In the highlands of Ethiopia, the temperature dips to an average 37 degrees at night. A typical family's one-room house has no chimney, and the stove consists of three stones supporting a pot over an open wood fire. The mother fixes dinner as her toddlers edge closer, trying to stay warm in the swirling smoke.
And as they do, the air they breathe may be killing them.
A recent study estimated that in the next 25 years, 10 million women and children in sub-Saharan Africa will die prematurely from the smoke produced by the most basic and comforting of sources: the family cookstove.
"The actual physical exposure to smoke is off the scale, as far as our standards are concerned," Kim Mulholland, a clinical pediatrician at the London School of Hygiene and Tropical Medicine, said in a recent telephone interview.
The chemical compounds and minute particles that make up wood smoke affect growing and full-grown lungs in different, but equally dangerous, ways. Among children younger than 5, indoor smoke makes them much more susceptible to respiratory infections; in adults, the result is chronic obstructive pulmonary disease.
The lower respiratory infections in children usually end up as pneumonia, Mulholland said. Worldwide, such infections are responsible for 19 percent of deaths in children younger than 5, the World Health Organization (WHO) reported recently.
Public health workers have been trying for decades to reduce the toll of cooking fires by introducing less-polluting stoves. But a recent study by U.S. scientists concluded that in sub-Saharan Africa, where 80 percent of families depend on wood for fuel, the best approach must address energy and environmental issues, as well as the impact on women's and children's lungs.
The smoke from cooking fires hangs indoors because few stoves in sub-Saharan Africa vent to the outside, said Majid Ezzati, one of the authors of the study published last month in the journal Science. Chimneys are impractical because often "the roofs are made of grass, and there's the possibility of fire." In the semi-nomadic communities, he said, houses are constructed quickly and cheaply. Even if a family has the money for chimney materials, it is unlikely to invest in a home it plans to leave soon.
Because of the time a mother spends over the cookstove, her exposure to smoke is substantially greater than her children's, Mulholland said. He noted that research has shown that carbon monoxide, one of the compounds in wood smoke, reduces a pregnant woman's placental blood flow, and she is more likely to bear an underweight child.
Although chimneys would improve indoor air quality, they would just move the problem outdoors. By 2050, the study's authors estimated, smoke from wood fires will release at least 7 billion tons of greenhouse gases into the environment each year. That is about 6 percent of the total expected to be produced from the African continent.
Today, Africa produces only about 5 percent of the world's greenhouse gases, said Daniel M. Kammen of the University of California at Berkeley, another of the study's authors. But it will probably produce much more, he wrote in an e-mail, because "after decades of trailing Asia, Africa now has the fastest rate of urbanization in the world," and city dwellers use more energy.
The best-case scenario would be for Africans to switch from "biomass" fuels, such as wood, charcoal or dung, to fossil fuels, such as kerosene or liquid propane gas, for cooking, said Robert Bailis, a graduate student at UC-Berkeley and the study's principal author.
"If you promote fuel switching, then not only are you going to have health benefits, you are going to have these global benefits," Bailis said.
He acknowledged that talk of nonrenewable oil-based fuels "raises a greenhouse-gas flag," because emissions from fossil fuels are responsible for the majority of greenhouse gases believed to contribute to global warming.