Every year, air pollution kills about 6.5 million people worldwide — linked to everything from lung cancer to heart disease to strokes. It’s an honest-to-goodness public health crisis. So the International Energy Agency just put out a huge report on how pollution got so bad and what we might do about it.

Here’s a striking chart showing where air pollution is deadliest, with countries ranked by deaths per 100,000 people. This includes deaths linked to sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), and indoor air pollution from burning solid fuels. It doesn’t include greenhouse gases or climate change:

The statistics are from the World Health Organization here. A couple things stand out here:

1) These are mortality rates, not absolute figures. Georgia, Bosnia and Herzegovina, North Korea, and Bulgaria top the list, but countries like China and India have more total deaths from air pollution since they have so many more people.

2) These blunt rankings can obscure a lot of variation within countries. In China, for instance, heavier coal pollution north of the Huai River shaves about 5.5 years off life expectancy compared with the south, according to one study.

3) The United States certainly has room for improvement, but it has some of the cleanest air around. One reason the country has lower mortality from pollution than, say, France or Germany is that we have fewer diesel cars on the road. Diesel engines can be more fuel efficient than gasoline engines, but they also emit more soot, particulates, and NOx. Europe has been struggling to clean up these pollutants, particularly since lax testing procedures have allowed many cars to emit more than the legal limits.

4) How did the republic of Georgia get to be No. 1 anyway? Neither the IEA nor WHO offers much detail. But via Twitter, Tim Kovach pointed me to Georgia’s recent State of the Environment report for some clues. The country has seen a sharp rise in the number of old, dirty diesel vehicles on the road over the last decade — and pollution testing is virtually non-existent:

In Georgia the public transport system is not sufficiently developed and as a consequence a significant proportion of the population uses private vehicles as the preferred mode of transport. As a result, the number of private vehicles has grown rapidly over the past decade and has almost doubled in the last five year period. Most of the cars purchased are secondhand cars imported from aboard and the average age of the fleet in Georgia is 10-15 years. Diesel engine cars are very popular. Roadworthiness testing of vehicles is not in operation at the moment in Georgia and as a result many cars on the roads are in bad mechanical condition. Periodic testing of exhaust gases has also been suspended. Some low quality fuels available on the market can cause damage quickly to the catalytic converters of vehicle exhausts. Car owners tend to have the damaged catalytic converters removed and not replaced, resulting in higher emissions from the vehicle. Traffic management is still problematic in the cities of Georgia and traffic jams happen quite often. All of these factors lead to high emissions from motor transport in Georgia.

Kovach also points out a legacy of Soviet industry: Old manganese mines, for instance, still have very high levels of air pollution. Quality of health care may also play a role in differing mortality rates.

5) None of this is inevitable! As countries get richer, they tend to invest in clean-up technologies that improve air quality — something China is currently focusing on. Indeed, much of the IEA report is devoted to outlining policies and programs that will reduce overall air pollution deaths by 3.3 million by 2040.

That includes installing emissions-control technologies on cars and power plants in the developing world, as well as giving people access to clean fuels for the cooking and heating stoves (a major source of deadly indoor pollution). I’ll just say a bit about that last one.

Why cleaning up indoor air pollution is so difficult

I’ve written before about indoor air pollution, which is responsible for about half the world’s pollution deaths each year. The basic issue is that 2.7 billion people, mostly the rural poor, still burn wood, dung, or other solid fuels as energy for their lighting, cooking, and heating needs. The uncontrolled particulate pollution from these kerosene lamps and wood stoves is absolutely lethal.

Getting rid of this indoor air pollution, and saving some 3.5 million lives each year, would basically require increasing access to cleaner stoves and fuels. That includes stoves with better ventilation. Or stoves that use liquefied petroleum gas (LPG), which burns more cleanly than wood. Or hooking them up to the grid so that they have electricity.

So what’s the holdup? The IEA report has a good discussion of some of the barriers. For now, at least, burning wood or coal in traditional stoves is considerably cheaper than the alternatives, even if the emissions are far higher. Here’s a comparison of costs in India:

Investing in a traditional three-stone wood-burning stove costs less than 5 percent of an Indian household’s monthly income. An "improved cookstove" with ventilation is cleaner, but the investment might cost around 15 percent. Investing in an even-cleaner LPG stove might cost about 40 percent. And going full electric is more expensive still.

Some governments have started providing subsidies for cleaner cookstoves or LPG, but that’s not always enough. "LPG providers typically face difficulties establishing a market in areas where consumption prospects or population density are low or the road infrastructure is poor," the report says. "Until networks are sufficiently well established to ensure reliable supply, consumers typically (and rationally) prefer to refrain from switching."

What’s more, many people aren’t even aware of the health effects of burning wood or dung indoors, so they don’t bother switching even if they could. From the report: "This is why distribution campaigns for improved cookstoves tend to have an important public awareness component, to educate not only on proper use of the improved stoves but also the health benefits of doing so."

There are also knock-on environmental benefits to consider. Back in 2013, Sunil Nautiyal wrote a fascinating paper in the Journal of Mountain Science about how the switch from wood to LPG fuel in parts of India helped cut down on deforestation.

One final tidbit: The IEA estimates that providing universal access to cleaner stoves would cost the world around $55 billion by 2040. That’s a pittance beside the $1.1 trillion it will take to bring (at least a little bit) of electricity to the 1.2 billion people who don’t have it. And clean stoves would save 3.5 million lives a year in the process. The world ought to do both, obviously, but tackling indoor pollution in particular would have enormous payoffs.

Further reading: A closer look at indoor air pollution, the world's deadliest environmental problem.

A visual tour of the world's CO2 emissions