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Outdoor air pollution might get more attention because of global warming, agricultural problems and the deadly diseases it causes, but indoor air pollution is actually more dangerous — killing approximately four million people worldwide each year, according to research from the World Health Organization.

Outdoor ozone and particulate air pollution contributes to about two million deaths a year, according a study recently published in the journal Environmental Research Letters. As climate change warms the planet, some researchers believe this figure will increase.

Indoor air pollution is largely caused by burning biomass fuels from sources such as wood, agricultural waste or coal. More developed countries have shifted away from these fuels in favor of petroleum products and electricity, but 13 percent of global energy was still derived from biomass fuels as recently as 2000. There is evidence that use is increasing among families in poverty, according to a study from the University of Michigan — potentially raising health risks.

Smoke from cook stoves is believed to be the largest contributor of indoor air pollution, which can cause childhood respiratory infections, COPD, asthma, cancer and infant mortality. People in developing countries, where alternative fuel sources are not readily available, spend between 3 and 7 hours a day exposed to high levels of indoor pollution, meaning indoor air pollution affects about 50 percent of the world's population.

“The most important [health problems] appear to be childhood acute lower respiratory infections, which remain the single most important cause of death for children aged under 5 years in developing countries,” the study authors said.

(MORE: Air Pollution Linked to Millions of Deaths)

WHO officials compared the effects of indoor air pollution to the global burden caused by tobacco and unsafe sex. The four million deaths per year is more than AIDS-related deaths, malaria and tuberculosis combined, according to WHO data.

Future research should look at the benefits of reducing indoor air pollution exposure, and evaluate how poor households can cut their exposure, the study authors said. Installing clean cook stoves that use chimneys could be one way to solve many of the problems linked to indoor air pollution.

Because poverty creates a need to rely on cheaper, more dangerous fuels, researchers also called for socioeconomic development to combat the worldwide problem of unhealthy household environments.

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