This is an interim intervention report. We have spent limited time to form an initial view of this program and, at this point, our views are preliminary. We may consider undertaking additional work on this program in the future.
- What is the problem? Traditional cookstoves used in developing countries typically involve an open fire fueled by biomass fuels such as firewood or dung. They are often poorly ventilated, and the smoke they produce causes a substantial amount of indoor air pollution that carries health risks. Women and children suffer the highest levels of exposure.
- What is the program? Interventions distribute improved "clean cookstoves” to affected households. There are various types of cleaner cookstoves, some of which continue to use traditional biomass fuels while others are powered by gas.
- What is its evidence of effectiveness? Results from multiple randomized controlled trials (RCTs) suggest that distributions of clean cookstoves do not have clear evidence of effectiveness at reducing health problems attributable to air pollution. The evidence we have reviewed in our preliminary investigation finds limited impacts on women’s health and no clear impacts on children’s health under typical use. Distributions of clean cookstoves may have been less effective than expected due to implementation challenges, such as low compliance with using the replacement stoves and failure of the cleaner stoves to reduce air pollution sufficiently.
- How cost-effective is it? We have not produced a cost-effectiveness model for clean cookstoves because we have not yet seen strong enough evidence to model a health benefit of the intervention.
- Bottom line: We do not currently consider this program to be a priority program. We would consider revisiting this intervention if we found strong causal evidence showing a large reduction in morbidity or mortality, or possibly if we found a particular giving opportunity with robust monitoring data demonstrating very high levels of stove usage.
Published: July 2019
What is the problem?
The World Health Organization (WHO) estimates that about 3 billion people worldwide use traditional cookstoves that typically involve an open fire fueled by coal or biomass fuels such as firewood, dung, or crop waste.1 They are often poorly ventilated, and the smoke they produce causes a substantial amount of indoor air pollution, particularly in the form of small particles of soot that penetrate the lungs.2
Exposure to large quantities of smoke from cooking is correlated with a number of health risks, including acute respiratory illnesses (pneumonia) and non-communicable diseases such as heart disease and cancer that may lead to premature death, and these risks disproportionately affect women and young children.3
What is the program?
Interventions distribute cleaner cookstoves to affected households. There isn't a single version of a "clean cookstove" or an "improved cookstove." Trialed interventions range from providing chimney ventilation, to cleaner-burning stoves that reduce emissions through improved combustion efficiency, to gas-fueled stoves.6 New standards for cleaner cookstove performance were released in 2018, and it is possible that these could impact future interventions.7
Many of the cleaner cookstoves provided to date continue to use the same solid biomass fuels as traditional open fires. The WHO believes that gas stoves are likely to be more effective at achieving household air quality guidelines.8
Does the program have strong evidence of effectiveness?
Results from multiple randomized controlled trials (RCTs) suggest that distributions of clean cookstoves do not have clear evidence of effectiveness at reducing health problems attributable to air pollution. The evidence we have reviewed in our preliminary investigation finds limited impacts on women’s health and no clear impacts on children’s health under typical use. Distributions of clean cookstoves may have been less effective than expected due to implementation challenges, such as low compliance with using the replacement stoves and failure of the cleaner stoves to reduce air pollution sufficiently.
Evidence from RCTs
This section focuses on the evidence for health impacts of clean cookstove interventions. We have identified a number of randomized controlled trials (RCTs) conducted in Africa, South America, and South Asia that study key health outcomes, and these do not find strong effects of clean cookstove distributions on women’s or children’s health at the usage levels observed in the field. At the time we compiled this evidence, we did not find an up-to-date meta-analysis summarizing this evidence, and we did not complete our own meta-analysis at this early stage of the research process.9 Instead, we informally summarize the evidence below, generally assigning more weight to the results of larger and more intensive RCTs.10 Please refer to this spreadsheet for a full list of relevant literature we have reviewed on this topic.11
We also briefly considered whether there could be economic impacts of cleaner cookstoves unrelated to health, such as cost savings on fuel associated with the intervention. While we have not spent the time to form a complete viewpoint on this question, it does not seem likely to be a major benefit, since households appear to value traditional stoves for being a free technology,12 and improved stoves continue to require fuel.
