It is a problem that affects more than a third of the world's population but often goes unnoticed.
Across the world, nearly 2.5 billion people routinely use wood or dried dung mixed with straw to cook with simple clay stoves and open fires.
The fires burn unevenly and frequently need fanning, producing both smoke and quantities of invisible toxic gases. Those clustered around the stove inhale the toxins and, over time, many develop lung diseases -- some fatal.
Carlos Dora, a doctor and environmental health policy expert with the Geneva-based World Health Organization (WHO), calls cooking-smoke a huge problem in homes across the developing world:
"The last estimate is 3.5 million deaths a year are due to indoor air pollution," Dora says. "That includes half of all the pneumonias in children, and pneumonia is the greatest cause of death in children under five. Half of those cases are caused by indoor air pollution."
"In many places in the world it is the woman's role and responsibility to take care of the household cooking, and fuel provision often also, as well as childcare," says Fiona Lambe, a researcher at the Stockholm Environment Institute in Sweden.
"So, in homes where women are multitasking and really taking care of several different things to keep the household running, they will often have their children right next to them as they are cooking over smoky fires."
Lambe says about half a million children are killed each year by home air pollution.
Clean Cooking Technology
But if this death toll is frightening, it also may be unnecessary. That is because new technologies offer an answer that is both low-priced and easy to use.
The answer is a clean stove that already has been developed in multiple forms but usually follows a simple model. It replaces the traditional clay stove with a stainless-steel canister in which wood and dung cakes burn so efficiently they do not release quantities of smoke and toxins.
"Basically, you end up with a lot of smoke when fuel is burned inefficiently, so the whole idea with improved cookstoves, biomass cookstoves, is to improve on the efficiency to make sure that as much fuel as possible is combusted within the stove," says Lambe.
Tests show these clean cooking stoves reduce smoke by 50 to 80 percent compared to clay stoves and can require only half as much wood for fuel. The key to their efficiency is that they provide constant ventilation to the fire so it burns hot and consumes the fuel completely. The ventilation is produced by an inexpensive battery-powered fan fixed to the stove.
As for price, different models of clean stoves range from $12 to $50. Often, the price of producing the stoves locally is subsidized by international aid organizations.
Researchers say that clean cooking technologies have been around for at least 30 years. But they are still novel because in much of the developing world efforts to promote them and reduce their cost are quite recent.
The biggest campaign, the U.S.-backed Global Alliance for Clean Cookstoves, in partnership with the UN, was launched in 2010. It has set the goal of getting 100 million homes around the world to adopt cleaner stoves and fuels by 2020.
Meanwhile, in many countries, the number of people who could benefit from the new technology remains huge.
In Afghanistan, 95 percent of the population uses solid fuels for cooking. The number of deaths per year from the household pollution they cause is estimated by the Global Alliance for Clean Cookstoves at 54,000 people.
In Pakistan, the percentage of the population using solid fuels is 81 percent. An estimated 56,000 people die there every year from household air pollution.