Indoor air pollution gets surprisingly little attention for such a lethal public health problem. It kills more people each year than HIV/AIDS and malaria combined, but few countries treat it as a crisis on the same level.

The basic story: About 3 billion people around the world — mostly in Africa and Asia, and mostly very poor — don't have access to modern energy and still cook and heat their homes by burning coal, charcoal, dung, wood, or plant residue indoors. These homes often have poor ventilation, and the smoke can cause a horrible array of respiratory diseases, including lung cancer.

All told, indoor air pollution kills between 3.5 million and 4.3 million people each year. To put that in perspective, HIV/AIDS kills around 1.6 million people each year and malaria around 627,000. (By the way, this is separate from outdoor air pollution from coal plants, vehicles, and other sources, which affects nearly every nation and has separately been linked to another 3 million deaths per year.)

In theory, this should be straightforward to fix: Just give people access to cleaner heating fuels or cookstoves. (Indoor air pollution isn't a major problem in wealthy countries, where we barely give our quiet gas stoves or electric heaters a second thought.) But it's not always that simple. As a new report on pollution from the United Nations Environment Programme notes, even countries that have tried to promote cleaner fuels and stoves have often struggled to persuade people to switch.

How indoor air pollution kills millions of people

Back in 2014, The Lancet Respiratory Medicine published a big report on indoor air pollution, including a map showing where the deaths occur. India and sub-Saharan Africa are most heavily affected, but it's a problem almost everywhere outside of the wealthiest countries:

Indoor air pollution deaths (per million people)

The study spends a lot of time detailing the various respiratory problems linked to indoor air pollution — from chronic lung and cardiovascular disease to lung cancer. It also made a few important broader points:

1) Indoor air pollution has a wide variety of causes. In China, tens of millions of rural households still burn coal directly inside their homes to cook. But in India and Africa, wood and charcoal are far more common. And in countries like Kenya or Ethiopia where wood is scarce, animal dung is used. Different fuels lead to different health problems.

Lighting is another big source of indoor air pollution — particularly the use of unvented kerosene lamps. The study noted that the growing use of LED lighting is helping here, but "it remains a major problem."

There are also different reasons for poor ventilation in homes. "In extreme climates (eg, Nepal, north India), ventilation is deliberately minimised to conserve energy, resulting in extremely toxic amounts of [indoor air pollution for much of the year]," the paper noted. "Urban poor people in Africa often bring a simple cooking stove indoors to keep their sleeping area warm at night."

2) Women and children in extreme poverty are most affected. The paper notes that poor people are most likely to be affected, as they typically lack access to electricity and have to resort to cheaper fuels that produce more indoor smoke. What's more, the cheapest homes, made of mud or thatch, rarely have chimneys.

In many countries, women still do most of the cooking, so they're most exposed — and young children and infants are often nearby. That's especially troubling since indoor air pollution does a lot of damage to young kids.

3) All that soot is also a contributor to climate change. There's a climate angle here, too. Previous studies have found that soot particles from these cookstoves can settle on nearby glaciers, absorbing sunlight and melting the ice — particularly in combination with global warming. The loss of glaciers can, in turn, put pressure on local water supplies.

Cleaner stoves can help — but it's tough to persuade people to use them

Over the past decade, public health researchers have been exploring different ways to cut down on indoor air pollution. There are plenty of simple, proven technologies out there, from cleaner fuels (like propane or butane) to more efficient heating and cooking stoves with better ventilation. Yet adoption has been frustratingly slow.

There is some good news here: the UNEP report finds that 97 countries (out of 193) have increased the proportion of households with access to cleaner cooking and heating fuels, such as gas or electricity, between 2008 and 2013. Costa Rica, for instance, has been subsidizing liquefied petroleum gas (LPG) for many households and is promoting solar power lighting for households not on the grid. Chile runs a program allowing people to trade in their old wood stoves for cleaner models.

But that still leaves 96 countries that have seen no progress on the issue — including many sub-Saharan African nations. The map below shows the countries (in orange or red) that have few or no policies to promote cleaner fuels or stoves:

What's more, simply enacting policies to promote cleaner fuels isn't always enough. The UNEP report notes that governments in the Caribbean, Central America, and South America have pushed to expand access to cleaner fuels, yet indoor air pollution remains stubbornly high. Many people continue to burn wood indoors because it's a cheaper or more dependable option.

Governments and aid groups have also struggled to convince people to switch to cleaner stoves at times. One 2012 study looked at what happened when randomly selected households in Orissa, India, were given cookstoves with chimneys that diverted the smoke outside the house. After one year, smoke inhalation dropped compared with control groups. But after four years, there was no meaningful improvement in public health. Why? Because the households stopped using the stoves after a while — they required too many repairs, and the chimneys needed constant sweeping.

That doesn't mean it's all hopeless. A separate 2011 study gave randomly selected households in Guatemala a wood stove with chimney ventilation. That, researchers found, did seem to reduce cases of childhood pneumonia. The Lancet study notes that similar randomized control trials for other technologies are ongoing around the world.

The Lancet study also noted some other promising technologies in the future. Some groups have held out hope for clean solar-powered cookstoves, although the paper notes that these stoves don't offer nearly as precise control for cooking, and they're not wanted if they can't be used at night. Indeed, the authors note that people are unlikely to adopt a new technology just because it's healthier — it has to be more convenient too.

Ultimately, this is another reason why energy access is such a crucial issue. People in the United States (or even the wealthier parts of China) don't need to worry about indoor air pollution because we heat our homes and cook with electricity and gas. But there are still 1.2 billion people around the world who don't have any access to electricity at all. It's not just an inconvenience — it's a public health crisis.

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