Moisture damage and childhood asthma: a population-based incident case–control study

A large number of studies have observed an association between home dampness and respiratory symptoms such as wheezing, coughing and asthma 1, 2. As concluded in the recent US Institute of Medicine report on indoor dampness and health, there is sufficient evidence regarding the exacerbation of asthma symptoms, but whether exposure to dampness also causes development of asthma is still unclear 2. Most studies have been cross-sectional and relied on self-reported exposure or health or both. However, occupants' reports on dampness or mould growth are often inaccurate 3–5. The parents of asthmatic children may systematically over-report home dampness and thereby create an artificial association between home dampness and asthma 6, or the presence of dampness and mould in the home may lead to over-reporting of symptoms 7. Therefore, although it is acknowledged that home dampness exacerbates existing asthma, it is not known whether it is also associated with development of asthma 2.

A few studies on asthma or asthma-related symptoms among children using site visits or other standardised measurements of home dampness or mould exposure have recently been conducted. Most of these studies 5, 8–11 have been based on birth cohort studies that have followed the children only to the age of 1 or 2 yrs, which makes them difficult to interpret given the transient nature of asthma-related symptoms in early age 12, 13. A case–control study of prevalent asthma reported an association between self-reported, but not observed, damp and mould 4. A recent cohort study using self-reported exposure reported an association between incident asthma and mould odour 14. There are no studies on the specific association of home dampness with atopic versus nonatopic asthma.

The present authors therefore conducted a case–control study of newly diagnosed childhood asthma, using randomly selected population-based controls. The severity and location of moisture damage were assessed during a detailed home visit by trained civil engineers. In addition to overall analyses, atopic versus nonatopic asthmatics and those with early- versus late-onset asthma were analysed separately, as atopic, late-onset asthma is more likely to persist 12, 13.

METHODS

RESULTS

Cases were aged 12–84 months and controls 12–92 months (table 1⇓). There were no major differences in parental education, exposure to tobacco smoke, day-care attendance aged <12 months and living on a farm, but asthmatic children had fewer pets currently and more asthmatic parents and tended to have fewer siblings. There were no major differences in income or in breastfeeding (data not shown).

There were no statistically significant differences between the characteristics of the homes of cases and controls. However, the homes of asthmatic children tended less often to have a basement, a flat roof and natural ventilation, but more often to have condensation on windows, humidifiers and additional heating (table 1⇑).

Initially, moisture-damage variables characterising the whole house were analysed (table 2⇓). Cases tended less often to have any moisture damage. This difference persisted when houses with a basement were excluded from the analyses (data not shown). The overall area of damage in the residence tended to be larger among cases, and there was a tendency for the asthma cases to have more visible mould and mould odour in their homes. Past, but not current, reports of mould odour by the occupants was also more prevalent among the cases (data not shown).

Second, all observed damage sites, coded by their location, were analysed. Moisture damage and visible mould in the main living quarters was associated with increased risk of asthma (table 3⇓). There were no significant associations for all the other areas.

When the damage observations in the main living quarters of the house and in the child's bedroom were analysed in more detail (table 4⇓), there appeared to be a dose-dependent increase in the risk of asthma with severity of moisture damage and with maximum severity of the damage. Cases also tended to have larger damage sites and more mould odour.

The associations of moisture damage in the main living area or in the child's bedroom with asthma were mostly very similar for nonatopic and for atopic asthma, and for children aged ≤30 months or >30 months (table 5⇓). However, for visible mould the association was significantly (p<0.05) stronger with atopic than with nonatopic asthma.

DISCUSSION

In the present population-based case–control study, an association was observed between the risk of developing new asthma among children aged 1–6 yrs and moisture damage at home. Only moisture damage in the main living quarters of the home was associated with asthma. Trained engineers visited all homes to assess the severity and location of moisture damage and mould growth. The present results suggest that moisture and mould problems at home may increase the incidence of new asthma in early childhood.

Previous studies have usually found similar or somewhat lower odds ratios for childhood asthma 2 than the present study. These studies have, however, mostly been cross-sectional and have used data on self-reported exposure. A few studies have also used objective measurements of home dampness or moulds. In a case–control study 4, an association between chronic respiratory symptoms and asthma and self-reported dampness, but not dampness observed by an inspector, was observed. A crude odds ratio of 2.6 was reported between both self-reported and confirmed moisture problems and repeat bronchial obstruction by the age of 2 yrs, but in adjusted analyses confirmed moisture problems were a stronger predictor 5. In a cross-sectional study, there was a positive association of mould spore counts in dust with atopic sensitisation but not with asthma 8. Three recent birth cohort studies have reported positive associations between measured levels of fungi and wheezing 9, 11 and lower respiratory tract illness 10 in the first 12 months of life. A recent cohort study using self-reported exposure reported an association between incident asthma and mould odour, but not other indicators of moisture damage 14. There are no previous studies on the specific association of home dampness with atopic versus nonatopic asthma.

In the present study, a detailed examination of all the homes was carried out by a civil engineer 17. This allowed an analysis of the effect of location and extent of damage on the risk of asthma to be carried out. It was observed that only moisture damage in the main living areas or in the child's bedroom was associated with asthma. This issue has rarely been considered previously 4, although it is evident that the child spends most of his/her time in these rooms, which means that the exposure times are remarkably longer than in other locations. Thus, moisture damage in the basement or in the attic is likely to contribute much less to the exposure of the child. Children may also be exposed less to moisture damage in the kitchen and especially in bathrooms, as the exposure times are shorter.

