In Kogi State, Nigeria, a 39-year-old widow named Mama Caro relied on her earnings as a food vendor to support her daughters' schooling. Each day, as recounted in Nigeria Health Watch, Caro would fry garri, a popular snack made of fermented cassava flour, over an open flame.  

While serving customers one day, Caro began gasping for breath and collapsed, dying just a few hours after being transported to a health facility. Health-care workers concluded that a chronic lung condition—chronic obstructive pulmonary disease (COPD)—caused Caro's death and could have been linked to the smoke she inhaled while cooking. 

Every day, billions of people around the world, like Caro, light fires to make food for their families. These fires burn solid fuels such as wood, dung, charcoal and coal, or agricultural residues, and produce household air pollution (HAP), which contains harmful particles and gases such as carbon monoxide. In 2021, particulate matter, PM2.5, produced by cooking indoors was responsible for more than 3 million deaths. 

Despite considerable attention and investment, exposure to household air pollution has remained a leading risk factor for disease and early death. Institute for Health Metrics and Evaluation (IHME) reports via a new study, published March 18 in The Lancet, that although the proportion of those exposed to HAP has fallen from half the global population in 1990, current exposure is still extremely high, affecting an estimated 2.7 billion in 2021, the latest year with an available analysis. 

Sourced from IHME's Global Burden of Disease Study, the report presents the first comprehensive findings on HAP in more than a decade—and emphasizes how this pollution remains a grave threat to global health and deserves renewed attention.  

Indoor Air Pollution and Noncommunicable Diseases 

The study examined several health outcomes linked to HAP, including cataracts, COPD, cardiovascular disease, lower respiratory infections such as pneumonia, lung cancer, stroke, and type 2 diabetes. The study's authors also explored how HAP shapes pregnancy duration and low birthweight, which accounted for almost one-third of recorded death and disability worldwide. Cooking with solid fuels disproportionately affects the very young and very old, meaning that HAP is a threat not only to countries with high fertility, but also to those with aging populations.  

The number of people cooking with solid fuels varies geographically. People in sub-Saharan Africa and South Asia currently have the highest exposure to HAP and levels of related disease, though these two regions highlight important differences in addressing the challenge of HAP.

The 53% of people in South Asia still exposed as of 2021 represent remarkable progress: 85% were exposed in 1990. 

Sub-Saharan Africa is a different story. Although the proportion of population encountering HAP had fallen from 91% in 1990 to 79% in 2021, population growth outpaced efforts to reduce this exposure, and the region's number of related deaths increased from 685,000 in 1990 to 741,000 in 2021. 

Women bear the brunt of cooking globally, including hours spent collecting and transporting fuel, and face an increased likelihood of burns while cooking. However, the study finds that exposure from cooking with solid fuels harms all members of a family, not just those by the stove. In fact, disease and death from HAP was higher for men than for women. Although women on average are more exposed to HAP than men, the differences are relatively small. Importantly, because men have higher rates of cardiovascular and chronic respiratory diseases, even for the same level of exposure, HAP's effect is greater for them than for women. 

HAP from cooking with solid fuels is low in high-income countries and has declined throughout much of the world. Less than 20% of the population is exposed in North Africa and the Middle East, Latin America and the Caribbean, central Europe, eastern Europe, and Central Asia. Strict fuel use regulations and improved electricity access have resulted in dramatic declines in China: 85% of the population was exposed in 1990, but only 30% was in 2021. In India, increased access to gas cylinders has resulted in a reduction from 86% to 60% over the same 31 years.   

Yet the remaining challenge is large, especially in sub-Saharan Africa and South Asia. Although solid fuel usage in Asia is expected to continue to decline, Africa's consumption is projected to rise in the absence of robust intervention, given increasing populations in the region.  

Switching to Clean Cooking 

The IHME study found an association between HAP and adverse reproductive outcomes, but recent randomized controlled trials on the health of newborn infants have not clearly demonstrated the benefits of switching from solid fuels to cleaner cooking using gas. Some onlookers could interpret these findings to suggest that household air pollution is not harmful, but the IHME study documented strong evidence of an association between exposure to household air pollution and COPD, moderate evidence of an association for type 2 diabetes, ischemic heart disease, stroke, lung cancer, and birthweight, and weak evidence of an association for lower respiratory infections and short gestation.

No randomized trials to date have explored the link to the adult noncommunicable diseases that make up the greatest proportion of the overall disease burden from HAP. Although specifics of study designs could be an explanation for the apparent discrepancies between observational studies and the results of randomized trials, these trials were notably unable to reduce PM2.5 concentrations to the level the World Health Organization recommends. Given that HAP contributes to ambient air pollution, community-level exposure from those continuing to use solid fuels in the region or from other sources of air pollution could have canceled the benefits of emission reductions in individual homes. 

An additional challenge in such intervention studies is that families often use stove stacking—some combination of clean cooking modes using gas or electricity—for tasks such as boiling water or heating soups but solid fuels such as wood for cooking flatbreads. Understanding barriers to clean fuel access and specific practices within households could improve the design of high-quality randomized control trials and more effective policy options.  

Along with offering health benefits, cleaner cooking with gas or electric stoves reduces outdoor air pollution and emissions related to climate change. Eliminating household pollution sources would also reduce ambient air pollution, which causes more than 4 million deaths per year, and improve the health of whole communities. In India, mitigating household pollution could allow the country to achieve its ambient air quality standards. 

Reducing HAP, therefore, is not only a powerful tool to improve global health but also low hanging fruit in reducing the influence of the climate crisis. Past trials have shown that when nongovernmental organizations and governments provide the resources, families will switch to cleaner fuels.