Population and Health Studies, East-West Center, 1601 East-West Road, Honolulu, HI 968481601, USA. E-mail: MishraV{at}EastWestCenter.Org
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Abstract |
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Methods Analysis is based on 3559 children age 059 months included in the 1999 Zimbabwe Demographic and Health Survey (ZDHS). Children who suffered from cough accompanied by short, rapid breathing during the 2 weeks preceding the survey were defined as having suffered from ARI. Logistic regression was used to estimate the odds of suffering from ARI among children from households using biomass fuels (wood, dung, or straw) relative to children from households using cleaner fuels (liquid petroleum gas [LPG]/natural gas, or electricity), after controlling for potentially confounding factors.
Results About two-thirds (66%) of children lived in households using biomass fuels and 16% suffered from ARI during the 2 weeks preceding the survey interview. After adjusting for childs age, sex, birth order, nutritional status, mothers age at childbirth, education, religion, household living standard, and region of residence, 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).
Conclusions Household use of high pollution biomass fuels is associated with ARI in children in Zimbabwe. The relationship needs to be further investigated using more direct measures of smoke exposure and clinical measures of ARI.
Accepted 14 May 2003
Acute respiratory infections (ARI) are a leading cause of childhood illness and death worldwide, accounting for an estimated 6.5% of the entire global burden of disease.1 In Zimbabwe, as in many other developing countries, ARI are the leading cause of childhood mortality.2 This study examines the association between household use of high pollution biomass fuels (wood, dung, or straw) and ARI prevalence in children in Zimbabwe using data from a recent nationally representative household survey.
Biomass fuels are at the low end of the energy ladder in terms of combustion efficiency and cleanliness.3 Smoke from biomass combustion produces a large number of health-damaging air pollutants including respirable particulate matter, carbon monoxide (CO), nitrogen oxides, formaldehyde, benzene, 1,3 butadiene, polycyclic aromatic hydrocarbons (such as benzo[a]pyrene), and many other toxic organic compounds. 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.4 The fuels are typically burned in simple, inefficient, and mostly unvented household cookstoves, which, combined with poor ventilation, generate large volumes of smoke indoors. Moreover, cookstoves are typically used for several hours each day at times when people are present indoors, resulting in much higher exposure to air pollutants than from outdoor sources.5
In such settings, daily average and peak exposures to air pollutants often far exceed safe levels recommended by the World Health Organization.6 A comparison of typical levels of carbon monoxide (CO), particulate matter 10 µm (PM10), and
2.5 µm (PM2.5) in developing-country homes using biomass fuels with the US Environmental Protection Agencys standards for 24-hour average concluded that indoor concentrations of these pollutants in biomass-fuel-using developing-country homes usually exceed the guideline levels several-fold.4 Exposure levels are usually much higher among women who tend to do most of the cooking7 and among young children who stay indoors and who are often carried on their mothers back or lap while cooking.8
A recent baseline survey conducted in two districts of Zimbabwe found that women and young children spend an average of 5 hours per day in the kitchen area, where air pollution levels from biomass fuel combustion for cooking tend to be very high. The measured levels of CO in the kitchen were in the range of 3001000 p.p.m.9
Exposure to biomass smoke has been strongly associated with ARI in preschool age children1014 and with chronic obstructive pulmonary disease (COPD) or chronic bronchitis in women.1521 However, a few studies of preschool age children2223 and several studies of school age children2426 have failed to find a relationship between biomass smoke and ARI. For a comprehensive review of studies on indoor air pollution and ARI see Smith et al.27 Biomass smoke has also been associated with other major respiratory diseases including tuberculosis2830 and asthma,3136 but the evidence for tuberculosis is limited and the evidence for asthma is conflicting.
An earlier study in Zimbabwe compared 244 children under age 3 who visited a hospital with lower respiratory disease with 500 children of similar nutritional and socioeconomic background reported to a local Well Baby Clinic and found a significant association between the presence of woodsmoke pollution in the house and lower respiratory disease.37 This hospital-based study of 244 ARI cases is the only study to date that has tried to link biomass smoke with ARI in children in Zimbabwe.
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Methods |
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For each child under age 5, the mother was asked if the child had been ill with a cough in the 2-week period preceding the survey interview. For children who had been ill with cough in the last 2 weeks, the mother was additionally asked if the child, when ill with cough, breathed faster than usual with short, rapid breaths. Children who suffered from cough accompanied by short and rapid breathing at any time during the last 2 weeks are defined as having suffered from an acute respiratory infection. This reported prevalence of ARI is the response variable in our analysis.
Exposure to cooking smoke was ascertained indirectly by type of fuel used for cooking. The survey used an eightfold classification of main cooking fuelwood, dung, straw, charcoal, kerosene, electricity, liquid petroleum gas (LPG)/natural gas, and a residual category of other fuels. The question was, What type of fuel does your household mainly use for cooking? Information on fuel types was used to group households into three categories representing the extent of exposure to cooking smokehigh pollution fuels (wood, dung, or straw), medium pollution fuels (kerosene or charcoal), and low pollution fuels (LPG/natural gas or electricity). The small residual category of other fuels (0.1% of the sample) was excluded from the analysis due to unknown nature of fuels in that category.
