Search

Background: Estimated indoor climate conditions in unheated summer cottages offers house-dust mites unfavorable temperature and feeding conditions while giving them optimum humidity conditions. We aimed to estimate the exposure level of house-dust mites in summer cottages., Methods: A total of 37 summer cottages were sampled for house dust and storage mites in three locations and the results compared with samples from 33 patients suspected of house-dust mite allergy and living in ordinary houses. The processed dust samples were investigated by microscopy and exposure levels given as number of mites per 0.1 g dust., Results: The summer cottages were without exception all heavily infested with threefold or higher concentrations of both house dust and storage mites in comparison with ordinary houses. In unheated summer cottages were found tropical high concentrations of house dust and storage mites with a median concentration of house-dust mites in mattress dust of 2000 house-dust mites/g of dust which corresponds to an average of 40 microg allergen/g of mattress dust., Conclusions: As a probable consequence of high indoor air humidity conditions in unheated summer cottages in winter, dust samples contained threefold or higher concentrations of house dust and storage mites. Indoor humidity conditions thus more than compensated for lack of continuous supply of skin scales from human beings and an unfavorable low indoor air temperature. The uniform high mite exposure in summer cottages is likely to imply disease deterioration in patients already allergic to mites.

Objective: The aim of this study was to investigate whether window pane condensation and indoor vapour contribution >/= 3 g/m3 could be used as indicators of defective air change rate, high indoor humidity and high mite allergen concentration in mattress dust., Methods: Actual ventilation rate, indoor temperature, air humidity (AIH/RH) and concentrations of mite allergen were measured in 59 houses and compared with received outdoor temperatures and air humidity. Indoor vapour contribution defined as the difference between the indoor and the outdoor vapour concentration was calculated. Sensitivity, specificity, predictive values and accuracy were calculated for window pane condensation and high vapour contribution (>/= 3 g/m3), as indicators of defective ventilation (< 0.5 ACH), high indoor humidity (>/= 7 g/kg and >/= 45% RH) and high mite allergen concentration in mattress dust (>/= 2 microg/g)., Results: All houses with high humidity and high mite allergen concentrations were positive for the two indicators (high sensitivity), but with a specificity of only 50% so that half of the houses with reported condensation and high vapour contribution turned out to be low pollution houses with adequate high ventilation levels. Both indicators had high negative predictive values and absence of the two indicators almost certainly (97-100%) excluded high indoor pollution with high humidity and high mite concentrations. Overall more than 70% of the dwellings were correctly classified by the two indicators., Conclusion: Absence of window pane condensation on double-glazed windows and low indoor vapour contribution (< 3 g/m3) during the winter are true markers of a dwelling without high indoor air humidity and without high mite allergen concentrations in mattress dust in houses in a cold temperate climate with subzero outdoor temperatures. The presence of the two indicators is associated with a 18-45% risk of high humidity and mite allergen concentrations so in this latter group further measurements are needed for correct classification.

Increased exposure to house-dust mites entails a greatly increased frequency of sensitization and increased frequency of mite asthma in the population. The available case-control studies demonstrate large variations worldwide in sensitization and disease risk dependent on the actual level of population exposure. In areas with comparatively low population exposure to house-dust mites, there is a strong association between exposure and effect, while other areas with a more or less uniformly high population exposure do not demonstrate significant differences between patients and controls because the unexposed groups are too small. As to the existence of a hygienic maximum threshold exposure level, current data all point to a single value of 100 mites/g of dust, corresponding to 2 microg allergen/g of dust. This applies to the risk of sensitization and risk of disease, and when programs of prevention imply exposure below this value, convincing clinical improvement is always observed. Today, the magnitude of the health problem related to population exposure to house-dust mites in many areas is comparable to the effects of active tobacco smoking and traffic accidents.

Available epidemiologic data on the occurrence of house-dust mites in dwellings demonstrates a clear association between increased indoor air humidity and the increased occurrence of house-dust mites in house dust. Furthermore, in temperate climates, there is a threshold level of indoor air humidity of 7 g/kg (45% relative humidity at usual indoor air temperatures). Indoor air humidities below this level for extended periods will eradicate house-dust mites from dwellings. A reduction in inhabitant exposure to house-dust mites is implemented by reduction of indoor air humidity by controlled mechanical ventilation. Individual ventilation levels are estimated from the actual size of house, number of inhabitants, and average outdoor air humidity in winter. In contrast, more humid areas of the world with average outdoor humidities above 6-7 g/kg in winter will support uniformly large populations of house-dust mites, and reductions in indoor air humidity will have a comparatively minor effect on the occurrence of house-dust mites. Present-day building of energy-efficient houses with increased sealing of the building envelope, paralleled by a similar renovation of older houses, has increased indoor air humidity and is probably the cause of the almost fourfold increase in the occurrence of house-dust mites in Danish dwellings.

