Fine and coarse particulate air pollution in relation to respiratory health in Sweden

Introduction

There is a large body of epidemiological evidence on health effects of particulate air pollution, including increased respiratory symptoms, decreased lung function and increased hospital admissions and mortality from cardiopulmonary causes [1–5]. Mortality and cardiovascular events have been linked primarily to fine particulate matter (≤2.5 μm aerodynamic diameter range (PM_2.5)), possibly because it can penetrate deep into the lungs and cause systemic inflammation. Coarse particulate matter (commonly defined as particles in the 2.5–10-μm aerodynamic diameter range) has mostly been associated with non-fatal respiratory effects [5]. However, a review by Brunekreef and Forsberg [6] indicated that coarse PM is associated with cardiovascular disease and increased mortality as well. It is difficult to separate health effects of long-term exposure to exhaust emissions and wear particles from road traffic, and to separate effects of locally generated and long-range transported pollutants. Further epidemiological evidence to disentangle these relationships is needed. Although the background concentration of particulate air pollution in the Nordic countries is lower than in the rest of Europe [7], there are major concerns about the health effects of exposure to particulate air pollution from traffic [8–10]. The use of studded winter tyres on cars and the gritting of roads are common in northern Europe, and road wear constitutes an important source of coarse PM [11]. There are only a few large population studies investigating the health effects of air pollution from road traffic, which include urban as well as non-urban adult populations, and use spatially resolved air pollution data. Within the ROADSIDE project, we have data on air pollution exposure and health outcomes for ∼26 000 people aged 18–80 years over the whole country of Sweden. The main objectives of the present study were to investigate associations between exposure to total PM₁₀, long-range transport particles, locally generated combustion particles and locally generated road wear particles at the subject’s home address, and self-reported respiratory health outcomes in a general population sample in Sweden.

Results

Female and middle-aged respondents were slightly over-represented in the study population, with little regional variation (table 1). The largest proportion of persons with higher vocational or university education was in the Stockholm region. Smokers constituted 18% of the total study population, with a lower proportion in the North region (14%). Passive smoking was most common in the southern region Scania. Moisture damage was present in ∼15% of the homes, while visible mould was reported for 5%. More than 90% of the study population was of Nordic origin. The highest proportions of participants of non-Nordic origin were in Scania and Stockholm.

The estimated residential outdoor levels of particulate air pollution in Sweden could be modelled for 25 846 of the 25 851 study participants (table 2 and figs 1b and 2). Total PM₁₀ levels for our total study population appeared to have a Gaussian distribution with a mean of 11.3 μg·m⁻³, a 5–95% range from 8.0 to 16.1 μg·m⁻³, and a few addresses between 20 and 40 μg·m⁻³. Highest mean concentrations of total PM₁₀ were seen in the regions Scania, Stockholm and the West, which contain the three largest cities in Sweden: Stockholm, Gothenburg (region West) and Malmö (region Scania). In all regions, the total PM₁₀ level was dominated by long-range transport particles, with an overall average of 10 μg·m⁻³ (88% of the total PM₁₀ level). Long-range transport particle levels (regional PM₁₀ contribution) ranged from ∼4 μg·m⁻³ (region North) to 14 μg·m⁻³ (region Scania) (fig. 1b). Locally generated PM₁₀ level had a skewed distribution with an average of 1.3 μg·m⁻³. Overall, the locally generated PM₁₀ level was dominated by wear particles (62%) with an average of 0.8 μg·m⁻³, the rest being traffic exhaust and other combustion particles with an average of 0.5 μg·m⁻³. The highest levels of locally generated combustion particles were found in the regions Stockholm, Mälardalen, and the South East and Gotland. The highest levels of locally generated road wear particles were found in the regions Stockholm, Mälardalen and West. The differences in response rate across geographic and socioeconomic substrata of the population were not related to estimated total PM₁₀ levels (data not shown).

• Download figure
• Open in new tab
• Download powerpoint

Figure 2–

Distribution of modelled outdoor levels of total particulate matter <10 μm aerodynamic diameter range (PM₁₀) and of different particle fractions at the home addresses of the study population in the Swedish Environmental Survey of 2007. Data are presented as % of addresses sampled in the survey. a) Total PM₁₀, b) long-range transport particles, c) locally generated PM₁₀, d) locally generated wear particles and e) locally generated combustion particles.

The prevalence of respiratory outcomes is shown in table 3. Asthma symptoms and the use of asthma medication in the past 12 months seemed to be very similarly reported, by about 8% of the total study population. There were distinct regional differences, with the highest prevalence in the Mälardalen region and lowest in Scania and West. Blocked nose or hay fever for >4 days a week in the past 12 months was reported by 18.0%, with smaller regional differences. Being woken by chest tightness or cough in the past 3 months was reported by 19.4% and restricted activity days due to respiratory symptoms in the past 3 months by 10.9% of the population, both also with smaller regional differences.

