Our findings are based on 18,554 adults from 25 sites who participated in the large population-based study BOLD (Burden of Obstructive Lung Disease) and had acceptable post-bronchodilator spirometry. We found that in adults, from low-/middle- and high-income countries, airflow obstruction was not associated with self-reported use of solid fuels for cooking or heating. This finding brings into question the extent to which high mortality rates attributed to COPD in low-income countries, where consumption of cigarettes is relatively low, are explained by use of solid fuels for cooking or heating.

Five systematic reviews, published between 2010 and 2014, reported that adults exposed to the burning of solid fuels were more likely to have chronic obstructive pulmonary disease (COPD) compared to those not exposed to this type of indoor pollution. However, these reviews suffered from some degree of publication bias and high heterogeneity across studies. Moreover, the diagnosis of COPD in many of the studies was not based on post-bronchodilator spirometry. More recent and larger studies failed to replicate the findings of the systematic reviews published so far. Overall, the evidence of an association between COPD and use of solid fuels for cooking or heating is conflicting and inconsistent.

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death worldwide and is particularly common in low-income countries (1). The most important single risk factor for COPD is cigarette smoking (2, 3). However, cigarette smoking is still uncommon in many low-income countries and >20% of people with this disease do not have a history of smoking (4, 5). Exposure to household air pollution from solid fuel burning for domestic purposes has been put forward to explain high COPD mortality, especially among nonsmokers and where the use of solid fuels for cooking or heating is widespread (5).

Five systematic reviews, published before 2015, reported an overall 1.9- to 2.8-fold increased risk for COPD in adults exposed, as compared with those not exposed, to solid fuel burning (6–10). In three of these reviews, the authors acknowledged evidence of publication bias toward the reporting of positive findings. These reviews also demonstrated very high levels of heterogeneity across studies, indicating either residual confounding or strong effect modification. A study performed on over 300,000 never-smokers from the China Kadoorie Biobank reported that airflow obstruction (a principal COPD feature) was positively associated with cooking with coal, but not with other types of fuel and only among women (11). Other studies have also reported differences between men and women in the effects of solid fuel burning both for cooking (12), and heating (13). An earlier report from the BOLD (Burden of Obstructive Lung Disease) study, mostly undertaken in high-income countries, also failed to show an association between airflow obstruction and use of solid fuel (14). Results from trials of solid fuel use reduction are so far inconclusive in relation to the effects on lung function (15, 16). Overall, the evidence supporting an association of COPD (or airflow obstruction) with use of solid fuels for cooking or heating is conflicting and inconsistent.

The main aim of the present analysis was to assess the association of airflow obstruction with self-reported use of open fires burning biomass, or coal, for cooking or heating in the large international, population-based BOLD study. In addition, we performed similar analyses for spirometric restriction, chronic cough, and chronic phlegm.

In sensitivity analyses, we: 1 ) restricted the main analysis to never-smokers; and 2 ) further examined the association of each outcome with use of solid fuels for cooking. These further analyses were stratified by fuel (“charcoal, coal, or coke” or “wood, crop residues, or dung”), use of solid fuels for less than 20 or 20 years or greater, by those usually spending greater than 1 h/d cooking, and by those with or without ventilation (the use of ventilation was assessed by asking whether the participant’s stove or fire was vented to the outside [e.g., through chimney or window]); 3 ) excluded participants with less than 10 years of use of solid fuels; and 4 ) used the Global Lung Function Initiative (GLI) 2012 multiethnic equations to calculate the LLN ( 24 ). In addition, we assessed the association of airflow obstruction with duration of use of solid fuels (per 10 yr of use).

We assessed, by sex, the association of airflow obstruction, spirometric restriction, chronic cough, and chronic phlegm with use of open fires burning solid fuels for cooking/heating using logistic regression models, which were adjusted for age (yr), body mass index (<18.5, 18.5 to <24, 24 to <30, and ≥30 kg/m 2 ), pack-years of smoking, and cumulative exposure to dust in the workplace (yr). The association of each outcome with use of solid fuels was estimated for each site using probability weights to allow for the sampling design ( 21 ), and then combined in a random-effects meta-analysis stratified by gross national income (low-/middle- vs. high-income countries) ( 22 ). The level of heterogeneity was summarized using the I 2 statistic ( 23 ). We also regressed FEV 1 /FVC (%) and FVC (L) as continuous variables against the same independent variables.

Participants were considered to have: 1 ) chronic cough if they answered “yes” to both “Do you usually cough when you don’t have a cold?” and “Do you cough on most days for as much as three months each year?”; and 2 ) chronic phlegm if they answered “yes” to both “Do you usually bring up phlegm from your chest, or do you usually have phlegm in your chest that is difficult to bring up when you don’t have a cold?” and “Do you bring up this phlegm on most days for as much as 3 months each year?”