In temperate climates, energy-conserving measures may increase indoor humidity, enhancing house-dust mite (HDM) growth. Movement of families to "healthy" homes with mechanical ventilation systems reduced HDM exposure. The effect on asthma control of moving to the "healthy" homes was studied in 14 asthmatic patients allergic to HDM. Base-line evaluations of lung function, asthma symptoms, and medication requirements were made before moving and again after 5 and 15 months' residence. A control group of 11 mite-sensitive asthmatic patients who did not move were examined contemporaneously with the study group at base line and at the first follow-up investigation. After 5 months, the residents of the "healthy" homes improved in forced expiratory volume in 1 s (FEV1), medicine score, and serum IgE. These changes were significantly different from control group measurements. After 15 months, statistically significant improvements from base line were found in FEV1, average daily peak expiratory flow values, medicine score, symptom score, and serum IgE. Insignificant trends toward improvement were seen in provocation concentration of histamine and blood eosinophils. A significant relation was found between reduction in medicine score and fall in HDM exposure. The present study shows that a specific HDM-avoidance procedure can result in an overall, clinical improvement in HDM-sensitive asthmatic patients.

Exposure to house-dust mites (Dermatophagoides spp.) was investigated in 30 asthmatic patients before and after moving to homes with mechanical ventilation systems. Median house-dust mite concentration was 110 mites per gram of mattress dust at the initial investigation and 20 mites per gram at the first follow-up after a mean of 4.7 months in the new homes. This was lower (P < 0.05) than counts in a control group of 23 asthmatic patients that had unchanged counts. At a second follow-up, after 15.0 months, mite counts in the study group were further reduced (P < 0.01) from initial values. Among 16 patients, with initial mite counts above the recommended threshold limit value (TLV) of 100 mites per gram of dust, the mite counts fell, in most cases, below this TLV. Air-exchange rates increased (P < 0.001) from a median value of 0.40 air changes per hour (ach) to 1.52 ach at the second follow-up investigation. Furthermore, a reduction (P < 0.01) was found in indoor absolute air humidity, with a median value of 5.6 g of water/kg. No such changes were found in the control group. The present study indicates that reduction of air humidity through an increased supply of fresh air may significantly diminish and, in some cases, even eliminate house-dust mites in homes.

The concentration of house-dust mites (Dermatophagoides pteronyssinus) was investigated in 96 Danish homes with one or more members suffering from asthma. The air-exchange rates, humidity, and temperature were measured in all the homes. A positive correlation was found between indoor air humidity and mite concentration (P < 0.01), and an inverse correlation (P = 0.027) between house-dust mite concentration and indoor air-exchange was found. Homes of mite-allergic persons had a higher concentration of house-dust mites than did those of the non-mite-allergic group. House-dust mite concentration was above the proposed limit of 10 mites/0.1 g dust in 76% of the homes of persons allergic to mites and in 48% of the homes of nonsensitive persons. The high proportion of homes infested with house-dust mites suggests an increased prevalence of house-dust mites in Danish homes. The results support the concept that reduced ventilation in homes involves a risk of increased house-dust mite exposure.

The occurrence of mites in house dust from the homes of 25 patients with newly diagnosed house-dust mite asthma was much greater than that in the house dust from homes of 75 randomly selected matched control subjects. The difference in exposure corresponded to a relative risk of about 7.0, and a clear dose-response relationship could be demonstrated. The 25 patients lived in older and more humid houses than the control subjects did, which explains the greater number of house-dust mites found and suggests possibilities for and the necessity of introducing hygienic standards for the indoor environment as a measure of primary prevention in house-dust mite allergy.

The concentration of house-dust mites (Dermatophagoides spp.) was investigated for four seasons in three locations in each of 50 Danish apartments. Simultaneously the absolute humidity was recorded and the previously known correlation between mite counts and indoor humidity was confirmed. It appeared, however, that apartments which had a low absolute indoor humidity in the winter period (due to low household load of water vapour) did not contain noticeable concentrations of house-dust mites in the summer and autumn despite the fact that the indoor absolute humidity in these apartments could be high enough to allow for a high peak-population of mites. Because of this it is suggested that in a temperate climate avoidance measures against house-dust mites should be supplemented at least by a drying out period in the winter, when this process is convenient to perform because of the low outdoor absolute humidity.