in a population of house dust mites in a North American city.J ALLERGY CLIN IMMUNOL. 1979; 64: 266-268Abstract Full Text PDF PubMed Scopus (70) Google Scholar, 13Arlian LG Bernstein IL Gallagher JS The prevalence of house dust mites, Dermatophagoides spp., and associated environmental conditions in homes in Ohio.J ALLERGY CLIN IMMUNOL. 1982; 69: 527-532Abstract Full Text PDF PubMed Scopus (172) Google Scholar, 14Platts-Mills TAE Hayden ML Chapman MD Wilkins SR Seasonal variation in dust mite and grass-pollen allergens in dust from the houses of patients with asthma.J ALLERGY CLIN IMMUNOL. 1987; 79: 781-791Abstract Full Text PDF PubMed Scopus (252) Google Scholar Studies in Ohio and Virginia demonstrated an increase in mite numbers and mite allergen levels during the summer months.9Arlian LG Woodford PJ Bernstein IL Gallagher JS Seasonal population structures of house dust mites, Dermatophagoides spp. (Acari: Pyroglyphidae).J Med Entomol. 1983; 20: 99-102PubMed Google Scholar, 13Arlian LG Bernstein IL Gallagher JS The prevalence of house dust mites, Dermatophagoides spp., and associated environmental conditions in homes in Ohio.J ALLERGY CLIN IMMUNOL. 1982; 69: 527-532Abstract Full Text PDF PubMed Scopus (172) Google Scholar, 14Platts-Mills TAE Hayden ML Chapman MD Wilkins SR Seasonal variation in dust mite and grass-pollen allergens in dust from the houses of patients with asthma.J ALLERGY CLIN IMMUNOL. 1987; 79: 781-791Abstract Full Text PDF PubMed Scopus (252) Google Scholar Mite allergen levels were determined monthly for a year in 12 homes in central Virginia. This study demonstrated that more than 20-fold variations in antigen P 1 equivalents can occur in house dust samples collected in various sites within homes. Mite allergen levels increase in July, with mean levels of antigen P1 equivalents in house dust significantly higher in the months of August to December than in April and May. This study also demonstrated that mite allergen levels remain high even after a decline in the mite population, especially in sofas.[14]Platts-Mills TAE Hayden ML Chapman MD Wilkins SR Seasonal variation in dust mite and grass-pollen allergens in dust from the houses of patients with asthma.J ALLERGY CLIN IMMUNOL. 1987; 79: 781-791Abstract Full Text PDF PubMed Scopus (252) Google Scholar Arlian et al.[13]Arlian LG Bernstein IL Gallagher JS The prevalence of house dust mites, Dermatophagoides spp., and associated environmental conditions in homes in Ohio.J ALLERGY CLIN IMMUNOL. 1982; 69: 527-532Abstract Full Text PDF PubMed Scopus (172) Google Scholar demonstrated that seasonal fluctuations in mite numbers significantly correlated with the changes in relative humidity (p < 0.01) in five homes in Ohio. Mean mite numbers increased during May through July and were maximum in August of 1977 and in late June to early July of 1978. Mite populations declined during the fall and were lowest during December through April. The authors suggested that house dust mites survive dry winter months in homes as desiccation-resistant pharate protonymphs. [13]Arlian LG Bernstein IL Gallagher JS The prevalence of house dust mites, Dermatophagoides spp., and associated environmental conditions in homes in Ohio.J ALLERGY CLIN IMMUNOL. 1982; 69: 527-532Abstract Full Text PDF PubMed Scopus (172) Google Scholar Data from 20 mite seasonal variation studies performed in different parts of the world and analyzed by M. J. Colloff [8]Colloff M Practical and theoretical aspects of the ecology of house dust mites (Acari: Pyroglyphidae) in relation to the study of mite mediated allergy.Rev Med Vet Entomol. 1991; 79: 611-630Google Scholar demonstrated that the geometric mean of the maximum mite density in mattresses was 13.3 times greater than the mean minimum density. This study demonstrated that the mite population is at its peak between July and October in northern, temperate latitudes and that in subequatorial regions the peak occurs early in the year. This change reflects the seasonal climatic reversal in the southern hemisphere. The seasonal variations in mite numbers ranged from a minimum of 2.1 times, reported by Chang and Hsieh,[15]Chang YC Hsieh KH The study of house dust mites in Taiwan.Ann Allergy. 1989; 62: 101-106PubMed Google Scholar to a maximum of 108 times, reported by Lascaud.[16]Lascaud D Étude Écologique de acariens pyroglyphides de la poussière de maison dans la region grenobloise.Ann Parasitol Hum Comp. 1978; 53: 675-695PubMed Google Scholar On the basis of these data, it seems that in temperate climates late winter and early spring are the best times of the year to clean mattresses and carpets very aggressively to kill the few mites that survived the winter. Theoretically, this would reduce the chances of having a large infestation during the summer months. Environmental control measures to reduce symptoms in individuals allergic to mites have been contradictory. Asthma symptoms and bronchial hyperreactivity in subjects allergic to mites have been shown to improve after removal of the subject from mite-infested environments.17Platts-Mills TAE Tovey ER Mitchell EB et al.Reduction in bronchial hyperreactivity during prolonged allergen avoidance.Lancet. 