Lung function was assessed by spirometry technicians who were certified before data collection, received regular feedback on quality, and were required to maintain a prespecified quality standard. FEV 1 and FVC were measured using the ndd EasyOne Spirometer (ndd Medizintechnik AG) before and 15 minutes after administration of salbutamol (200 μg) from a metered-dose inhaler through a spacer. Each spirogram was centrally reviewed and scored based on the American Thoracic Society and European Respiratory Society acceptability and reproducibility criteria ( 18 ). We defined: 1 ) airflow obstruction as a post-bronchodilator FEV 1 /FVC less than the lower limit of normal (LLN) ( 19 ), based on reference equations for white individuals from the third U.S. National Health and Nutrition Examination Survey (NHANES) ( 20 ); and 2 ) spirometric restriction as a post-bronchodilator FVC less than LLN, based on the same reference population.

The use of solid fuels was defined based on whether the participant had used an open fire with charcoal, coal, coke, wood, crop residues, or dung as the primary means of cooking or heating the house or water for greater than 6 months in their lifetime. Levels of exposure (years of use and hours per day spent cooking on an open fire) were also assessed.

The BOLD study design and rationale have been described elsewhere ( 17 ). Representative samples of adults aged 40 years or older were recruited from sites in low-, middle-, and high-income countries. Information on respiratory symptoms and exposure to risk factors was collected through face-to-face interviews conducted by trained and certified staff in the participant’s native language. Four sites did not use the questionnaire on use of open fires: Bergen (Norway); Hannover (Germany); Sydney (Australia); and Uppsala (Sweden). In the 29 remaining sites, 27,534 participants responded to the core questionnaire, of whom 23,250 had acceptable post-bronchodilator spirometry, and 20,746 also provided information on the use of open fires for cooking/heating. Sites where the prevalence of ever having used open fires for cooking/heating was either less than 0.5% (Mumbai [India]) or greater than 99.5% (Tirana [Albania]), Srinagar [India], and Adana [Turkey]) were excluded from the analysis. The present study population consisted of 18,554 individuals from 25 sites ( Table 1 ). All sites received approval from their local ethics committee, and participants provided written informed consent.

At an aggregate level, there was no strong or significant correlation between the prevalence of airflow obstruction (men: r , −0.146; P , 0.5) (women: r , −0.353; P , 0.08), spirometric restriction (men: r , 0.171; P , 0.4) (women: r , 0.273; P , 0.2), chronic cough (men: r , −0.004; P , 1.0) (women: r , −0.326; P , 0.1), or chronic phlegm (men: r , −0.044; P , 0.8) (women: r , −0.386; P , 0.06), and use of solid fuels for cooking/heating ( Figures 1 and 2 ). The weak correlation with spirometric restriction was strongly influenced by four sites in high-income countries (Iceland, the Netherlands, Canada, and Austria) with low levels of restriction, a finding typical of high-income countries, and low use of solid fuels.

Overall, chronic phlegm was not associated with the use of solid fuels among either men (OR, 1.23; 95% CI, 0.99–1.54) or women (OR, 1.16; 95% CI, 0.93–1.45). However, among never-smokers, women who ever used solid fuels were 28% more likely to have chronic phlegm compared with women who never used solid fuels (OR, 1.28; 95% CI, 1.04–1.58; Table 2 ). Among men, the association of chronic phlegm with use of open fires was significant in those who used charcoal, coal, or coke for 20 years or greater and in those who used wood, crop residues, or dung and had been exposed for less than 20 years. Among women, the association was stronger in those who used either of the two groups of solid fuels for 20 years or greater ( Table 8 ).

Chronic cough was not associated with use of solid fuels (men: OR, 0.98 [95% CI 0.71–1.34]; women: OR, 1.04 [95% CI, 0.77–1.41]; Table 2 ). No association between chronic cough and use of solid fuels was found in any of the sensitivity analyses, either restricting the analysis to never-smokers ( Table 2 ) or by type of cooking fuel, cumulative time of exposure, or the presence of ventilation ( Table 7 ).

There was no association between the FVC and use of solid fuels (Table E1). Exclusion of participants with greater than 6 months, but less than 10 years of solid fuel use (Tables E2 and E3), and use of the GLI2012 LLN equations did not change the results ( Table 4 ).

There was no association between spirometric restriction and use of solid fuels among either men (OR, 0.89; 95% CI, 0.75–1.06) or women (OR, 1.03; 95% CI, 0.87–1.21) ( Table 2 ). This pattern was similar across low-/middle- and high-income sites. Among male never-smokers, there was evidence of an inverse association between spirometric restriction and use solid fuels (OR, 0.72; 95% CI, 0.57–0.91). An association between spirometric restriction and use of solid fuels for cooking was still not present after examining the association by cooking fuel, cumulative time of use for cooking, and the presence of ventilation. Women who had ever used open fires burning charcoal, coal, or coke for 20 years or longer, more than 1 h/d, and without ventilation, were more likely to have restriction, whereas men who had ever used an open fire burning wood, crop residues, or dung were less likely to show restriction ( Table 6 ).

