a Department of Epidemiology and Public Health, University of Newcastle upon Tyne, UK.
b Department of Statistics, University of Newcastle upon Tyne, UK.
Tanja Pless-Mulloli, Department of Epidemiology and Public Health, The Medical School, Framlington Place, Newcastle upon Tyne NE2 4HH, UK. E-mail: Tanja.Pless-Mulloli{at}ncl.ac.uk
Abstract
Background Public concern about respiratory conditions prompted the investigation of asthma and other respiratory diseases in children living near and away from opencast coal mining sites.
Methods We selected all 4860 children aged 111 years from five socioeconomically matched pairs of communities close to (OC) and away from (CC) active opencast sites. A postal questionnaire collected data on health and lifestyle. Outcomes were the cumulative and period prevalence (2 and 12 months) of wheeze, asthma, bronchitis and other respiratory symptoms.
Results The cumulative prevalence of wheeze varied from 30% to 40% across the ten communities, it was 36% in OC and 37% in CC. The cumulative prevalence of asthma was 22% in both OC and CC, varying between 12% and 24%. We found little evidence for associations between living near an opencast site and an increased prevalence of respiratory illnesses, or asthma severity. Some outcomes such as allergies, hayfever, or cough varied little across the study communities. Others, such as the use of asthma medication, the number of severe wheezing attacks in the past year or tonsillitis showed large variation. These similarities and variations were not explained by differences in lifestyle factors or differences in health services delivery and remain unexplained.
Conclusions There was little evidence of an association between residential proximity to opencast mining sites and cumulative or period prevalence of respiratory illness, or asthma severity. Some variations in health outcomes between communities remained unexplained.
KEY MESSAGES
Keywords Opencast coal mining, respiratory health, cumulative prevalence, air pollution, dust, asthma, children
Accepted 2 November 2000
While the aetiology of asthma, wheeze, cough and phlegm remain under discussion, there is now widespread agreement that the prevalence of childhood asthma has risen over the past decades.17 Much of the recent evidence points towards a smaller than previously expected role of air pollution in the causation of asthma,811 while there is evidence to suggest that air pollution is associated with the exacerbation of asthma.11,12 Other recent evidence points towards differences in the potency for respiratory effects from different types of particulate matter with a diameter of less than 10 micron (PM10). Heinrich et al. reported differences in the lifetime prevalence in respiratory disorders and allergic sensitization in children from communities with industrial pollution related to mining and smelting compared to children from communities with pollution from chemical and power plants in East Germany.13
Investigations of respiratory diseases around point sources of pollution have occasionally found links between proximity of residence to a polluting industry and symptoms.1419 However, most of the reported associations were weak. Opencast coal mining has been one of the specific industries implicated as possibly causing elevated asthma levels in residents living nearby, particularly in children.2022 One study directly related to opencast coal mining operations was conducted with workers on opencast mining sites. It reported a prevalence of asthma among workers similar to the general population, but a frequency of chronic bronchitis that was higher than the general population.23,24 Temple et al. reported on an audit of asthma cases in general practice conducted without a control group and without exposure measurements.22 This work was consequently criticized for a lack of scientific rigor. No previous epidemiological study had therefore investigated the association between residential proximity to opencast coal mining and the prevalence of asthma and other respiratory symptoms in detail.
While there is clear evidence of wide variations in the prevalence of asthma and other respiratory symptoms between countries,9,25 the variation within countries is much less clear cut and sometimes unexplained. The ISAAC study reported a less than 1.3-fold variation in any symptoms for 1214-year-old children across the UK, without a north-south or east-west gradient.8 However, Wieringa et al. reported an unexplained up to 2.7-fold variation across six neighbouring centres in the Netherlands and Germany in young adults.26 This potential variation needs to be considered when designing studies.
