By Alex Kirby
BBC News Online environment correspondent
Smoke from indoor cooking fires kills one person every 20 seconds in the developing world, UK campaigners say.
Smoke shroud for a Nepali village
The Intermediate Technology Development Group says smoke in the home kills more people than malaria does, and almost as many as unsafe water and sanitation.
The problem affects more than two billion people who burn wood, charcoal, vegetation and dung for heating food.
The United Nations says inefficient stoves can be as bad for health as smoking two packs of cigarettes a day.
ITDG says 2.4 billion people burn biomass (organic matter) for cooking and heating, and when coal is included 3 billion people - half the world's population - rely on solid fuel.
It says smoke in the home is the fourth greatest cause of death and disease in the world's poorest countries, killing 1.6 million people annually. Nearly a million of them are children.
It says: "In poor people's homes throughout the developing world levels of exposure to pollutants are often 100 times greater than recommended maximums.
"Illnesses caused by indoor air pollution include acute lower respiratory infection. A child is two or three times more likely to contract it if exposed to indoor air pollution.
"Women who cook on biomass are up to four times more likely to suffer from chronic obstructive pulmonary disease, such as chronic bronchitis.
Solid fuel can be a killer
"Lung cancer in women in China has been directly linked to use of coal burning stoves.
"In addition there is evidence to link the pollution to asthma, tuberculosis, low birth weight and infant mortality and cataracts."
More than half the people who cook on biomass live in India and China, but in many sub-Saharan African countries more than 90% of people do so.
On current trends 200 million more people globally will rely on biomass by 2030, the International Energy Agency says.
In parts of central Asia, where gas and electricity used to be available in Soviet days, people are having to revert to biomass use.
Since 1991 the incidence of acute respiratory infection, the world's top child killer, has risen by 35% in Tajikistan, mainly because of the burning of wood indoors.
The answer is to switch to cleaner fuels, but most people at risk are too poor to afford them.
But ITDG says they can reduce their exposure to the pollution, for instance by using well-designed chimney stoves, or smoke hoods able to reduce indoor pollution by up to 80%.
It says: "For relatively little outlay, massive health benefits and savings in life could be achieved.
"The total cost of providing three billion people with access to healthy indoor air would be in the region of $2.5bn annually over the next 12 years.
But smoke hoods are one cure
"It is estimated that government spending and international development aid would be about 20% of this total, around $500m a year - less than 1% of total Western aid spending."
Cowan Coventry, ITDG's chief executive, said: "Poverty condemns more than a third of humanity to cook on a small bonfire in the middle of the home.
"It's a technology that has changed little since the Stone Age and turns homes into death traps for women and children.
"It's an international scandal that while the world spends millions of dollars combating levels of pollution in Western cities, it has neglected to tackle the death toll caused by lethal levels of smoke in the homes of the poor world."
ITDG is calling for support for the Partnership for Clean Indoor Air, backed by the World Health Organisation, World Bank, US Environmental Protection Agency and others.
It wants the United Nations to draw up a global action plan "in line with the international community's response to hunger, HIV/Aids, dirty water, poor sanitation and malaria".