There was no association between the FEV 1 /FVC and use of solid fuels ( see Table E1 in the online supplement). Exclusion of participants with fewer than 10 years of solid fuel use (Tables E2 and E3) and use of GLI2012 LLN equations did not change the results ( Table 4 ). There was no significant exposure–response trend per 10 years of use ( Table 5 and Table E4).

Participants who used solid fuels were not more likely to have airflow obstruction than those who did not use solid fuels ( Table 2 ). The adjusted odds ratio (OR) and 95% confidence interval (CI) for the association between airflow obstruction and use of solid fuels was 1.20 (0.94–1.53) for men and 0.88 (0.67–1.15) for women. The estimates for this association were similar across low-/middle- and high-income sites. Among never-smokers, there was no evidence of an association of airflow obstruction with use of solid fuels (men: OR, 1.00; 95% CI, 0.57–1.76) (women: OR, 1.00; 95% CI, 0.76–1.32). The lack of a statistically significant association was also evident when examining it by cooking fuel, cumulative time of use for cooking, and the presence or absence of ventilation ( Table 3 ).

The characteristics of the 18,554 participants included in this study are presented in Table 1 . There were more females than males, and the mean age ranged from 50.3 to 59.6 years. Cumulative smoking history (i.e., pack-years) varied across sites, and most participants from low-/middle-income sites were never-smokers. The proportion of people who had used solid fuels for cooking/heating varied from 16.3% in Salzburg (Austria) to 99.1% in Guangzhou (China) and Naryn (Kyrgyzstan). The mean duration of use varied from 11.1 years in Reykjavik (Iceland) to 39.9 years in Vadu (India). The prevalence of the outcomes also varied: airflow obstruction from 3.2% in Riyadh (Saudi Arabia) to 19.3% in Uitsig/Ravensmead (South Africa); spirometric restriction from 8.4% in Vancouver (BC, Canada) to 84.1% in Colombo (Sri Lanka); chronic cough from 0.4% in Ile-Ife (Nigeria) to 19.5% in Lexington (KY); and chronic phlegm from 0.4% in Ile-Ife (Nigeria) to 16.8% in Lexington (KY).

Discussion Section: Choose Top of page Abstract Methods Results Discussion << References CITING ARTICLES

In this population-based study of adults, airflow obstruction was not associated with self-reported use of solid fuels for cooking/heating. The same was true for spirometric restriction and chronic cough. These findings were similar in low-/middle- and high-income sites, and are unlikely to be confounded by smoking, as they were also observed among never-smokers. The only significant association was for a 28% increase in risk of chronic phlegm among women who had never smoked, but had used solid fuels for cooking/heating. The findings were similar, but not significant, for men and for all participants regardless of smoking status.

The strengths of this study are: 1) its large sample size and the inclusion of many sites; 2) the use of a standardized protocol for spirometry and questionnaires for collecting data on risk factors across sites; 3) the use of post-bronchodilator spirometric measurements; and 4) the central quality control of all the spirometry and rigorous training of all study staff.

Nevertheless, this study also has limitations. As this is a cross-sectional study, we are unable to address temporality and draw firm conclusions in terms of causation. A longitudinal study showing no greater rate of lung function decline in the exposed group would be less open to confounding, and a negative randomized trial would be even stronger evidence. The information on solid fuel use was self-reported, and this may have led to exposure misclassification. Even nondifferential (unbiased) misclassification of the exposure will tend to reduce the estimate of the association between the exposure and the outcome. It may also be argued that the reporting of solid fuel use differs between low-/middle- and high-income countries. This is most likely to have influenced the ecological analyses, but is unlikely to have had much influence on the other analyses, as there was little evidence of heterogeneity in the results from different sites.

Assessment of lifetime exposure to open fires burning solid fuels was based on participants’ recall. Although direct measurement of the concentrations of pollutants at an individual level would provide more precise assessments of current levels of exposure, these are less relevant to the study of chronic conditions that develop over many years, and all studies of chronic long-term effects have relied on a history of fuel use. We did not find an association between solid fuel use and airflow obstruction among people who had used this type of fuel for 10 years or greater, nor between increasing duration of use and any of the outcomes. Further restricting analyses to those who had been exposed for at least 20 years, for greater than 1 h/d, and with no ventilation did not change these conclusions. However, we had limited power to assess the effect of ventilation.