There appear to be two main hypotheses for the program’s potential health benefits:
- Impacts on women as the primary users of cookstoves, expected to reduce rates of heart and lung diseases13 and typically measured by blood pressure and lung function in trials.14
- Health impacts on young children exposed to large quantities of smoke both in utero and in early life, expected to manifest in poorer birth outcomes (low birthweight, stillbirth, etc.) and childhood cases of pneumonia.15
We have identified the following RCT evidence on each of these outcomes:
- Women’s blood pressure. Key RCTs do not find evidence of large reductions in blood pressure. A large RCT conducted in India (Hanna et al. 2016) did not find effects of stove distributions on blood pressure.16 A trial in Nigeria found a modest, statistically significant 2.8 mm Hg (mercury) reduction in diastolic blood pressure during pregnancy and reduced incidence of hypertension in the treatment group, but no effect on systolic blood pressure.17 These studies differed in that the Nigeria study distributed ethanol-fueled gas stoves and focused on pregnant women, while the India study distributed improved biomass stoves and looked at all women who regularly cooked, so its results are more broadly applicable. It seems that these different stove types and target populations may explain the slight differences in outcomes that were observed, but in either case the measured blood pressure improvements were not large.
- Women’s lung function. There is no causal evidence of improvements in measured lung function based on two RCTs. One of these RCTs finds limited evidence of improvement in women's self-reported respiratory symptoms. Hanna et al. 2016 did not find any improvement in measured lung function in Indian women who cook regularly.18 The second RCT, the RESPIRE trial in Guatemala, also failed to measure any improvement in women’s lung function in response to a confirmed reduction in carbon monoxide exposure. However, there was a statistically significant improvement in self-reports of wheeze, though results on other respiratory symptoms were not statistically significant.19
- Birth outcomes. Three RCTs did not find higher birth weights or improvements in other birth outcomes in treatment groups where cleaner stoves were provided to women during pregnancy. Two of these trials compared traditional fires to cleaner biomass stoves, and one trial included a gas stove intervention.20
- Pneumonia in children. Three RCTs of cleaner biomass stoves failed to find a reduction in pneumonia cases among young children in treatment households. The large CAPS [Cooking and Pneumonia Study] trial in Malawi found very similar pneumonia incidence rates between treatment and control groups.21 The RESPIRE trial in Guatemala found a statistically significant reduction in the secondary outcome of severe cases of pneumonia, but not in pneumonia incidence.22 The third RCT combined improved biomass cookstoves with sanitation interventions in Peru and found no effects on pneumonia or children's growth rates up to 36 months.23
Our review to date covers direct, randomized evidence for the provision of cleaner cookstoves on important health outcomes. This question is distinct from investigating the association between actually achieving targets for healthy levels of household air pollution and health, as we discuss in more detail below. Due to the limited nature of this review, we did not focus on reviewing the literature measuring relative smoke exposures from different types of cookstoves and/or attempting to model expected health impacts of reductions in smoke exposure indirectly. We also did not review the evidence on the environmental impacts of transitioning to more efficient stoves.
Factors that may limit the program’s impact
There are several factors that may have limited the effectiveness of distributions of cleaner cookstoves to date:
- Stove usage. If households prefer cooking with traditional stoves, stove distributions are likely to involve substantial wastage that reduces program impact. An RCT testing demand for cleaner stoves in Bangladesh finds that households there placed low values on non-traditional stoves.24 Also, while we have not looked at stove demand and usage rates comprehensively, we note that several of the trials described in the evidence section reported low stove usage rates.25 The evidence we considered above evaluates the health impacts of clean cookstoves under typical use, meaning that the impacts are averaged over all households in the treatment group, regardless of whether they actually used the new stoves. It is thus possible that these stoves could have greater health effects on those who actually use them.
