Indoor air pollution still gets fairly little attention for such a spectacularly lethal public-health problem.

Indoor air pollution now kills 4 million people a year — more than AIDS and malaria combined

Here's the basic version: About 3 billion people around the world — mostly in Africa and Asia, and mostly very poor — still cook and heat their homes by burning coal, charcoal, dung, wood, or plant residue in their homes. These homes often have poor ventilation, and the smoke can cause all sorts of respiratory diseases.

All told, indoor air pollution kills between 3.5 million and 4.3 million people each year. To put that in perspective, that's more deaths than are caused by HIV/AIDS (around 1.6 million per year) and malaria (around 627,000) — combined.

This month, The Lancet Respiratory Medicine published a big new report taking a detailed look at indoor air pollution — with 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)

There are a couple of key points in the Lancet study:

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.

Women and children in extreme poverty are most likely to be affected

Lighting is another big source of indoor air pollution — particularly the use of unvented kerosene lamps. The study notes 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 notes. "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) Indoor air pollution kills between 3.5 million and 4.3 million people a year. A 2012 Global Burden of Disease study found that household air pollution killed 3.5 million people a year — making it the deadliest environmental problem.

A follow-up study by the World Health Organization in 2014 upped that estimate to 4.3 million deaths, mainly by including cardiovascular deaths associated with exposure to air pollution.

(Also note that outdoor air pollution from coal plants, vehicles, and other sources is linked to about 3 million deaths per year — not quite as deadly, but still quite deadly.)

The Lancet study spends a lot of time exploring the various respiratory diseases linked to indoor air-pollution exposure. Lung cancer, for instance, is strongly associated with indoor coal-burning. Other chronic lung diseases are associated with indoor wood-burning. Meanwhile, the effects of smoke exposure on young children are still being explored.

4) All that soot is also a contributor to climate change: This isn't mentioned in the Lancet paper, but 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.

As a result, many green groups have turned their attention to tamping down on household air pollution as a way to boost public health and slow the pace of climate change at once.

So what's the best way to reduce indoor air pollution?

Over the last decade, public-health researchers have been exploring more and more ways to help minimize the effects of indoor air pollution. And it turns out to be surprisingly difficult.

Outside efforts to persuade households to adopt cleaner technologies often fail

There are plenty of proven technologies out there — including cleaner-burning cookstoves with better ventilation that use wood or crop pellets. But efforts by aid groups to persuade communities to adopt these technologies have often failed.

For example, one 2012 study looked at what happened when randomly selected households in Orissa, India, were given cleaner-burning cookstoves with chimneys. After a year, smoke inhalation had dropped compared to control groups. But after four years, there was no meaningful improvement in public health. Why? Because the households stopped using the stoves after awhile — they required too many repairs and the chimneys needed constant sweeping.

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

The Lancet study also notes 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.

And, 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 big public health issue.

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