Figure 3 shows associations between total PM₁₀, as well as the different PM fractions, and respiratory health outcomes in the total study population. Total PM₁₀ concentration was positively associated with chest tightness or cough and restricted activity days in the past 3 months. The long-range transport part of PM₁₀ was not associated with any outcome except for a non-expected marginally significant inverse association with asthma symptoms (OR per 10-μg·m⁻³ difference 0.69, 95% CI 0.49–0.99). Locally generated PM₁₀ was significantly associated with being woken by chest tightness or cough in the past 3 months and with restricted activity days in the past 3 months, but also with blocked nose or hay fever in the past 12 months. When separating the locally generated particles into wear and combustion particles, both fractions showed elevated risks for respiratory symptoms. Locally generated wear particles were significantly associated with blocked nose or hay fever (OR per 10-μg·m⁻³ difference 1.40, 95% CI 1.02–1.90), chest tightness or cough (OR 1.91, 95% CI 1.40–2.60) and restricted activity days (OR 1.50, 95% CI 1.05–2.16), while locally generated combustion particles were significantly associated with blocked nose or hay fever (OR per 1-μg·m⁻³ difference 1.09, 1.02–1.16) and being with woken by chest tightness or cough (OR 1.08, 95% CI 1.01–1.15).

• Download figure
• Open in new tab
• Download powerpoint

Figure 3–

Adjusted associations between different particulate matter (PM) fractions and self-reported current respiratory health outcomes in the Swedish Environmental Survey of 2007. One-pollutant models adjusted for the individual confounders sex, age, educational level, smoking, passive smoking, mould and moisture exposure in the home and country of birth, and the contextual confounders proportion of highly educated and unemployed people in the municipality. a) Total particulate matter <10 μm aerodynamic diameter range (PM₁₀), b) long-range transport particles, c) locally generated PM₁₀, d) locally generated wear particles and e) locally generated combustion particles. Data are presented as odds ratios and error bars represent 95% confidence intervals.

In order to compare the effects of locally generated wear and combustion particles we also calculated associations for an exposure difference between the fifth and 95th percentile of the distribution in the study participants. These exposure differences were 3.22 and 1.74 μg·m⁻³, respectively. The corresponding odds ratios were fairly similar: blocked nose or hay fever 1.11 (95% CI 1.00–1.23) and 1.15 (1.03–1.29), chest tightness or cough 1.23 (1.11–1.36) and 1.14 (1.02–1.28), and restricted activity days 1.14 (1.01–1.28) and 1.10 (0.97–1.27), respectively.

Exposure to locally generated wear and combustion particles were highly correlated (r=0.75) and in regression models including both exposure parameters, the effect of the combustion particles disappeared while the effect of the wear particles was strengthened (data not shown).

When we performed analyses restricting the population to those resident at the current address for ≥5 years, the results were essentially the same.

Discussion

We have investigated the associations between exposure to different fractions of particulate air pollution and current or recent respiratory symptoms in Swedish adults. Our results indicate significant effects of exposure to total PM₁₀ and locally generated PM₁₀, including wear and combustion particles, on the prevalence of restricted activity days due to respiratory symptoms, being woken by chest tightness or cough, and blocked nose or hay fever (only statistically significant for locally generated PM₁₀). However, we did not find any consistent effects of PM exposure on asthma outcomes in the present study.

Our results are broadly in line with other studies showing respiratory effects from coarse particles. A systematic review of the effects of coarse airborne particles on health observed that in studies of chronic obstructive pulmonary disease (COPD), asthma and respiratory admissions, coarse PM has a stronger or equally strong short-term effect as fine PM, suggesting that coarse PM may lead to adverse responses in the lungs triggering processes leading to hospital admissions [6]. Among recent studies, Chen et al. [21] showed that ambient coarse PM was positively associated with respiratory hospital admissions among the elderly population in Vancouver, Canada. Lin et al. [22] found a detrimental effect of coarse particulate matter on hospitalisation for respiratory infections in children. A study conducted in France also showed positive associations between exposure to PM_2.5–10 and hospitalisation for respiratory infections in children [23]. Another French study, using a dispersion model, found positive associations between PM₁₀ exposure and asthma and allergies in children [24].

It was possible to explore the effects of different particle size fractions, which would have been more problematic in smaller areas because of the locally very high correlation between the fractions. In our study, locally generated wear particles appear less harmful than combustion particles on a mass basis, but when the associations were calculated for difference between the fifth and 95th percentile of the distribution of the two fractions the results were rather similar. When both these types of locally generated particles were entered into the same model, the effect of the combustion particles disappeared while the effect of the wear particles was strengthened. However, it should be noted that in our study the two fractions, to a large extent, come from the same source, and thus are substantially correlated (r=0.75), which implies that the results regarding the specific role of each fraction have to be interpreted with caution.