- Inadequate technology. Indoor air quality may not improve sufficiently with the introduction of cleaner stoves to have strong health benefits, especially if health responds non-linearly to exposure to pollutants. Several of the RCTs we describe above tested air pollution in intervention households, and it appears that air pollution remained above healthy benchmarks in these studies. Further, actual reductions in air pollution in the field seemed to be smaller than forecasted based on laboratory tests of the cleaner stoves.26
- Ambient air pollution. Switching an individual household to a cleaner cookstove may reduce the exposure of its members to indoor air pollution, but ambient outdoor air quality may remain poor if its neighbors continue to use traditional stoves.27
Is the program cost-effective?
We have not produced a cost-effectiveness model for clean cookstoves because we have not yet seen strong enough evidence to model a health benefit of the intervention. As a result, we do not believe that this program currently meets the cost-effectiveness standards of our other priority programs.
We may revisit the cost-effectiveness of clean cookstoves in the future if more evidence of causal impact becomes available.
Organizations that implement this program
We have not conducted a comprehensive search for charities distributing clean cookstoves. The Clean Cooking Alliance is a large public/private partnership focused on promoting clean cookstoves, and we expect that other charities may implement this program as well.
Key questions for further investigation
- Are there stronger health effects of clean cookstoves for those who use them regularly compared to typical usage? If so, are there any incentives or stove designs that would increase demand for clean cookstoves?
- Are there air quality thresholds that, if achieved, might have a larger effect on health outcomes?
- Are there any promising stove designs that may be capable of reducing pollution by much more than the stoves used in trials?
- Would achieving high community coverage and usage of cleaner cookstoves substantially increase the effectiveness of the intervention by reducing ambient pollution?
- Do clean cookstoves have positive impacts on long-term health outcomes such as lung cancer and heart disease? Factors that may have limited the effectiveness of the intervention on short- and medium-term health outcomes seem likely to influence long-term outcomes as well, but the direct evidence we have seen so far does not cover long enough time spans to be conclusive on these potentially important health benefits.
- How durable are clean cookstoves?
We searched for Cochrane Collaboration systematic reviews on the effectiveness of clean cookstoves, but none were available. We searched the Cochrane Collaboration library using the search term “clean cookstove” and reviewed all of the completed field trials of clean cookstoves that studied health effects listed there. We also conducted a medium-depth primary literature search for randomized trials of clean cookstoves on health using Google Scholar with the terms “clean cookstoves RCT” and “clean cookstoves randomized controlled trial” and reviewed the sections of the World Health Organization (WHO) website on Cleaner Cookstoves and Household Air Pollution. We also reviewed the sources referenced here.
"Around 3 billion people cook using polluting open fires or simple stoves fuelled by kerosene, biomass (wood, animal dung and crop waste) and coal." WHO, Household air pollution and health, 2018
"These cooking practices are inefficient, and use fuels and technologies that produce high levels of household air pollution with a range of health-damaging pollutants, including small soot particles that penetrate deep into the lungs. In poorly ventilated dwellings, indoor smoke can be 100 times higher than acceptable levels for fine particles." WHO, Household air pollution and health, 2018
- "Indoor biomass cooking smoke is associated with a number of diseases, including acute respiratory illnesses and even cancer, with women and young children affected disproportionately. They are exposed to levels of indoor cooking smoke, in the form of small particulates, up to 20 times higher than the maximum recommended levels of the World Health Organization. It is estimated that smoke from cooking fuels accounts for nearly 2 million deaths annually, which is more than the deaths from malaria or tuberculosis; by 2030 over 4,000 people will die prematurely each day from household air pollution." World Bank 2011, p. ix.