The current study was initiated in 1993 in response to concerns expressed by local community groups about a possible link between living close to opencast coal mining sites and respiratory ill-health, especially asthma. At the time, no direct evidence existed to suggest an association between opencast mining operations and the prevalence of asthma. However, contemporary time series studies were reporting associations between the daily variation in particulate matter and health outcomes, providing indirect evidence for a possible association between a dust producing industry and respiratory health. The study as a whole was designed to compare the chronic and acute morbidity of children living in non-urban communities near to and some distance from active opencast sites. Its aims were: to characterize and compare their exposure to particulate matter; and to assess the link between morbidity and exposure (for details of study design see27). This paper presents the findings on parent's reports on their children's current and past ill-health, and on social and lifestyle characteristics known to influence health. It covers a wide range of general and respiratory health outcomes, including lifetime prevalence and 2- and 12-months period prevalence.
Methods
Study subjects
The study was based in five pairs of rural and semi-urban communities (or parts thereof) in Northern England, varying in population size between 2000 and 20 000. The communities were matched for socioeconomic characteristics and geographical features: five were close to operational opencast sites and five paired control communities were some distance away. The criteria for the selection of sites were: (1) Distance between centre of the community and the site boundary less than 750 m; (2) No intervening dust source (i.e. major road, other industry); (3) Prevailing wind direction from the site to the community; (4) Site active at time of study and (5) Population size of community at least 2000, equivalent to 300 children.27 Once an opencast community was chosen the identification of matching control communities involved the use of routinely collected Census information on housing tenure, unemployment and car ownership, consultation with local authorities and site visits. We identified all 4860 children aged 111 years resident within specified geographical areas and registered with a General Practitioner (GP): 2443 in five communities near active opencast sites, 2417 in socioeconomically matched control communities within the same district.
Exposure assessment
For the questionnaire survey, exposure was defined as currently living near an active opencast site. The duration of exposure varied with the age of the children and with the length of activity at the opencast sites (from 6 months to many years prior to the study) (Table 1). Particulate matter (PM10) monitoring was also carried out in the communities and is reported elsewhere.28
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Questionnaires were mailed with or without GP letters of support, and two reminders were posted where appropriate after 2 and 4 weeks. The survey dates are shown in Table 1. Communities in Pair 1 could not be compared on very recent prevalences (2 months) because the surveys were done at different times of year.
Statistical analysis
Results are presented as proportions of those in each community with a particular characteristic. Comparisons were made between and within pairs. The extent to which there was a difference between opencast and control communities across all pairs was estimated by the odds ratio (OR) and associated 99% CI. A 99% CI was chosen to partly compensate for the large numbers of comparisons being made. Logistic regression fitted models with terms for residence in an opencast community (yes/no) and pair (1,2,3,4,5) and previously defined covariates. These were: sex, number of people in the household, population stability, smokers in the household, presence of moulting pets, use of polluting fuels to heat or cook, damp problems, housing tenure, unemployment, child's age, family history of asthma, eczema or hayfever and propensity of parent to worry. Season and temperature were not included in the model because their effect was controlled at the design stage by using matched pairs of communities. If a significant interaction between pair and residence in an opencast community was detected, OR were estimated separately for similar groups of pairs.
Results
In all, 89 GP practices were approached to provide a covering letter, of which 56% gave their support. Overall, 82% of children received letters of GP support: 74% in opencast and 90% in control communities. Parents of 1639 children in opencast communities and 1577 children in control communities returned the questionnaire (response rate 69% and 68%). The very similar response rates in the two groups suggests that any response bias as a result of the presence or absence of a GP letter was unlikely. Questionnaire respondents in opencast and control communities were, overall, well matched for factors linked with the occurrence of respiratory illnesses. Table 2 shows that the characteristics of children in the opencast and control communities were very similar, except for the greater use of polluting cooking fuels (largely mains gas). Nevertheless, any comparisons between opencast and control communities were adjusted for demographic and lifestyle factors.
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Period prevalence over the past 12 months
Results on the period prevalence fall into two groups: those where little variation was found between communities, and those where considerable variation was found (Table 4). Little variation was found for asthma, wheezing attacks, whether an asthma attack had woken the child at night, limited the child's speech, or occurred on exercise. A comparison of prevalence over the last year with the cumulative prevalence (in Table 3
), showed that Pair 4 communities reported period prevalences similar to other pairs, but their cumulative prevalence estimates were lower than other pairs. The use of either reliever or preventer medication for asthma varied between 13% in Control Community 4 to 30% in Control Community 1. The proportion of those who had many wheezing attacks varied between 3% and 21%. There was little evidence overall of an association between living in an opencast community and prevalence of respiratory ill health over the last year. For the outcome asthma reliever or preventer medication' there was evidence of interaction between community pair and proximity to an opencast mining site (P = 0.006). It was therefore necessary to estimate the OR separately for each pair of communities for this outcome, where a negative association was found for Pair 1 only.
