Study area and recruitment

The study area was defined as the affected neighbourhood of Herrgården and a selected part of the adjoining neighbourhood of Törnrosen with similar infrastructure. Both study neighbourhoods are subdistricts within Rosengård, Malmö, in southern Sweden. The buildings are of similar age and construction, but the selected area in Törnrosen had had appropriate upkeep in contrast to the main part of the buildings in Herrgården, where upkeep had been severely neglected by the proprietors for many years.

Recruitment of study participants was carried out in two ways. In early 2010, electronic patient registries between 2007 to 2010 at the two local primary healthcare centres (PHCs) serving the area were searched for all children between the ages of 0 and 13 years (the cut-off date was 1 May 2010) with diagnoses of asthma or other respiratory diseases and who were registered as living in the defined study area at the time of the documented healthcare contact. The specialised asthma nurses’ records were also checked as were the enrolment lists of information sessions for families with children with asthma that had been held at the PHCs. This identification of index children was carried out by the same team of investigators in both PHCs with similar procedures. To obtain a subgroup where children with respiratory symptoms were not oversampled, the sampling was expanded to include children from the study area who were aged between 6 and 13 years from class lists that were obtained from the local school. All families of the children received written invitations from the researchers and the PHCs to participate in the study. It was explained that the study would entail a home visit and that data would be collected on the children (regardless of whether or not they were symptomatic at the time of the home visit), their siblings, and parents living in the same household. If no replies to the written invitations were received, the investigators attempted to contact the families by telephone. If families chose to participate, a home visit was scheduled during which health communicators fluent in the family's native language visited the apartment. All initial visits were carried out between 27 May 2010 and 29 May 2011. At the start of the home visit, informed written consent in the native language of the family for participation in the study was obtained from the parents.

Interview questionnaires were used to collect demographic information for all core family members in the household (including country of origin, place of birth, and duration of residence in Sweden), physical apartment characteristics (number of rooms, number of regular occupants, presence of pests), subjective exposure assessment in the various rooms (dampness, mould) and information about lifestyles and behaviours (smoking, water pipes, incense, pets). In addition, questionnaires were used to collect health information for children aged 0–13 years at the time of visit with a main focus on respiratory, allergic, and dermal symptoms, using questions from a national Children's Environmental Health Survey of 2003. The questionnaires also gathered information on physician-diagnosed diseases such as asthma, allergic rhinitis, and eczema. The mothers were also asked about breast feeding practices for all children. Age of the mother at the time of birth of the child as well as the number of siblings were calculated from the available data. Finally, a standardised visual assessment of multiple areas of all homes was carried out at the time of the home visit by the health communicators who had undergone training in home assessments. Documented in the form of a checklist, the assessment scored the extent of moisture damage, visible mould, and mouldy odour in all rooms to which access was granted.

Background variables

We used the variables age at the time of the health visit, sex, born in Sweden (yes/no), mother's age at birth, number of older siblings, number of months with exclusive breast feeding, father's education (university level or lower), mother's education (university level or lower), father working full time (yes/no), and mother working full time (yes/no) for sociodemographic description.

Outcome variables

We used self-reported data (with maternal report as proxy) on general health (‘Good’ and ‘Very good’ vs ‘Fair’, ‘Poor’, and ‘Very poor’), current asthma (defined as a doctor's diagnosis in Sweden or abroad and at least one episode of wheeze during the past 12 months or wheeze more than four times in the past 12 months or asthma medication during the past 12 months and at least one episode of wheeze during the past 12 months), the use of asthma medication in the past 12 months, emergency care in the previous 3 months (2 times or more vs 0 or 1 time), colds in the past 3 months (≥2 vs 0 or 1), otitis media in the past 3 months (≥1 vs 0), headache in the past month (yes/no), stomach pain or nausea in the past month (yes/no), feelings of stress in the past month (yes/no), difficulty concentrating in the past month (yes/no), difficulty falling asleep in the past month (yes/no), and fatigue (yes/no). SPTs were performed within 3 weeks after the home visit and read and documented by experienced specialised nurses. SPTs with a standard panel for aeroallergens, including moulds, house dust mites, plants, animal dander, and, in addition, cockroach (Bla g 2), were performed on all participants who were willing to be included. All tests were read after 15 min. Control tests were carried out with histamine and saline. If a positive reaction occurred to histamine with a negative reaction to normal saline, the SPT was deemed valid. If the reaction to an antigen was at least half the size of the histamine reaction, the test was judged as positive. Any smaller or no reaction was deemed a negative result.