Our study was based on a large sample (nearly 26 000 people) distributed over a large geographical area (449 964 km²), including densely populated areas, the three major cities Stockholm, Gothenburg and Malmö, as well as rural areas all over Sweden. This resulted in a greater than usual exposure contrast for a Swedish population and in a high statistical power for the analyses.

The response rate of the initial environmental health survey was reasonable (59.4%). From a non-response analysis of the question “How do you assess your general health status, compared to others of your age?”, it was concluded that the study population well represented the target population of long-term Swedish residents aged 18–80 years [12]. The difference in response rates over the socioeconomical and geographical strata was not related to air pollution exposure, which implies that no bias was introduced by the lack of total participation in the survey.

A limitation of this study was the cross-sectional study design, which makes it difficult to study effects of long-term exposure. Subjects who have developed respiratory problems may have moved away from areas with high air pollution exposure, which could result in apparent protective effects from air pollution. We therefore focussed on the effects of air pollution exposure on current and recent respiratory symptoms (in the past 3 months or in the past year). Restricting the analyses to subjects living at their address for >5 years did not substantially change the results.

The different air quality gradients, as simulated by the SIMAIR model system, vary in quality. The gradient of the regional contribution over Sweden was estimated from a combination of model results and measurement data, and is expected to have high validity for the model year (2004). The validity of the urban contribution of PM is highly dependent on the information on traffic in the 1×1-km grid, which was of good quality [13, 14]. The total contribution from residential wood combustion to PM exposure, as well as its spatio-temporal distribution, is less well known. As for the local impact of traffic emissions within a radius of 250 m, the spatial distribution of the emissions should be well described for all state-owned roads and for the major city roads. However, traffic on minor city roads was to a great extent simulated, and thus described with poorer quality. The modelling was performed with hourly input of meteorology for the model year 2004. The questionnaire data are from 2007 but very little is expected to have happened to any of the gradients in the years before and after the model year.

The SIMAIR model output has been extensively evaluated against monitor data from 12 urban background and 15 street site locations [25]. The results of these evaluations show that the SIMAIR model performs substantially better than the European Union Directive's quality criteria for PM₁₀, that require a maximum deviation between the simulated and monitored annual average concentration levels of <50% for 90% of individual monitoring points [26]. Furthermore, the results of the comparison of the SIMAIR model with monitored data at six urban background and four street site locations in different parts of Sweden showed that the difference between simulated and monitored annual averages of PM₁₀ levels were below ∼20% for urban background and ∼10% for street level concentrations, with correlation coefficients for the time-series of daily average PM₁₀ varying between 0.56 and 0.76.

An important uncertainty may be found in basing the exposure determination on outdoor levels at the home address only. People spend most of their time indoors, but indoor PM levels in Sweden are similar to outdoor levels [27], and the home address is a common approach found to constitute a fairly good proxy, especially for PM_2.5 exposure [28]. However, other environments that may give significant contributions are school, the workplace and traffic. Future studies could benefit from including estimates of these contributions to individual exposure.

Health outcome variables used in this study were self-reported and derived from a questionnaire containing questions on both living environment and personal health. Self-reporting of health is clearly subject to error [29]. Biased reporting of the outcomes associated with the estimated air pollution is, however, unlikely as the data on air pollution levels used in our analyses were derived independently from the questionnaire. Our study has been conducted in a very heterogeneous study area varying from very sparsely populated rural areas to the urban areas of Stockholm, Gothenburg and Malmö. Furthermore, the exposure to air pollution and the prevalence of the investigated respiratory symptoms vary over the country as well. Earlier studies in Sweden have found that the socioeconomic context of the area of residence is associated with levels of air pollution [30] and disease risk [31]. Potential contextual confounding by the socioeconomic position of the area of residence has been taken into account by conducting multilevel analyses, adjusting for the percentage of highly educated persons and of unemployed people in the municipality. These two contextual variables were both associated with air pollution levels and the investigated respiratory symptoms, and including them in the models slightly attenuated the risk estimates.

The apparent protective effect of long-range transport particles on asthma outcomes is probably due to the fact that asthma is more prevalent in the north, while long-range transport particle concentration shows the opposite gradient. Earlier studies in Sweden have also shown an increased prevalence of asthma and COPD in adults and children in northern Sweden compared with southern Sweden [32–35]. Climate factors such as temperature and humidity, and lifestyle factors such as nutrition or indoor air quality may explain these differences in prevalence. However, the exact reason is still unknown, underlining the need for great caution in the interpretation of studies with large-scale differences in both environmental contaminants and health effects. In conclusion, our data indicate that exposure to locally generated road wear particles increases the risk of respiratory symptoms in adults.