A number of multivariate logistic regression models were estimated to assess the effects of cooking fuel type (representing exposure to cooking smoke) and other factors on ARI. The analysis was carried out using the GENMODE procedure in the SAS System.38 Control variables, identified in previous research as covariates of ARI, included age of child in months (05, 611, 1223, 2435, 3659), sex of child (boy, girl), birth order of child (1, 2, 3, 4+), nutritional status of child (stunted, not stunted), mothers age at childbirth (1524, 2534, 3549), mothers education in completed years of schooling (<3, 36, 6+), mothers religion (Christian, non-Christian), household standard of living (low, medium, high), and region of residence (Manicaland, Mashonaland Central, Mashonaland East, Mashonaland West, Matabeleland North, Matabeleland South, Midlands, Masvingo, Harare, Bulawayo). See footnotes to Table 1 for definitions of childs nutritional status and household standard of living variables.
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Human subjects informed consent
The analysis presented in this paper is based on secondary analysis of existing survey data with all identifying information removed. The survey obtained informed consent from each respondent (in this case, mothers of the children included in the analysis) before asking questions.
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Results |
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Sixteen per cent of children under age 5 suffered from an ARI during the 2 weeks preceding the survey. The reported prevalence of ARI was much higher among children living in biomass-fuel-using households (18%) than among those living in households using low pollution fuels (10%) (Table 1). Children aged 623 months were more likely to have suffered from ARI than children under 6 months of age or older children. This pattern of childhood disease rates peaking at 623 months is typical for ARI and diarrhoea in many developing countries. This may be partly due to start of supplementary feeding around 6 months of age, which increases the likelihood of consuming contaminated foods and removes the protection provided by breast milk. Also, children start crawling around this age and are more likely to be carried outdoors, which exposes them to infections. The disease rate typically declines as children grow older and start developing resistance.
Children from high standard of living households were considerably less likely to have had ARI (11%) than those from low or medium standard of living households (1718%). The prevalence of ARI is much lower in urban areas (11%) than in rural areas (18%). By region of residence, the prevalence of ARI ranged from Bulawayo (4%) and Matabeleland North (5%) at the low end to Midlands (23%) and Mashonaland Central (28%) at the high end. The pattern of ARI prevalence by region of residence is generally consistent with the patterns of diarrhoea prevalence and infant and child mortality rates in Zimbabwe, and may reflect differential access to health care and climatic conditions. Prevalence of ARI did not vary much by sex of child, birth order of child, nutritional status of child, mothers age at childbirth, mothers education, or mothers religion. Children of birth order 2 and 3, children of younger mothers (aged 1534), children of mothers with <3 years of education, and children of Christian mothers were somewhat more likely to have suffered from ARI than other children.
The unadjusted odds of having suffered from ARI are almost two times higher among children living in households using high pollution biomass fuels than among those living in households using low pollution LPG/natural gas or electricity for cooking (OR = 1.95, 95% CI: 1.40, 2.71) (Table 2, Model 1). As expected, children from households using medium pollution charcoal or kerosene also had higher ARI prevalence (OR = 1.58, 95% CI: 0.96, 2.71) than those from households using LPG/ natural gas or electricity. The effect of biomass fuel use on ARI remains large and statistically significant when childs age, sex, birth order, and nutritional status are controlled in Model 2 (OR = 2.13, 95% CI: 1.45, 3.12). Controlling for childs demographic and physical growth characteristics sharpens the effect of biomass fuel use to some extent. When mothers age at childbirth, mothers education, mothers religion, and household living standard are additionally controlled in Model 3, the effect of cooking with biomass fuels remains virtually unchangedlarge and statistically significant (OR = 2.12; 95% CI: 1.19, 3.77). In the full model (Model 4), when region of residence is additionally controlled, the effect of cooking with biomass fuels on ARI prevalence sharpens further, and remains large and statistically significant (OR = 2.20, 95% CI: 1.16, 4.19).
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Discussion |
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The mechanism by which cooking smoke can increase the risk of ARI is not fully understood. Exposure to biomass smoke has been associated with compromised pulmonary immune defence mechanisms.3943 Tobacco smoke also has been shown to cause depressed immune system responses.4448 Of the specific pollutants in biomass smoke, exposure to respirable particulate matter (PM10) has been shown to induce a systemic inflammatory response that includes stimulation of the bone marrow, which can contribute to the pathogenesis of the cardiorespiratory morbidity.4952 Other evidence indicates that exposure to polycyclic aromatic hydrocarbons (PAH)especially benzo[a]pyrene (B[a]P), which is found in large quantities in biomass smokecan cause immune suppression and can increase the risk of infection and disease.5355 Moreover, acute and long-term exposures to oxides of nitrogen, commonly found in biomass smoke, can increase bronchial reactivity and susceptibility to bacterial and viral infections.56,57 It is, therefore, possible that extended exposure to high levels of cooking smoke can impair the pulmonary defence mechanisms, compromise the lung function, and render children more susceptible to ARI.