Exposure variables

The following exposure variables that were objectively assessed by the health communicators were used in the current study: ‘Dampness in kitchen, bathroom, or toilet’, ‘Dampness in bedrooms or living room’, ‘Mould smell or visible mould in bedrooms or living room’, ‘Visible mould anywhere in apartment’, and ‘Mould smell anywhere in apartment’. The variables were defined as ‘yes’ if any of a list of observations were made. For example, ‘Dampness in kitchen, toilet, or bathroom’ was defined as a ‘yes’ if at least one of the following were observed: floor drain to the toilet; bubbles in the plastic mats in the toilet, kitchen, or bathroom; a dripping faucet in the bathroom or kitchen; visible damp patches in the bathroom or kitchen; visible moisture stains in the bathroom or kitchen; visible moisture stains on the toilet; difficulty turning off the faucet in the bathroom or kitchen; leaking or dripping water pipes in the bathroom or kitchen; or inefficient floor drainage in the bathroom. We also created a variable describing the total number of factors from the list of factors aforementioned indicating dampness in the kitchen, bathroom or toilet as well as for mould or mould smell in bedrooms or living room. To describe exposure to cockroaches, we were restricted to self-reported data and the variables ‘Cockroaches in the apartment now (yes/no)’ or ‘Apartment sanitised from cockroaches (yes/no)’. Furthermore, we defined a variable on environmental tobacco smoke (ETS) as anyone smoking at home (mother or father or someone else smoking at home every day, yes/no) and a variable for the average number of persons living in the apartment per room (bedrooms and living room) in the apartment as an indicator of crowdedness.

Statistical analysis

We used descriptive statistics of the background variables and the exposure variables to describe differences in the background variables between the participating children of the two neighbourhoods, Herrgården and Törnrosen. The differences between the two study neighbourhoods were tested with Fisher's exact test for binary variables, and with the Mann-Whitney test for continuous variables. For the outcome variables, we stratified the children according to neighbourhood, as well as according to whether the child was an index child, a sibling of an index child, or if the child was recruited from the local schools or was a sibling to a child recruited from the local schools. This was necessary because index children were sampled from local health registers, and children with respiratory health problems could thus be expected to be oversampled in index children and in siblings of index children.

In order to investigate associations between the outcome variables and the exposure variables, we chose two objectively assessed exposure variables described above: ‘Mould smell or visible mould in bedrooms or living room’ and ‘Dampness in kitchen, toilet or bathroom’. The reason for analysing mould and dampness separately was that indicators for dampness were very common in the study setting, and that we wanted to also use a more conservative measure. The overlap between the variables was large; all but one apartments categorised with ‘Mould smell or visible mould in bedrooms or living room’, were also categorised with ‘Dampness in kitchen, toilet or bathroom’. We defined a variable for exposure to cockroaches according to the self-reported variables ‘Cockroaches in the apartment now’ or ‘Apartment sanitised from cockroaches’, and a ‘yes’ in either of these two variables was coded as a ‘yes’ and a ‘no’ in both variables was coded as a ‘no’. We analysed the data with logistic regression analysis and report crude frequencies as well as unadjusted and adjusted ORs with 95% CIs. Family ID was used as a repeated subject because the exposure variables were identical for children from the same family. In another analyses, categorical variables for ETS (‘yes’/‘no’), water pipe use (‘yes’/‘no’), incense (‘yes’/ ‘no’), maternal and paternal participation in labour market (working full time or not) and continuous variables for the number of persons per room in the apartment, child's age, exclusive breast feeding, mother's age at birth, and number of older siblings and were included as independent variables into the model to account for any potential bias from confounding variables. Pets with fur were present in less than 1% of the homes so this variable was excluded from the analysis. We did an additional analysis where we adjusted estimates associated with general health and mental well-being for asthma. We performed a sensitivity analysis with different definitions of exposure to mould and dampness. All analyses were run on complete case data, that is, individuals with missing data were excluded. All analyses were performed with SAS v9.4 for Windows 8.1.