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For the interpretation of the findings of the current study it is useful to compare prevalence of ill-health with recent studies in the region. This allows consideration of whether the non-urban communities near and away from opencast mining sites were typical. There were a number of studies of children's health from the North East of England (Table 6), which included questions on general and respiratory health comparable to the current study.8,15,16,18,3034 Some of these were local surveys, others were regional components of national surveys. The response rate of just under 70% was higher than similar previous studies in the same region which had sent questionnaires directly to homes,15,16,18 but lower than a school-based study, which had a response rate of 80%.34
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A minority of outcomes showed considerable variation between the ten communities. There was little evidence of a consistent pattern, but some variation between community pairs could be due to differing affluence: that within pairs remains largely unexplained. The variation within community pairs was unexpected given the close match for socioeconomic factors, which was one of the main design features of this study. A difference in diagnostic labelling may be the explanation in some cases. For instance, in the area of Pair 4, the reported rates of asthma (12% and 14%) were much lower than the overall average (21%), whereas the reported prevalence of wheezing was in line with the study average. Considerable, and largely unexplained, levels of variation in asthma prevalence across small geographical areas have been reported for adults in the EC Respiratory Health Survey (ECRHS)25 and from the Netherlands,26 and for children in the former East and West of Germany.35
The current questionnaire survey provided information on lifestyle and parent-reported general and respiratory health. To make an unbiased comparison between the health of children in opencast and control communities, it was necessary that the populations within each of the five study pairs were comparable on factors associated with respiratory morbidity, such as lifestyle and demographic variables. It has been shown that opencast and control communities were generally similar in terms of these factors. The level of smoking in the households, the proportion of moulting pets and the proportion of households using polluting heating methods were all found to be similar. However, higher levels of damp and use of polluting cooking fuels were found in control communities. Children in opencast communities were slightly less likely to have lived at the present address for most of their life. It might be speculated that traditional mining areas, where opencasting now takes place, have gone through a period of restructuring which has sometimes led to new housing being built, and hence an influx of new residents.
The statistical analysis found little evidence for an association between living near an opencast site and asthma or wheeze but did not exclude a small effect in either direction. A similar picture emerged for other respiratory illnesses and regular respiratory symptoms (data not shown). No significant associations were found between living in an opencast community and the period prevalence of respiratory symptoms over the past 12 and past 2 months. No evidence was found to support the hypothesis that asthmatic children in opencast communities had more numerous or more severe asthma attacks than children in control communities. Anecdotal evidence from residents and GPs near opencast sites had suggested that asthmatic adults and children experienced a worsening of their condition. In a recent public inquiry it was argued that either anxiety or dust levels had worsened the asthma of an adult asthmatic.36 It is possible that the current results failed to support the view that living near opencast sites exacerbated asthma because children are less likely to be influenced by anxiety about opencast activity. On the other hand, a recent report indicated that asthma severity was not an indicator for sensitivity to air pollution.37
Exposure was assessed in this part of the study as current residential proximity to an opencast mining site. However, PM10 concentrations recorded shortly after the surveys were found to be 17.0 µg/m3 in opencast communities and 14.9 µg/m3 in control communities (geometric mean) over the 6-week monitoring periods in each pair of communities during 19961997.28 The PM10 concentrations were only slightly but significantly higher in opencast communities. Shale, which is indicative of the geological layer from which coal is extracted, was found to be a measurable contributor to the additional PM10 load in opencast communities in this study. The cumulative exposure to particulates over the lifetime of children is not known. Opencast sites had been operational for different periods of time before the study took place, which complicated an assessment of lifetime exposure. Based on the small difference in daily PM10 concentrations measured during the study, it would appear not likely that opencast activity added a substantive amount to the lifetime exposure. However, we cannot exclude that there have been times in the past when the differences between PM10 concentrations in opencast and control communities were larger.