Several measurement constraints should be kept in mind when considering the findings of this study. First, because the survey did not collect information on tobacco smoking by household members, we were unable to control for environmental tobacco smoke (ETS), which is a known risk factor for acute respiratory illness in children.58 Although data on mothers smoking were collected, maternal smoking is not a significant confounding factor because women rarely smoke in Zimbabwe (only 1.47% in the ZDHS sample).
Second, many households in Zimbabwe typically use a combination of cooking fuels, whereas we have information only on the primary cooking fuel. Our estimated effects are attenuated to the extent that a mix of biomass fuels and cleaner fuels is actually used by many households instead of biomass fuels alone.
Third, there is a possibility of some selection in the sample due to ARI-related mortality. To the extent that children living in poorer biomass-fuel-using households are more likely to die from ARI, our estimates of effect of cooking smoke on ARI are downwardly biased. However, given high prevalence of ARI and relatively small number of deaths in the sample, the impact of this bias on our estimated effect is likely to be small.
Fourth, there is also a possibility of underreporting of ARI due to lack of awareness that the child had the disease during the 2-week reference period. To the extent that underreporting due to lack of awareness is greater among those living in households using biomass fuels, it would contribute to underestimation of the effect of cooking smoke on the prevalence of ARI.
Fifth, we were unable to control directly for urban/rural residence in the analysis owing to its collinearity with cooking fuel type (r = 0.95). However, our analysis includes controls for measures of socioeconomic status (SES) such as childs nutritional status, mothers education, and household living standard, which may provide some gradient of exposure within urban and rural populations. Also, the analysis controls for residence in the 10 provinces of Zimbabwe, including two exclusively urban provinces of Harare and Bulawayo. We feel that controlling for SES and region adequately controls for potential confounding effects of urban/rural residence. Moreover, other similar studies of national survey data have found no statistically significant effects of urban/rural residence on respiratory health after controlling for SES, and in these studies controlling for urban/rural residence did not alter the effects of cooking fuel.28,31
Sixth, we were unable to control directly for crowding in the household because the ZDHS did not collect data on number of rooms in the household. Indoor crowding tends to be correlated with biomass fuel use and may affect the risk of ARI. However, indoor crowding is controlled in our analysis to the extent it is correlated with birth order of child and with various measures of SES included in the analysis.
Finally, we were also not able to control directly for extent of use of medical services, because the survey did not collect any information on this subject. However, our set of control variables includes measures of SES, which are correlated with access to and use of medical services.
As mentioned earlier, information on ARI is based on mothers reports and no clinical measurements were undertaken, and smoke exposure was ascertained from type of fuel used for cooking. Although the symptomatic definition used here is intended to measure acute lower respiratory infections (ALRI) in children, some acute upper respiratory illness may have been included in the reported prevalence. Because it is not possible to separate ALRI from these data, we use the term ARI in this study, not ALRI. In developing countries such as Zimbabwe, where clinical data on ARI are usually not available or very weak, the symptomatic definition of illness used here has been shown to provide a fairly accurate assessment of ARI in the population.59 Moreover, indoor air pollution measurements in several developing countries have shown fuel type to be the best single indirect indicator of household pollution levels.60 Despite these problems in the measurement of smoke exposure and ARI, the consistency in the size of crude and adjusted effects of biomass fuel use on childhood ARI suggests a possible exposureresponse relationship. To validate this relationship, our research needs to be followed by carefully designed epidemiological studies, with direct measures of smoke exposure and clinical measures of ARI. Such research is important because a large proportion of households in Zimbabwe and other developing countries rely on biomass fuels for household energy and ARI are a leading cause of ill health and death in young children.
Given heavy reliance on biomass fuels for household energy and high prevalence of ARI in children in Zimbabwe, the findings of this study have important implications for public health programmes and policy. The most important intervention to reduce exposure to indoor air pollution is to promote widespread use of cleaner fuels, such as LPG and electricity. However, given that many poor households in Zimbabwe that currently rely on biomass fuels are unlikely to be able to afford cleaner fuels any time soon, and given poor infrastructure to supply cleaner fuels to most rural households, widespread adoption of cleaner fuels is unlikely to occur in the short term. Therefore, the efforts need to focus on providing improved cookstoves designed to reduce exposure to smoke by means of improved combustion and improved venting, and designing public information campaigns to inform people about the health risks of exposure to indoor smoke. For such programmes to be effective, local needs and community participation should receive high priority.
KEY MESSAGES
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Acknowledgments |
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References |
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