1982; 69: 290-292Google Scholar, 18Vervloet D Penaud A Razzouk H et al.Altitude and house dust mites.J ALLERGY CLIN IMMUNOL. 1982; 69: 290Abstract Full Text PDF PubMed Scopus (160) Google ScholarDales RE Burnett R Zwanenberg H Am Rev Respi Dis. 1991; 143: 505-509Crossref PubMed Scopus (240) Google Scholar Other studies have evaluated the influence of allergen avoidance by relocating patients to a higher altitude.19Boner AL Peroni DG Piacentini GL Venge P Influence of allergen avoidance at high altitude on serum markers of eosinophil activation in children with allergic asthma.Clin Exp Allergy. 1993; 23: 1021-1026Crossref PubMed Scopus (88) Google Scholar, 20Sporik R Holgate ST Platts-Mills TAE Cogswell JJ Exposure to house-dust mite allergen (Der p I) and the development of asthma in childhood. A prospective study.N Engl J Med. 1990; 323: 502-507Crossref PubMed Scopus (1446) Google Scholar One of these studies analyzed serum markers of eosinophil activation in children with allergic asthma.[19]Boner AL Peroni DG Piacentini GL Venge P Influence of allergen avoidance at high altitude on serum markers of eosinophil activation in children with allergic asthma.Clin Exp Allergy. 1993; 23: 1021-1026Crossref PubMed Scopus (88) Google Scholar Histamine provocative concentration causing a 20% fall in forced expiratory volume in 1 second was unaltered during the study period, whereas serum levels of eosinophil cationic protein and eosinophil protein X showed significant increases with exposure to mite allergens. The serum levels of myeloperoxidase, as well as chemotactic factors for both neutrophils and eosinophils, were unaltered during allergen exposure. The total IgE level significantly increased during exposure. The authors concluded that serum markers of eosinophil activity (eosinophil cationic protein and eosinophil protein X) are sensitive indices of allergen exposure in children with atopic asthma and that allergen avoidance reduced the levels of these markers. Successful studies on dust avoidance have used rigid avoidance measures in bedrooms, including removal of carpets, covering of mattresses, and regular washing of bedding in hot water. A successful mite control program should combine the killing of mites and the removal, immobilization, or denaturation of mite allergens throughout the house. The reduction of dust mite allergen level required to achieve clinical improvement has not been determined. However, Der p I concentrations of approximately 10 μg/gm of dust have been considered a risk factor for development of symptomatic mite allergic asthma in children.[20]Sporik R Holgate ST Platts-Mills TAE Cogswell JJ Exposure to house-dust mite allergen (Der p I) and the development of asthma in childhood. A prospective study.N Engl J Med. 1990; 323: 502-507Crossref PubMed Scopus (1446) Google Scholar Several methods to kill mites have been proposed. They include the use of acaricides and physical methods, such as heating, freezing, and drying. The use of acaricides presents potential problems of toxicity to human beings and pets. Several chemicals are currently in use. Pirimiphos methyl (Actelic, Imperial Chemical Industries, Ltd., Macclesfield, Cheshire, U.K.), a mosquito insecticide with acaricidal activity is currently used to protect grain against storage mites.[21]Mitchell EB Wilkins S Deighton JM et al.Reduction of house dust mite allergen levels in the home: use of the acaricide, pirimiphos methyl.Clin Allergy. 1985; 15: 235-237Crossref PubMed Scopus (67) Google Scholar A compound of benzoic acid esters and acrylate polymers is an effective acaricide and is currently used as a household acaricide in Europe and the United States.22Kniest FM Young E Van Praag MCG et al.Clinical evaluation of a double-blind dust-mite avoidance trial with mite-allergic rhinitic patients.Clin Exp Allergy. 1981; 21: 39-42Crossref Scopus (58) Google Scholar, 23Dietemann A Bessot JC Hoyet C Ott M Verot A Pauli G A double-blind placebo controlled trial of solidified benzyl benzoate applied in dwellings of asthmatic patients sensitive to mites: clinical efficacy and effect on mite allergens.J ALLERGY CLIN IMMUNOL. 1993; 91: 738-746Abstract Full Text PDF PubMed Scopus (79) Google Scholar A benzyl-tannate complex solution, a combination of an acaricide and an allergen-reducing agent, is currently being marketed in Australia.[24]Green WF Nicholas NR Salome CM et al.Reduction of house dust mites and mite allergens: effect of spraying carpets and blankets with Allersearch DMS, an acaricide combined with an allergen reducing agent.Clin Exp Allergy. 1993; 19: 203-207Crossref Scopus (48) Google Scholar A study demonstrated the acaricidal effect of pure caffeine. Caffeine-treated D. pteronyssinus cultures showed a significant decrease in mite numbers and in mite allergen after 2 months.