|Fig. 1: Example of a crude, wood-burning stove. Pictured is a woman from the Attapeu province of Laos. Stoves such as this are common throughout Laos. They are tripods made of metal or ceramic with space underneath to burn logs. (Source: Wikimedia Commons)|
Globally about 2.7 billion people depend on coal and biomass such as wood, dung, and crop reside to supply domestic energy.  These materials are typically burnt in crude stoves whose poor ventilation does not allow for complete particle combustion.  For this reason, women and young children are exposed to high levels of indoor pollution every day. (See Fig. 1.) This high exposure to indoor air pollution increases the risk of chronic obstructive pulmonary disease and of acute respiratory infections in women, children, and men. According to the International Energy Agency, exposure to indoor air pollution could be responsible for nearly 3.5 million premature deaths in developing countries and for around 2.7% of the global burden of diseases such as asthma and lung cancer. 
Burning solid fuels produces extremely high levels of indoor air pollution.  According to the World Health Organization, 24-hour levels of PM10 (particles with diameters below 10 μm) in biomass-using homes in Africa, Asia, or Latin America range from 300-3000 micrograms per cubic meter (μg/m3).  Comparatively, the United States Environmental Protection Agency has set the standard annual PM10 level for outdoor air at 50 μg/m3 while the European Union has set their annual mean of PM10 at 40 μg/m3.  Many of the substances in biomass smoke can damage human health. The most important are particles, carbon monoxide, nitrous oxides, sulphur oxides (principally from coal), formaldehyde, and polycyclic organic matter, including carcinogens such as benzo[a]pyrene.  Particles with diameters below 10 μm, and particularly those less than 2.5 μm in diameter (PM2.5), can penetrate deeply into the lungs and appear to have the greatest potential for damaging health.  As cooking occurs every day, most people using solid fuels are exposed to levels of small particles much higher than the accepted annual limit. Traditionally responsible for household chores, such as cooking, women and children are usually the most affected by indoor air pollution.
In 2011, the World Health Organization estimated that 1.5 million premature deaths per year are directly attributed to indoor air pollution from the use of solid fuels.  That amounts to more than 4,000 deaths per day, over half of them being children under five years of age.  More than 85% of these deaths (~1.3 million people) are due to biomass use, the rest due to coal.  This means that indoor air pollution associated with biomass use is directly responsible for more deaths than malaria, almost as many as tuberculosis and almost half as many as HIV/AIDS.  In developing countries, only malnutrition, unprotected sex, and lack of clean water and sanitation are greater health threats. Though this study was made in 2011, its findings remain evermore relevant. Indeed, the study theorized that, due to population growth, by 2030 2.7 billion people would be relying on biomass. However, as delineated in the 2017 World Energy Outlook, we have already reached the number projected in the 2011 WHO study.
Due to the fact that most adverse health effects of indoor air pollution result from the absence of an effective stove and proper ventilation, adding chimneys to stoves would appear to be the most effective improvement to be made from the point of view of health.  However, increasing household ventilation, though very cost-effective requires the procurement of building materials such as bricks and mortar that are not always readily available. Fortunately, there exist other stove improvement technologies such as retained heat cookers, fan stoves, and rocket stoves.  Not prohibitively expensive, improved stoves range from $2 in Ethiopia to $10-$15 for rocket stoves in Guatemala.  Improved biomass stoves save from 10%-15% of biomass consumption for the same cooking service provided and can reduce indoor air pollution by up to one half.  While the cultural reception and financial feasibility for these stoves will be unique to each country, it is encouraging to recognize that, with these stoves, indoor air pollution can be decreased and, thus, reduce the corresponding number of premature deaths.
© Laura Feigen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Energy and Air Pollution," International Energy Agency, 2016
 "WHO Guidelines for Indoor Air Quality: Selected Pollutants," World Health Organization, 2010.
 I. Romieu et al., "Improved Biomass Stove Intervention in Rural Mexico - Impact on the Respiratory Health of Women," Am. J. Respir. Crit. Care Med. 180, 649 (2009).