- The WHO estimates that "3.8 million people a year die prematurely from illness attributable to the household air pollution caused by the inefficient use of solid fuels and kerosene for cooking. Among these 3.8 million deaths: 27% are due to pneumonia; 18% from stroke; 27% from ischaemic heart disease; 20% from chronic obstructive pulmonary disease (COPD); 8% from lung cancer." WHO, Household air pollution and health, 2018
“But in many developing countries worldwide, the poor still burn biomass energy to meet their household cooking needs. These open fires are fairly inefficient at converting energy into heat for cooking; the amount of biomass fuel needed each year for basic cooking can reach up to 2 tons per family. . . . By contrast, a family that uses LPG [liquefied petroleum gas] as its cooking fuel requires only about 0.2 tons per year.” World Bank 2011, p. 1.
“[H]ousehold pollutants, such as black carbon, also have climate warming effects.” WHO, Cleaner cookstoves
"While clean cookstoves . . . also comprise stoves using modern cooking fuels like electricity and Liquefied Petroleum Gas (LPG), so-called Improved Cooking Stoves (ICS) that still use biomass are considered a viable low-cost "bridging technology". ICS are designed to reduce the fuel consumption per meal and to curb smoke emissions. The definition of ICS ranges from more sophisticated bricked stoves with chimneys leading the smoke out of the kitchen to very simple portable clay or metal stoves that just improve the heating process." Bensch & Peters 2012, p. 4.
"ISO [International Organization for Standardization] has developed a series of International Standards and related documents for cleaner, safer cooking solutions that will provide a platform for new and existing technologies to develop and grow. These include an International Standard on laboratory testing, a technical report on sector-specific vocabulary and a technical report for benchmarking lab testing measurements. Two of them have just been published, with the third due to be released later this year." Clare Naden, Improving health with new standards for cleaner cookstoves, 2018
"Among the available technologies, cleaner fuels such as biogas, ethanol, LPG, and natural gas along with electricity are the best alternatives to solid fuels for reaching WHO air quality guideline levels for household air pollution." WHO, Cleaner cookstoves
A new meta-analysis, Thakur et al. 2018, has been published since we did our initial research on cleaner cookstoves. We have not reviewed it carefully or vetted the quality of evidence it includes, but its conclusions generally seem to match our synthesis of the evidence. This increases our confidence that we haven't missed any major pieces of contradictory evidence, despite only having conducted a medium-depth literature review at this stage.
We also discuss trials of gas stoves regardless of size, because we have only identified two relevant field trials of this technology so far, and it seems plausible that the impacts of these stoves could differ compared to biomass stoves with reduced emissions.
While we have simplified the evidence below by summarizing trials that we believe are most important, we do not think that the additional results presented in the spreadsheet are likely to change our bottom line conclusions. While some of these studies do report statistically significant impacts, these often arise after adjusting results for stove usage and/or other control variables, so they are not truly random or directly comparable to the main studies presented below. For details on these studies and the specific methodological limitations in each case, see this spreadsheet.
"[R]ural women in Bangladesh do not prioritize nontraditional cookstoves over other basic developmental needs despite demonstrating awareness of their potential negative health consequences. Because they overwhelmingly rely on a traditional cookstove technology that costs nothing and are accordingly not willing to pay much for a new nontraditional cookstove, non-health considerations are the most salient determinants of cookstove technology choices." Mobarak et al. 2012, p. 10816.
The WHO estimates that “3.8 million people a year die prematurely from illness attributable to the household air pollution caused by the inefficient use of solid fuels and kerosene for cooking. Among these 3.8 million deaths: 27% are due to pneumonia; 18% from stroke; 27% from ischaemic heart disease; 20% from chronic obstructive pulmonary disease (COPD); 8% from lung cancer.” WHO, Household air pollution and health, 2018
"Given the evidence that ambient particles increase blood pressure, we hypothesized that the intervention would lower blood pressure." McCracken et al. 2007, Abstract.
