Estimating the number of people that have died as a result of air pollution, or any type of pollution, isn’t the easiest task. Breathing in ultra-fine particulate matter and toxic gases can result in premature death, through respiratory or cardiovascular problems. Getting a clear cause-and-effect link, and quantifying that, is difficult.

Thankfully, scientists all over the world have worked on such estimates, and a new study – led by the Max Planck Institute for Chemistry in Mainz (MPI-C) and the London School of Hygiene & Tropical Medicine – has the latest. In 2015 alone, as many as 4.5 million people died prematurely from diseases linked to ambient (outdoor) air pollution, including 237,000 children below the age of five from respiratory afflictions. The findings are published in The Lancet: Planetary Health.

Before we go into the study itself, let’s put that 4.5 million number in context. That’s 12,329 people per day, on average. That’s like having just over four September 11 terrorist attacks every 24 hours. As another point of comparison, 1.3 million people die in road traffic accidents every year, which is 3.5 times fewer than those dying from ambient air pollution.

It’s a ludicrous figure, one that highlights just how much of a crisis this is. It neatly fits with another recent analysis by the World Health Organization (WHO), which estimates 4.2 million people die via ambient air pollution each year.

It’s largely about that particulate matter. It comes in all shapes and sizes when it emerges from smokestacks from power plants, agricultural activities, construction work, and vehicle exhaust pipes, but those smaller than 2.5 microns across – about 30 times finer than a human hair – are the real killers.

They’re so small that they easily get into your respiratory tract, and thereby kick up your odds of getting a whole host of illnesses, from heart attacks and strokes to lung cancer and obstructive pulmonary diseases.

That’s not all, of course. You’ve also got nitrogen dioxide, ozone, and sulfur dioxide, all of which can irritate and inflame the lining of your airways. So, with all this in mind, how did this new paper get to 4.5 million?

For this latest paper, the team took a multi-pronged approach. They used results from a complex computer model, one that simulated how chemicals move through the atmosphere. Health stats from the WHO were plugged in, and likelihoods of premature deaths due to air pollution – particularly due to afflictions of the lung – were calculated.

Remarkably, 4.5 million deaths isn’t even the most jaw-dropping figure.

This also equates to 122 million years of life being lost through premature deaths – so much time robbed unfairly. The paper didn’t even take into account household pollution, which for nations that rely on coal and wood-burning stoves, is a huge problem too.

The team also found that the mortality rate from ambient air pollution was highest in Asia, whereas the highest rate of years of lost life per person could be found in Africa. Low-income countries were affected the most.

In Pakistan and India, the mortality rate was 1.2 times higher for girls than for boys, which was linked to nutritional priorities being given to the latter. This reminds us that air pollution, like climate change, affects the already disadvantaged disproportionately.

It's true that, if you’re near a source of air pollution, no matter who you are, you’re at an increased risk of getting sick. Studies repeatedly show, however, that the old, very young, and infirmed are at a far greater risk. Girls and women – who suffer from societal discrimination all over the world – are more open to risk. The poor will easily suffer fare more than the affluent.

Another comprehensive analysis in The Lancet found that pollution of all kinds killed 9 million people in 2015. That's 15 times more people in 2015 than deaths from all wars and other forms of violence put together. How on Earth do we deal with this colossal killer?

Lead author Prof. Jos Lelieveld, a chemist at the MPI-C, told IFLScience that one potential way to deal with this would be to look to the UN.

“The UN could adopt ‘clean air’ as a sustainable development goal, which may have even higher priority as ‘clean water’, since many more people die from polluted air than from polluted water” by a factor of 3 to 6.

Lelieveld cautiously noted, however, that with few exceptions, each individual country is still personally responsible for environmental legislation implementation. Like climate change, then, this has to be an international effort, where everyone pulls their weight.