It is worth noting that moisture damage, especially in the bathroom, may frequently be due to excessive use of water. This may lead to different conditions for microbial growth and subsequent emissions 2. Also, kitchen and bathroom facilities are usually better ventilated and under negative pressure compared with the living area. The importance of the location of the moisture damage clearly needs further study but the present results suggest that it is one of the major determinants of the health effects of moisture damage 18, 19.

Home inspection gives a good indication of long-term exposure. In addition, focusing on the building, which is the source of the exposure, helps to understand the whole complex exposure situation and its determinants. In contrast, sampling and culturing airborne mold spores has poor reproducibility owing to large variations in time and space 20. The method also detects only viable fungi 1. Nonculture-based methods, such as measuring markers of microbes from settled dust 21 or using DNA-based methods such as quantitative PCR 22, appear more promising. However, the pathophysiological mechanism linking home dampness and asthma is currently unknown 1, and therefore it is unclear which microbes or components of microbes to measure. The toxicity of different microbes is also modified by differences in the materials upon which they live and in their growth conditions indoors 23, 24. Therefore, in addition to markers of specific microbes, building investigations, as in the present study, are needed.

Several possible mechanisms have been suggested for the observed associations between home dampness or indoor mould and asthma. Moulds can induce the production of specific IgE antibodies, which are related to severity of asthma 25. However, the association between moisture damage or mould growth and IgE production is very weak 26. Therefore, non-IgE-mediated mechanisms are more likely. Components of fungal cell walls, such as β(1–3)-glucans 27, or volatile organic compounds, may act as irritants in the airways. Fungi and bacteria produce microbial toxins in moisture damage situations 28–31, which deserve attention owing to the acute toxicity of these products 32. The evidence on the role of microbial metabolites in the causation of asthma is, however, very limited or nonexistent 1. For instance, the observed levels of volatile organic compounds in damaged homes are usually considerably lower than the levels that would cause irritation 33. Given the lack of evidence for IgE-mediated sensitisation 1, 26 and the low indoor levels of most measured irritants in moisture-damaged buildings 34, it is tempting to speculate that the underlying mechanism involves some, as yet unknown, immunological reactions. Unfortunately, due to the low number of specific IgE results among the controls, the present authors were not able to study the association between moisture damage and development of atopy or the potential effect modification by atopy.

Home dampness has previously been linked with house dust mites at home. However, in Finnish homes, house dust mites occur quite infrequently and in low numbers, mainly because indoor air relative humidity is well under 50% for most of the year. Also, in a previous study, dust mites occurred in beds and padded furniture in similar prevalence, regardless of whether moisture and mould problems were observed in buildings 35. As part of the present study, house dust mites were studied using the Acarex test in the beds of 50 asthmatics and 46 nonasthmatics. Mites were observed equally frequently in the beds of asthmatics (34%) and nonasthmatics (33%; unpublished data). Hence, house dust mites are not likely to confound the observed association between home dampness and asthma.

The present design has limitations. The home visits were made after the child was recruited to the study, i.e. only after the asthma was diagnosed. However, the home inspection provides a very good reflection of the moisture history of the house in the preceding years. The recruited children, especially the younger ones, were required to have lived practically all their life in the inspected home. Also, the development of moisture damage is often slow in cold climates (several months to years). Therefore, the present design resembles as closely as was practically possible a prospective birth cohort study.

Although the engineers making the home visits were blinded to the asthma status of the child, it is difficult to maintain this blinding during the home visit. Therefore, it could be suspected that engineers would find more moisture problems in the homes of asthmatics. However, in the present study, the engineers found more moisture problems in total in the homes of the controls, which speaks against this possibility, although the homes of asthmatics had more damage observed only in the living areas. The pre-designed checklist used during the visit also gives equal weight to all interior spaces of the house.

In an effort to cover the lifetime history of moisture damage, the current study focused on young children, among whom asthma is often of a transient nature 12, 13. In the present study, new cases of asthma were recruited from a hospital clinic by a paediatrician and were required to have had at least two doctor-diagnosed attacks of wheezing. Therefore, despite their young age, most of the children in the present study are likely to have persistent asthma. The association between moisture damage and asthma was explored separately by age (≤30 months or >30 months) and atopy, as wheezing in the first 3 yrs of life and among nonatopics has been reported to be transient more often than wheezing in later life and among atopics 12, 13. The associations were found to be comparable among atopic and nonatopic asthmatics and among those with early and late onset of the disease, further suggesting that moisture damage at home not only exacerbates asthma symptoms, but is also involved in the development of persistent asthma.

In conclusion, the present results demonstrate the importance of the location of the moisture damage and suggest that moisture damage at home not only exacerbates asthma symptoms, but is also involved in the development of persistent asthma. Although the exact mechanisms linking moisture damage and mould growth with the risk of asthma are unknown, the present results underline the urgent need to improve building designs, construction and maintenance practices in order to prevent moisture damage and consequent risks to occupant health.