"Exposure to household air pollution almost doubles the risk for childhood pneumonia and is responsible for 45% of all pneumonia deaths in children less than 5 years old. . . . There is also evidence of links between household air pollution and low birth weight." WHO, Household air pollution and health, 2018
Hanna et al. 2016, a 4-year RCT of free provision of laboratory-validated biomass cookstoves in India, stated: “We fail to find a positive impact on a wide variety of measured and self-reported health outcomes, including . . . probability of a cough, blood pressure, or even the probability of any illness in the last 30 days.” P. 83.
In an RCT of ethanol stove use in pregnant Nigerian women, "[t]he change in diastolic blood pressure (DBP) over time was significantly different between ethanol users and control subjects (P = 0.040); systolic blood pressure (SBP) did not differ (P = 0.86). . . . At the last visit, mean DBP was 2.8 mm Hg higher in control subjects than in ethanol users, . . . and 6.4% of control subjects were hypertensive (SBP ≥140 and/or DBP ≥90 mm Hg) versus 1.9% of ethanol users (P = 0.051)." Alexander et al. 2017, Abstract.
"[T]here is no difference in lung functioning (as measured by spirometry tests) between women who regularly cook in the treatment and control groups." Hanna et al. 2016, p. 83.
"Use of a plancha significantly reduced carbon monoxide exposure by 61.6%. For all respiratory symptoms, reductions in risk were observed in the plancha group during follow-up; the reduction was statistically significant for wheeze (relative risk = 0.42, 95% confidence interval: 0.25, 0.70). The number of respiratory symptoms reported by the women at each follow-up point was also significantly reduced by the plancha (odds ratio = 0.7, 95% confidence interval: 0.50, 0.97). However, no significant effects on lung function were found after 12–18 months. Reducing indoor air pollution from household biomass burning may relieve symptoms consistent with chronic respiratory tract irritation." Smith-Sivertsen et al. 2009, Abstract.
- The GRAPHS trial in Ghana randomized pregnant women into controls, a treatment arm that received improved wood-burning stoves, and a second treatment arm that received liquified petroleum gas (LPG) stoves. It did not find improvements in birth outcomes: ”In unadjusted intention-to-treat analysis for live births, birth weight did not differ for women using BioLite or LPG stoves compared to traditional fires (p=0.49 and 0.68)(Table). Secondary outcomes also did not significantly differ for either intervention versus control. Stillbirths were less frequent for LPG (6/346, 1.7%) compared with traditional fires (15/490, 3.1%) but did not reach significance (OR 0.6; 95%CI 0.2-1.5). CONCLUSION: A strategy of providing improved cook stoves or fuels to individual pregnant women prior to 28 weeks does not improve mean birth weight or other obstetric outcomes in this rural African population.” Wylie 2017, p. S23.
- Hanna et al. 2016, a 4-year RCT of free provision of laboratory-validated biomass cookstoves in India, stated: ”we fail to find a positive impact on a wide variety of measured and self-reported health outcomes, including infant birth weight, infant mortality rates, probability of a cough, blood pressure, or even the probability of any illness in the last 30 days. This does not appear to be due to a lack of power.” P. 83.
- Katz et al. 2016, a cluster-randomized trial of improved biomass cookstoves in Nepal, concluded that “[t]here was no evidence that installation of improved biomass stoves reduced adverse birth outcomes.” P. 576.
Mortimer et al. 2017 is a large (10,543 under-5 children) community cluster-RCT conducted in Malawi that provided treatment clusters with two cleaner biomass stoves. In summary, "10 543 children from 8470 households contributed 15 991 child-years of follow-up data to the intention-to-treat analysis. The IMCI pneumonia incidence rate in the intervention group was 15·76 (95% CI 14·89–16·63) per 100 child-years and in the control group 15·58 (95% CI 14·72–16·45) per 100 child-years, with an intervention versus control incidence rate ratio (IRR) of 1·01 (95% CI 0·91–1·13; p=0·80). . . .We found no evidence that an intervention comprising cleaner burning biomass-fuelled cookstoves reduced the risk of pneumonia in young children in rural Malawi." Abstract.
