Elsevier

The Lancet

Volume 359, Issue 9304, 2 February 2002, Pages 386-391
The Lancet

Articles
Asthma in exercising children exposed to ozone: a cohort study

https://doi.org/10.1016/S0140-6736(02)07597-9Get rights and content

Summary

Background

Little is known about the effect of exposure to air pollution during exercise or time spent outdoors on the development of asthma. We investigated the relation between newly-diagnosed asthma and team sports in a cohort of children exposed to different concentrations and mixtures of air pollutants.

Methods

3535 children with no history of asthma were recruited from schools in 12 communities in southern California and were followed up for up to 5 years. 265 children reported a new diagnosis of asthma during follow-up. We assessed risk of asthma in children playing team sports at study entry in six communities with high daytime ozone concentrations, six with lower concentrations, and in communities with high or low concentrations of nitrogen dioxide, particulate matter, and inorganic-acid vapour.

Findings

In communities with high ozone concentrations, the relative risk of developing asthma in children playing three or more sports was 3·3 (95% CI 1·9–5·8), compared with children playing no sports. Sports had no effect in areas of low ozone concentration (0·8, 0·4–1·6). Time spent outside was associated with a higher incidence of asthma in areas of high ozone (1·4, 1·0–2·1), but not in areas of low ozone. Exposure to pollutants other than ozone did not alter the effect of team sports.

Interpretation

Incidence of new diagnoses of asthma is associated with heavy exercise in communities with high concentrations of ozone, thus, air pollution and outdoor exercise could contribute to the development of asthma in children.

Introduction

Asthma is the most common chronic disease of childhood; asthma prevalence and incidence have been increasing in children in developed countries during the past few decades.1, 2 Causes for this epidemic are unknown, although changes in frequency and severity of early-life infections, diet, and exposure to indoor allergens and to indoor and outdoor air pollutants have all been linked with asthma.

Cross-sectional studies3, 4, 5 have shown that competitive athletes have a high prevalence of asthma and exercise-induced bronchospasm or bronchial hyper-reactivity. Possible mechanisms for this association include increased inhalation of cold air, allergens, or air pollutants, increased response to respiratory infections, and increased parasympathetic tone.3, 4, 5, 6 Various mechanisms could be linked with sports-associated asthma. However, few epidemiological investigations have all been done, and there have been no prospective studies of asthma in competitive athletes or children playing team sports.5

Acute exposure to ozone and other outdoor air pollutants exacerbates asthma;7 the chronic effects of air pollution have been less studied, but combustion-related air pollution is not thought to cause asthma.8 However, this conclusion is based on studies in which personal exposure was measured with community air pollution monitors. The true dose of outdoor air pollutants to the lung depends on local pollutant concentrations, which may vary within a community, and on personal habits such as time spent outside and physical activity. People exercising outside should receive greater doses of outdoor pollutants to the lung than those who do not, and thus be more susceptible to any chronic effects of air pollution. Because the onset of asthma might cause athletes to reduce their levels of exercise, cross-sectional studies are not an appropriate way to measure the causal relations between exercise, air pollution, and asthma.

We postulated that children engaged in team sports in polluted communities might also be at high risk of asthma. Because the amount of time spent playing sports is an individual factor that affects exposure to ambient pollution, this approach avoids many biases of studies of air pollution that have relied on between-community comparisons of rates of asthma and other illnesses. We assessed the association of playing team sports with subsequent development of asthma during 5 years of follow-up of participants in the Southern California Children's Health Study. Study communities were selected on the basis of concentrations of ambient ozone and other pollutants.9

Section snippets

Participants

We selected 12 communities in southern California for variability of concentrations of ozone, particles with aerodynamic diameter less than 10 μm (PM10), and nitrogen dioxide (NO2).9 In 1993, in each of the 12 communities, we recruited around 150 children aged 9–10 years, 75 aged 12–13 years, and 75 aged 15–16 years from schools in areas of the communities with stable, mainly middle-income populations. All children from targeted classrooms were invited to take part; participants completed a

Results

5762 (79%) of eligible children completed baseline questionnaires. 479 children were excluded because they were not at school at the time of a questionnaire administered during the entry year by an interviewer, and an additional 883 were excluded for a history of asthma. We excluded 312 children because of missing or “not known” answers to questions on wheezing, and 26 for a history of cystic fibrosis, severe chest injury, or chest surgery. 527 additional children were excluded who did not have

Discussion

Our results show that playing multiple team sports in a high ozone environment is associated with development of physician-diagnosed asthma. The results are consistent with a large increased risk both for new-onset asthma and for exacerbation of previously undiagnosed asthma, because playing multiple sports was associated with asthma in children with no lifetime history of wheezing at baseline and children with a previous history of wheezing. The larger effect of high activity sports than low

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