A frequent explanation that is given for negative findings in relation to indoor air pollution and lung function is that the exposure has been mismeasured and that regression-dilution bias may have led to underestimation of the risks. This is unlikely to explain the difference between our results and the results of the earlier meta-analyses (6–10). First, the assessments that we have made are not significantly worse than the measures that have been used in the past to support an association, but have been better standardized. Second, our conclusion is supported by the ecological analysis, which shows no significant association between the prevalence of the different outcomes and the prevalence of solid fuel use. As the exposure in this analysis is a summary of all the individual exposure measures in the sample, it is less prone to random error. Finally, the random error in answering simple questions on lifetime use of solid fuel is likely to be less marked than the random sampling error implicit in estimating levels of exposure over a lifetime from very short-term recent measurements. This may partly explain why associations reported from studies that have used an exposure history have not been replicated with measured exposures of air pollution (25).

Ecological data have been used in the past to argue for the potential importance of exposure to solid fuel burning in explaining the global distribution of mortality from COPD, but we have failed to show any clear association between the prevalence of spirometric measurements and the prevalence of use of solid fuel. In the absence of such an association, it is unlikely that a policy implemented at an area level to reduce exposure would have any marked effect on prevalence. We found no convincing evidence that the prevalence of airflow obstruction or any other abnormality was associated with the use of solid fuel after adjusting for the individual effects of smoking and other confounders. Although ecological analyses have their weaknesses, these are different from analyses based on individuals. The lack of association at both levels supports the negative finding.

Use of the NHANES reference equations for white subjects in our spirometry measurements may be thought to overstate lung function abnormality in some study sites, but is unlikely to affect these analyses. Reference equations do not define illness, but an arbitrary level of lung function (defined here as the upper bound for the lowest 5% of the “normal”—asymptomatic, nonsmoking—population). It is largely immaterial whether the definition uses the lower 1, 5, or 50%, and, as each site is analyzed separately in our analysis, the association with fuel use within each site will not be greatly affected by the choice of the cut point. To check this assumption, we reran our main results using the GLI2012 multiethnic reference equations and using the continuous outcome measures of FEV 1 /FVC and FVC, which are not dependent on any reference equation. None of these analyses showed a significant change in the conclusions.

Our findings on airflow obstruction disagree with five systematic reviews (6–10). However, these reviews assessed a mixture of noncommensurate outcomes, and demonstrated clear publication bias, as acknowledged by their authors. Two other large studies have recently failed to find a positive and consistent association between airflow obstruction/COPD and solid fuel use (11, 13).

Experimental studies have explored whether there is a causal relationship between biomass smoke and airflow obstruction by reducing exposure to biomass smoke. For example, a randomized, controlled stove intervention trial among Guatemalan women, with personal exposure and spirometry measurements, reported an exposure–response relationship between exhaled carbon monoxide, used as a surrogate of recent exposure to biomass smoke, and lung function (26), but failed to show an improvement in lung function after a reduction in wood smoke exposure (27). A similar study with Mexican women reported a reduction in the decline of FEV 1 among those who used the intervention stove, but no significant improvement in the FEV 1 /FVC after the intervention, and no effect in the more reliable analysis by intention to treat (15). A study in China reported a reduction in the risk of COPD defined as an FEV 1 /FVC less than 0.7 after improvement in the type of stoves and fuel, but this finding was not supported by results for the continuous outcome, FEV 1 /FVC (28). Although experimental studies are regarded as the gold standard for demonstrating causality, these broadly negative studies are not decisive. Airflow limitation develops over a long period of time, and these trials had limited power to show a change in decline in lung function over time.

A lack of association can never be proven, but the evidence that indoor air pollution is responsible for a substantial amount of the airflow obstruction in low-/middle-income countries comes from meta-analyses that have been overinterpreted. The observation in this study, that airflow obstruction, spirometric restriction, and chronic cough were not associated with use of solid fuels, does not mean that this exposure is not harmful to humans. We found that chronic phlegm is more likely to occur among people who used solid fuels and, although chronic bronchitis has a relatively weak effect on survival compared with the effect of poor lung function (29), chronic bronchitis has a serious impact on quality of life that may exceed the effects of poor lung function (30). Moreover, there are many other conditions that have been shown by at least some studies to be associated with high exposures to the burning of solid fuels, including childhood pneumonias and airway malignancies (31).

We cannot exclude a small effect of solid fuel use on lung function and, where this exposure is common, it could still pose a risk to health. However, there is no evidence that solid fuel use is likely to explain a substantial component of airflow obstruction or of COPD. These remain unexplained, even though they are among the most important causes of death in poorer regions of the world. An explanation for this excess mortality is still urgently needed.

In summary, in this population-based study, airflow obstruction was not associated with self-reported use of solid fuels for cooking/heating. However, this is not a definitive study. Future long-term longitudinal studies in low-income countries could inform whether airflow obstruction and mortality ascribed to COPD are temporally associated with exposure to solid fuel smoke and whether different fuels have different effects.