[25]Russel DW Fernández-Caldas E Swanson MC Seleznick MJ Trudeau WL Lockey RF Caffeine, a natural acaricide.J ALLERGY CLIN IMMUNOL. 1991; 87: 107-110Abstract Full Text PDF PubMed Scopus (12) Google Scholar The weekly washing of bed linens is of benefit to patients if the temperature of the water destroys mites (≥130° F). Warm or cold water does not kill mites, and washing live mites out of a fabric is difficult. Bleach also kills house dust mites. Electric blankets are also effective in reducing relative humidity and the concentration of house dust mites in the surface of mattresses. Encasing mattresses, pillows, and blankets in plastic covers is also an important strategy in avoiding mite allergens. Liquid nitrogen has been successfully used to control mite populations.26Colloff MJ Use of liquid nitrogen in the control of house dust mite populations.Clin Allergy. 1986; 16: 41-44Crossref PubMed Scopus (60) Google Scholar, 27Dorward AJ Colloff MJ MacKay NS et al.Effect of house dust mite avoidance measures on adult atopic asthma.Thorax. 1988; 43: 98-101Crossref PubMed Scopus (106) Google Scholar Dorward et al.[27]Dorward AJ Colloff MJ MacKay NS et al.Effect of house dust mite avoidance measures on adult atopic asthma.Thorax. 1988; 43: 98-101Crossref PubMed Scopus (106) Google Scholar demonstrated a 95% reduction of live mites after 8 weeks of liquid nitrogen treatment of the mattresses of 10 patients with asthma compared with a 19% reduction in the control group. The treated group had a significant improvement in symptom scores and lung function test results. Humidity is a major limiting factor on the growth of house dust mites. Mites are able to absorb water vapor from unsaturated air until the absolute indoor humidity is below 7 to 8 gm/m3 (equivalent to approximately 50% relative humidity [RH] at 20° C). At lower humidities, mites die of dehydration. Korsgaard et al. observed that an RH above 60% in the house limits the effectiveness of mite reduction measures.28Korsgaard J Preventive measures in mite asthma. A controlled study.Allergy. 1983; 38: 93-102Crossref PubMed Scopus (84) Google Scholar, 29Korsgaard J Iversen M Epidemiology of house dust mite allergy.Allergy. 1991; 46: 14-18Crossref PubMed Scopus (49) Google Scholar Because subjects allergic to mites improved significantly in a controlled environment with an RH of 40% to 50%, individuals allergic to mites should be advised to control the relative humidity in their homes. Low humidity (<50%) also prevents growth of mold and bacteria. Inadequate ventilation, a consequence of attempts to conserve energy, also seems to be an important risk factor for house dust mite sensitization in temperate regions.[30]Wickman M Nordwall SL Pershagen G Sundell J Schwartz B House dust mite sensitization in children and residential characteristics in a temperate region.J ALLERGY CLIN IMMUNOL. 1991; 88: 89-95Abstract Full Text PDF PubMed Scopus (144) Google Scholar Because RH is the most important abiotic factor influencing the reproduction, survival, geographic distribution, and seasonal fluctuations of domestic mites, the reduction of the RH in the bedroom of individuals with asthma who are allergic to mites is commonly recommended by allergists. However, little information is available on the effectiveness of humidity reduction within a home. At this point, this recommendation is based more on common sense than on scientific data. There is evidence that beds in rooms with RHs above 64% contain significantly more mites than those in rooms with lower humidities, that pyroglyphid mite populations are greater during months with higher humidity, and that damp houses have more mites than dry houses. The reduction in humidity by airing bedrooms (combined with extensive cleaning and replacement of floor covers) was used in a controlled clinical trial in Denmark.[29]Korsgaard J Iversen M Epidemiology of house dust mite allergy.Allergy. 1991; 46: 14-18Crossref PubMed Scopus (49) Google Scholar In this study there was only a limited reduction in indoor humidity in spite of an increased number of airing hours per day. The concentration of house dust mites did not diminish, and no clinical improvement was observed. It has been suggested that keeping the indoor humidity below 7 gm/kg by ventilation is sufficient to eradicate house dust mite populations.28Korsgaard J Preventive measures in mite asthma. A controlled study.Allergy. 1983; 38: 93-102Crossref PubMed Scopus (84) Google Scholar, 29Korsgaard J Iversen M Epidemiology of house dust mite allergy.Allergy. 1991; 46: 14-18Crossref PubMed Scopus (49) Google Scholar The complete removal of house dust mites was achieved in 11 of 16 mattresses in Denmark within a year by using a mechanical ventilation system in new flats.[31]Harving H Korsgaard J Dahl R Mechanical ventilation in dwellings as preventive measures in mite asthma.Allergy Proc. 1988; 9: 283Google Scholar The cold, dry winter climate in Scandinavia contributes su