The present study put great emphasis in its design and data collection on comparing children of similar socioeconomic and family background and lifestyle. Overall, this was well achieved and therefore strengthens any findings made. The two previous studies of the effect of opencast mining and coal dust on the prevalence of symptoms in children were only able to deal with confounding by socioeconomic factors at the analysis stage.20,38 The Liverpool study investigated proximity of children's schools to coal loading areas: the exposure was to coal dust, whereas exposure to opencast activity is likely to be dominated by shale particles. It reported a cumulative prevalence of ever-diagnosed asthma of 17% and 15.3% in the two control populations and 21.3% in the exposed population. They reported OR between proximity to coal loading and the cumulative prevalence of wheeze (OR = 1.37, 95% CI : 0.981.91), school absence due to cough, wheezing or breathlessness (OR = 1.59, 95% CI : 1.232.04) and ever diagnosed asthma (OR = 1.25, 95% CI : 0.881.76).38
The South Wales study involved three communities, one urban, one rural and one rural near an opencast site. They reported the cumulative prevalence of self-reported asthma of 21.3% (urban), 23.4% (rural near opencast) and 13.0% (rural), but no difference in the prevalence of respiratory symptoms. The cumulative asthma prevalence in West Glamorgan was similar to those found in this study, but no other data were collected which would allow a direct comparison. Lyons et al. interpreted the discrepancy between the different health outcomes as being caused by differences in doctor's diagnostic criteria, and consequently whether or not children's parents had been told that their child had asthma. They did not report the number of GPs the children were registered with, but given the relatively few children in the Glamorgan study from three small communities, this is a plausible explanation.20 The importance of making comparisons between populations near and further away from opencast sites in a number of communities (10 communities in 5 pairs in the present study), has been highlighted by finding unexplained variations between communities in some health outcomes which did not follow any discernible pattern. This design allows a comparison between opencast and control communities which is averaged over these unknown factors.
The logistic regression modelling investigated the possibility that the strength of the association between each outcome and proximity to opencast mining varied between pairs of communities. This could arise from variation between geographical features of the communities and/or the length of time the mine had been active. When such variation occurred, the OR were reported separately for each pair of communities, rather than averaged across all pairs. An interaction was found for only two outcomes: lifetime prevalence of tonsillitis and usage of asthma medication. Since these patterns were not found consistently across all outcomes, these may be chance findings.
While there are currently no long-term studies available on the association between air pollution levels and health in children, numerous studies have reported that associations between air pollutants and symptoms are stronger among children with pre-existing hyperactive airways, starting with the work by Dockery et al. in 1989.39 Air pollutants are therefore likely to lead to exacerbation of symptoms in those already suffering from asthma. The data from the current study, however, did not indicate that asthmatic children in opencast communities were likely to have suffered more frequent or more severe asthma symptoms in the past 12 months. It is possible that the difference in exposure to PM10 levels was too small to lead to a detectable difference in symptoms.
The overall picture emerging from the questionnaire survey can be summarized as follows: (1) the prevalence of respiratory symptoms was similar to those of other relevant studies; (2) considerable unexplained variation was observed for some outcomes without a discernible explanation; (3) there was little evidence for an association between the prevalence of asthma, wheeze, bronchitis, other respiratory illnesses, and proximity to opencast mining sites and (4) there was no evidence for an increase in the severity of asthma or an increase in the frequency of asthma attacks amongst asthmatic children living near opencast mining sites.
Acknowledgments
Maxine Craven did fieldwork during the surveys; Jacqui Tate contributed to the design and fieldwork until 1996; Susanne Young helped to co-ordinate the study and did part of the descriptive analysis. Funding was received from Local Authorities in Northumberland, Durham and Sunderland (May 1994December 1994) (Pilot Study), Northern and Yorkshire Regional R+D grant No. 94070020 (December 1994November 1996), DOH/DETR/MRC grant No. AIR/96/9 (August 1996 February 1999).
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