Study design and population
The SALIA study (Study on the influence of air pollution on lung function, inflammation and aging) was part of the Environmental Health surveys as an element of the Clean Air Plan introduced by the Government of North-Rhine Westphalia in Germany . Consecutive cross-sectional studies were performed between 1985 and 1994. The study areas (Dortmund (1985, 1990), Duisburg (1990), Essen (1990), Gelsenkirchen (1986, 1990) and Herne (1986)) were chosen to represent a range of polluted areas with high traffic load and steel and coal industries. Two non-industrial small towns, Dülmen (1985) and Borken (1985, 1986, 1987, 1990, 1993, 1994), were chosen as reference areas. Data from similar studies done in 1987, 1993 and 1994 in Cologne, Düsseldorf, Hürth, Dormagen and Wuppertal were not included in this analysis because of a low response, different type of exposure (chemical industry) and non availability of address-coordinates for GIS- based exposure estimation.
All women aged 54 to 55 residing in the selected areas were asked to participate in the study, which took place in March and April in the years specified. 4874 responded, every second responder was invited to have a lung function testing (N = 2593). We restricted the analysis to those 4757 women whose addresses were available and where the addresses could be merged with geographic coordinates. Men were not recruited for the study, to avoid bias due to occupational exposure from working in the mining and steel industry.
Questionnaire: diagnoses, symptoms and risk factors
Together with an invitation to participate in the study, a self-administered questionnaire was sent to the women. The investigating physicians checked the returned questionnaires. We asked whether a physician had ever diagnosed chronic bronchitis and for respiratory symptoms. Respiratory symptoms were asked as "chronic cough with: (a) phlegm production, (b) for > 3 month a year, (c) for more than 2 years". We evaluated "chronic cough" and "chronic cough with phlegm production". The diagnosis of chronic cough with phlegm production was positive, when each of the answers categories (a), (b) or (c) was positive. This symptoms complex classically defines chronic bronchitis.
We further asked about risk factors such as single room heating with fossil fuels, occupational exposure (dust and extreme temperatures) and education as indicator for socioeconomic status. We classified socioeconomic status into three categories using the highest school level achieved by either the women or her husband as low (< 10 years), medium (= 10 years) or high (> 10 years). Women were grouped according to their smoking habits as never smoker, passive-smoker (home and/or work place), past smoker or current smoker (<15 pack years; 15–30 pack years and >= 30 pack years).
Lung function testing and COPD
Forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC) were measured. Four maneuvers were performed, and the values, where the maximal FEV1 was reached, were used. All measuring instruments were calibrated prior to each testing by using a 3-liter-syringe. All personal were specially trained, the same type of measuring device was used (Vica Test 4 spirometer (Mijnhardt, Rotterdam, Holland)) and all maneuvers were performed in accordance to a standardized protocol . We also used the ratio FEV1/FVC, which is considered a sensitive measure of COPD on its own . A FEV1/FVC ratio <0.7 is the main criterion for COPD according to the newly developed criteria by GOLD . We used this criterion to define the disease. However, we did not use a post-bronchodilator measurement in our epidemiological study, therefore we excluded 168 women with asthma from further analysis of the association between lung function and air pollution, to avoid confounding. Asthma was considered present, when ever diagnosed by a physician or if asthma medication were used.
We used two ways to assess air pollution exposure, first, we used data from monitoring stations maintained by the State Environment Agency. They cover the area in an 8 km grid and are designed to mainly reflect broad scale spatial variations in air quality. Second, we used distance of residential address to the nearest major road, which reflects small-scale spatial variations in traffic related exposure.
All 7 monitoring stations used for this study were located within a distance of not more than 8 km to the women's home address. Given that there was no monitoring station available for Dülmen, the air pollution data from Borken was used, because of its proximity and comparability. Due to the incompleteness of air pollution data from Borken, where continuous measurements started in 1990, the data preceding this year were imputed by using measurements (1981–2000) from 15 monitoring stations in the Ruhr area assuming similar trends. Between 1985 and 1987 discontinuous measurements were performed in Borken and Dülmen (four days per month). These discontinuous measurements agreed well with the imputed values. Mean measured TSP between 1984–1987 was 70 μg/m3 and the imputed value for 1985 was 66 μg/m3.
The concentrations of nitrogen dioxide (NO2) was measured half-hourly by means of chemiluminescence. Total suspended particles (TSP) were gathered with a low volume sampler (air flow: 1 m3/h) and measured using beta-ray absorption. For the assessment of individual medium term air pollution exposure we used annual mean concentrations in the year of the investigation and for long-term air pollution exposure we used five-year means of measurements done before the investigation. To estimate the exposure of particulate matter of less than 10 μm dynamic diameters (PM10), we multiplied TSP measurements with a conversion factor of 0.71. This conversion factor was calculated from 7 monitoring sites in the Ruhr area, where parallel measurements of TSP and PM10 were performed between 1998 and 2004.
We further assessed the exposure to motor vehicle exhaust by the distance (< 100 m and >= 100 m) from each residential address to the nearest major road (> 10 000 cars per day) by using geographic information system (GIS) software Arc GIS 9.0 (ESRI Redlands, CA). Average daily traffic counts for the year 1997 and mean traffic load per square kilometer for the year 1987 (without Borken and Dülmen) were obtained from the North Rhine Westphalia State Environment Agency (LUA NRW).
The association of symptoms and diagnoses with ambient air pollution exposure was analyzed by logistic regression. Odds ratios (OR) with 95% confidence intervals (CI) are presented for an interquartile range increase in PM10 [7 μg/m3] and NO2 [16 μg/m3] exposure and for living nearer than 100 m respectively >= 100 m from a road with heavy traffic. FEV1, FVC and the ratio FEV1/FVC were approximately normally distributed and multiple linear regressions were used for analysis. The regression coefficients b were transformed to relative mean differences (MD) MD = 1+b/mean (lung function). We included a random area effect in the logistic as well as the linear regression analysis to account for possible clustering within areas.
Age, socioeconomic status, smoking, exposure to environmental tobacco smoke (ETS), occupational exposure to temperature (heat/cold) and dust and heating with fossil fuels were included as covariates in all models. FEV1 and FVC were adjusted for body mass index (BMI) and height additionally.
All statistical analysis was done with SAS for windows release 9.1 (SAS Institute, Cary, NC).