The RESPIRE RCT in Guatemala randomized households to receive a woodstove with chimney and recorded children’s health weekly up to age 18 months. It concluded, "[i]n a population heavily exposed to wood smoke from cooking, a reduction in exposure achieved with chimney stoves did not significantly reduce physician-diagnosed pneumonia for children younger than 18 months. The significant reduction of a third in severe pneumonia, however, if confirmed, could have important implications for reduction of child mortality." Smith et al. 2011, Summary.
"We conducted a community-randomized–controlled trial in 51 rural communities in Peru to evaluate whether an environmental home-based intervention package, consisting of improved solid-fuel stoves, kitchen sinks, solar disinfection of drinking water and hygiene promotion, reduces lower respiratory infections, diarrhoeal disease and improves growth in children younger than 36 months. . . . No effects on acute lower respiratory infections or children’s growth rates were observed." Hartinger et al. 2016, Abstract.
“Overall, we find a variety of congruent evidence suggesting that rural women in Bangladesh do not prioritize nontraditional cookstoves over other basic developmental needs despite demonstrating awareness of their potential negative health consequences. Because they overwhelmingly rely on a traditional cookstove technology that costs nothing and are accordingly not willing to pay much for a new nontraditional cookstove, non-health considerations are the most salient determinants of cookstove technology choices. Our price experiment confirms negligible adoption rates at full price and despite our very large price elasticity estimates (ranging between −8 and −10). This result implies that large discounts by themselves are unlikely to promote substantial adoption and use. Efforts to promote non- traditional cookstoves may be more successful by developing and emphasizing designs with features valued more highly, even ones unrelated to cookstoves’ health and environmental impact.” Mobarak et al. 2012, p. 10816.
- “In the early years, treatment households only cooked 3.5 more meals per week (or 25 percent of total meals) with an improved stove in good condition than the control households. This difference was halved to about 1.8 meals per week in year three, as the stoves deteriorated.” Hanna et al. 2016, p. 82.
- In an RCT of improved cookstoves in rural India, “Forty percent of the intervention homes continued using traditional stoves.” Aung et al. 2016, Abstract.
- ”Adherence to the intervention was low (50%).” Romieu et al. 2009, Abstract.
- An RCT of improved cookstoves on measurements of air pollution in rural India found that “[f]or intervention homes, absorbance-to-mass ratios suggest a higher proportion of black carbon in PM2.5 emitted from intervention compared with traditional stoves. Absent of field-based evaluation, stove interventions may be pursued that fail to realize expected carbon reductions or anticipated health and climate cobenefits.” Aung et al. 2016, Abstract.
- ”While there was a significant effect on smoke inhalation during the first year for the primary cooks in the household (though not for children), the treatment effect became statistically indistinguishable from zero in subsequent years as usage rates and maintenance declined. Further, even in the first year, the resulting effect (a 7.5 percent decrease in the carbon monoxide concentration of exhaled breath) was smaller than the reduction observed in laboratory-style settings with properly maintained stoves and near-perfect usage rates.” Hanna et al. 2016, p. 83.
- ”PM2.5 concentrations following installation of the improved stoves were still well above the WHO indoor air standard of 25 mg/m3.” Katz et al. 2016, p. 576.
- In the RESPIRE trial, ”Significant exposure-response associations contribute to causal inference and suggest that stove or fuel interventions producing lower average exposures than these chimney stoves might be needed to substantially reduce pneumonia in populations heavily exposed to biomass fuel air pollution.” Smith et al. 2011, Summary.
"From the original ‘framing’ of the risk factor as ‘indoor smoke’ in initial estimates, the risk factor has been broadened to ‘household air pollution’. This is an important distinction, because not everyone cooks outdoors and cooking with solid fuels can be a major source of ambient (outdoor) air pollution." Clean Cooking Alliance 2017