Articles
Cooking fuels and prevalence of asthma: a global analysis of phase three of the International Study of Asthma and Allergies in Childhood (ISAAC)

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Summary

Background

Indoor air pollution from a range of household cooking fuels has been implicated in the development and exacerbation of respiratory diseases. In both rich and poor countries, the effects of cooking fuels on asthma and allergies in childhood are unclear. We investigated the association between asthma and the use of a range of cooking fuels around the world.

Methods

For phase three of the International Study of Asthma and Allergies in Childhood (ISAAC), written questionnaires were self-completed at school by secondary school students aged 13–14 years, 244 734 (78%) of whom were then shown a video questionnaire on wheezing symptoms. Parents of children aged 6–7 years completed the written questionnaire at home. We investigated the association between types of cooking fuels and symptoms of asthma using logistic regression. Adjustments were made for sex, region of the world, language, gross national income, maternal education, parental smoking, and six other subject-specific covariates. The ISAAC study is now closed, but researchers can continue to use the instruments for further research.

Findings

Data were collected between 1999 and 2004. 512 707 primary and secondary school children from 108 centres in 47 countries were included in the analysis. The use of an open fire for cooking was associated with an increased risk of symptoms of asthma and reported asthma in both children aged 6–7 years (odds ratio [OR] for wheeze in the past year, 1·78, 95% CI 1·51–2·10) and those aged 13–14 years (OR 1·20, 95% CI 1·06–1·37). In the final multivariate analyses, ORs for wheeze in the past year and the use of solely an open fire for cooking were 2·17 (95% CI 1·64–2·87) for children aged 6–7 years and 1·35 (1·11–1·64) for children aged 13–14 years. Odds ratios for wheeze in the past year and the use of open fire in combination with other fuels for cooking were 1·51 (1·25–1·81 for children aged 6–7 years and 1·35 (1·15–1·58) for those aged 13–14 years. In both age groups, we detected no evidence of an association between the use of gas as a cooking fuel and either asthma symptoms or asthma diagnosis.

Interpretation

The use of open fires for cooking is associated with an increased risk of symptoms of asthma and of asthma diagnosis in children. Because a large percentage of the world population uses open fires for cooking, this method of cooking might be an important modifiable risk factor if the association is proven to be causal.

Funding

BUPA Foundation, the Auckland Medical Research Foundation, the Health Research Council of New Zealand, the Asthma and Respiratory Foundation of New Zealand, the Child Health Research Foundation, the Hawke's Bay Medical Research Foundation, the Waikato Medical Research Foundation, Glaxo Wellcome New Zealand, the NZ Lottery Board, Astra Zeneca New Zealand, Hong Kong Research Grant Council, Glaxo Wellcome International Medical Affairs.

Introduction

Despite much research, little is known about the cause of asthma. The international study of asthma and allergies in childhood (ISAAC) has documented a wide variation in asthma prevalence across the world and has also detected evidence of a continuing increase, especially in low-income and middle-income countries.1, 2 The possible role of air pollution in the development of respiratory diseases is a major focus of research. Several studies have investigated the association between indoor air pollution and asthma and chronic obstructive pulmonary disorder (COPD).3, 4 In high-income countries, the use of gas appliances for cooking has been implicated as a cause of respiratory symptoms, particularly in women.5 The use of gas as cooking fuel has also been implicated as one of the factors that might explain the higher asthma prevalence in Chinese children in Hong Kong compared with children in other Chinese cities.6 However, results from the European community respiratory health survey of more than 10 000 respondents did not show any relation between the use of gas for cooking and obstructive respiratory symptoms.7

Exposures to domestic fire burning of coal and biomass such as wood, animal dung, and crop residues for cooking or heating are widespread, especially in rural areas of poor countries. According to WHO, at least half the world's population live in households in which solid fuels or biomass are the primary fuel for cooking, heating, or both.8, 9 In resource-poor countries, cooking with biomass is typically done on unvented stoves without any form of ventilation system.10 In India, biomass burning has been shown to be associated with increased respiratory symptoms in children.11 A nationwide study in India showed that exposure to the combustion of biomass and solid fuels was associated with an increased risk of asthma in women.12 A study of 508 adults in the USA also showed a positive association between asthma and exposure to cooking indoors with wood and coal.13 WHO estimated that indoor air pollution from the burning of biomass causes almost 2 million deaths annually.8 Because the burning of biomass fuel or the use of gas for cooking are potentially modifiable factors, the study of their relation with asthma and wheezing illnesses in children is important.

Many studies of the association between cooking fuel and asthma have been of low statistical power. Furthermore, estimation of the individual exposure presents a major challenge because the proximity to the sources of exposure, the duration of exposure, and accurate assessment of ventilation are not easily quantifiable in large studies. The existing evidence about the association between household air pollution from biomass burning and asthma is conflicting, with more consistent positive associations in children than in adults.14, 15, 16, 17, 18 We investigated the relation between asthma and the use of a range of cooking fuels in study centres around the world. Using standardised methods, phase one of ISAAC documented large variations in asthma prevalence across the world. Phase two included objective measurements including skin-prick test and bronchial challenge test, providing further support of the importance of environmental factors in the development of asthma. The results reported here are based on a detailed environmental questionnaire administered to children in 47 countries to test different cause hypotheses of asthma as part of the phase three ISAAC study.

Section snippets

Study design

ISAAC phase three is an expansion using the same study design of the first phase of ISAAC, findings from which showed a wide variation in the prevalence of childhood asthma and related atopic disorders across the world.1, 2, 19 The details of the study protocol are available elsewhere.2, 19 Briefly, written questionnaires were self-completed at school by secondary school students aged 13–14 years who were then, in most centres, shown a video questionnaire on wheezing symptoms. 244 734 (78%)

Results

Data were collected between 1999 and 2004. In the initial statistical models, there were 198 398 children aged 6–7 years from 70 centres in 29 countries (figure 1) and 314 309 children aged 13–14 years from 108 centres in 47 countries (figure 2). Table 1, Table 2 show the distribution of the use of different types of fuel for cooking by region for the two age groups (see appendix for the prevalence rates of the various health outcomes in relation to the use of different types of cooking fuel in

Discussion

The findings from this large multicentre survey show that the use of open fires for cooking is associated with symptoms of asthma and ever reported asthma in school children of two age groups: 6–7 years and 13–14 years. The associations were consistent between sexes. Furthermore, the associations were similar using three different validated methods to assess the symptoms of current wheeze or ever reported asthma (self-completed written questionnaire and video questionnaires for children aged

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