Original ReportMaternal exposure to biomass smoke and reduced birth weight in Zimbabwe☆
Introduction
A recent study based on 1717 women and their newborn children in rural Guatemala reported an association between household use of wood fuels and reduced birth weight (1). The study found that babies born to mothers using wood fuels were 63 grams lighter, on average, than those born to mothers using gas or electricity, after adjustment for a number of socioeconomic and maternal factors. This study provided a first report of an association between biofuel use and reduced birth weight in a human population. To examine this relationship further, we designed a similar study based on data from a recent national household survey in Zimbabwe.
The 1999 Zimbabwe Demographic and Health Survey (ZDHS) collected information on birth weight for children born during the 5 years preceding the survey (2). Unlike most developing-country situations where a large majority of deliveries take place at home and only a small proportion of children are weighed at birth, in Zimbabwe, three-quarters of children are born in a health facility and weighed at birth. The ZDHS, a nationally representative probability sample, also asked about household use of cooking fuels, which is the best single indicator of exposure to smoke indoors.
Smoke from biomass combustion produces a large number of health-damaging air pollutants including respirable particulate matter, carbon monoxide, nitrogen oxides, formaldehyde, benzene, 1,3 butadiene, polycyclic aromatic hydrocarbons (such as benzo[a]pyrene), and many other toxic organic compounds (3). In developing countries, where large proportions of households rely on biomass fuels for cooking and space heating, concentrations of these air pollutants tend to be highest indoors. The fuels are typically burned in simple, inefficient, and mostly unvented household cookstoves, which, combined with poor ventilation, generate large volumes of smoke indoors. In such settings, daily average and peak exposures to air pollutants often far exceed safe levels recommended by the World Health Organization 3., 4.. Exposure levels are usually much higher among women who tend to do most of the cooking (5).
A recent baseline survey in two districts of Zimbabwe found that women, on average, spend 5 hours per day in the kitchen area. The measured levels of CO in the kitchen were in the range of 300 to 1000 ppm and levels of PM10 were in the range of 1000 to 4000 μg/m3 (6).
Two previous studies in Zimbabwe have examined health effects of biomass smoke. One study compared 244 children under age 3 who visited a hospital with lower respiratory disease with 500 children of similar nutritional and socioeconomic background and found a significant association between the presence of woodsmoke pollution in the house and lower respiratory disease (7). A second study of 3559 children aged 0 to 59 months, included in the 1999 ZDHS, reported that children in households using wood, dung, or straw for cooking were more than twice as likely to have suffered from ARI as children from households using LPG, natural gas, or electricity (OR = 2.20; 95% CI, 1.16, 4.19), after statistically controlling for several confounding factors (8).
In other settings, biomass smoke exposure has been associated with a host of respiratory diseases (9), but little is known about its effects on pregnancy outcomes. The Rural Guatemala study is the only study that seems to have reported an association between wood fuel use and reduced birth weight (1), although an Indian study reported significantly increased risk of stillbirths among women cooking with biomass fuels during pregnancy (10).
In addition to the large literature on active smoking, studies have linked exposure to passive tobacco smoke to fetal growth and adverse pregnancy outcomes, including low birth weight 11., 12., 13., 14., 15., 16.. Although there are differences, in nicotine content, for example, the incomplete combustion products thought to be responsible for most of the health effects in tobacco smoke are also found in smoke from other forms of biomass combustion, such as wood or dung. The concentrations found in typical household cooking situation, however, are often much higher than found in passive smoking studies, although much lower than active smoking (17).
Exposure to CO has been associated with fetal development and adverse pregnancy outcomes including reduced birth weight 18., 19., 20.. Levels of CO in homes using biomass fuels are sometimes high enough to result in carboxyhemoglobin levels comparable to those in smokers 5., 21..
In recent years, an increasing number of studies have found an association between adverse pregnancy outcomes, including intrauterine growth retardation and reduced birth weight, and maternal exposure to outdoor air pollution at levels of pollution substantially lower than found in biomass-burning homes 22., 23., 24., 25., 26., 27..
Section snippets
Methods and materials
The analysis is based on 3559 childbirths during the 5 years preceding the ZDHS. The survey collected demographic, socioeconomic, and health information from a nationally representative probability sample of 6369 households, representing all 10 provinces. The sample is a two-stage cluster sample with an overall response rate of 97.8%. In the first stage, 230 enumeration areas were selected with equal probability. In the second, households were selected using the probability proportional to size
Results
Table 1 shows the sample distribution of births in the 5 years preceding the survey by whether they were weighed at birth, cooking fuel, and other variables. Seventy-eight percent of all children born during the 5 years preceding the survey were weighed at birth. Children who were not weighed at birth are considerably more likely to be from biomass-fuel-using households compared with those who were weighed at birth (91% compared with 59%, respectively). This differential in the likelihood of
Discussion
Results suggest that exposure to cooking smoke from biomass combustion is associated with reduced birth weight, independent of child's sex, birth order, mother's nutritional status, pregnancy care, mother's education, household living standard, and other factors. These results are consistent with the earlier Guatemalan study linking wood fuels to reduced birth weight (1), and provide further evidence that cooking with high pollution unprocessed biomass fuels can increase the risk of adverse
Acknowledgements
The authors thank Nigel Bruce, Eva Rehfuess, and three anonymous reviewers for useful comments on an earlier draft of this manuscript.
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The analysis presented in this article is based on secondary analysis of existing survey data with all identifying information removed. The survey obtained informed consent from each respondent before asking questions. The study was supported in part by a grant from the National Institute of Child Health and Human Development (#1 R03 HD043929-01). An earlier version of this article was presented at the 131st American Public Health Association Annual Meeting in San Francisco, November 15–19, 2003.