Ask yourself for a moment: What environmental problem causes more deaths than any other today? Deforestation, overfishing, industrial pollution or smog?

The surprising answer is indoor air pollution, which kills between 3.5 and 4.3 million people worldwide every year –more than HIV/AIDS, malaria and suicide combined –according to a new study published this month in the medical journal The Lancet Respiratory Medicine.

Nearly all of those deaths occur in poor countries where vast numbers of people have no access to electricity, and where they cook and heat their homes with indoor stoves in which they use coal, charcoal, wood and animal dung as fuel.

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The report looks at the connection between indoor air pollution and respiratory infections, cancers and chronic lung diseases, but also offers a window into just how many people are exposed to toxic pollution every day.

How many? The report estimates that as many as 3 billion people around the world are exposed to what the authors call Household Air Pollution, or HAP for short, because they use what the report terms "solid fuels," often in homes without ventilation or chimneys. So what they burn, they breathe in every day.

Depending on where they live and what they burn –in China, for example, coal use is predominant, while animal dung is used much more often in rural communities in places like Nepal and Afghanistan –they may inhale carbon monoxide and particulate matter, or air that has the same toxicity as a traffic-clogged highway or an industrial smokestack.

Where indoor air pollution deaths occur: Countries in darker colors show higher incidences compared to the countries in lighter colors. See the legend at left for a guide.

(The Lancet)

Why are they using these as fuels for cooking? Especially in the world's poorest countries in Asia and Africa, as many as 1.2 billion people have no access to electricity grids like those common in the United States and Europe, so their only fuel comes from what they gather themselves, Vox.com points out.

Giving them access will require massive, years-long investment and effort, ThinkProgress notes, pointing out that about a third of South Asia's and two thirds of Africa's population lack access today to distributed electrical power.

And cooking isn't the only way pollutants enter their homes. Even when they use coal, charcoal or animal dung-powered stoves outdoors, "urban poor people in Africa often bring a simple cooking stove indoors to keep their sleeping area warm at night," The Lancet explains.

Though many efforts have been made over the years to bring new cooking stove technologies to poor nations –the report notes efforts to supply advanced cook stoves, LPG [liquefied petroleum gas] and solar-powered stoves –all "have thus far been insufficient."

“In communities where solid fuel cooking methods are currently the norm, cleaner fuel and cooking methods need to be at least as affordable, efficient, and long-lasting as the traditional style methods they replace," Professor Stephen Gordon of the U.K.-based Liverpool School of Tropical Medicine, one of the report's authors, said in a news release.

"They also need to be fit for the different cultures and regions in which they’re used – if families only partially adopt cleaner cooking methods, using them alongside more polluting technologies, we are potentially looking at an expensive failure, and no reduction in the millions of people currently at risk from household air pollution.”

Read the full study at The Lancet Respiratory Medicine.

MORE ON WEATHER.COM: The Causes of Indoor Air Pollution

How 3 Billion Cook & Heat Their Homes

A Pakistani woman displaced by floods stokes a fire to bake bread at a makeshift camp in Thatta district in September 2010. (RIZWAN TABASSUM/AFP/Getty Images)