A breakdown of the sources of many air pollutants that damage our health and ecosystems.

Millions of people die prematurely from air pollution every year. This problem has existed since humans started burning materials for fuel — first wood and biomass, then fossil fuels.

But it’s an environmental and public health problem that we can make progress on. We know this because the world has already been successful in reducing air pollutants, and many countries that used to be highly polluted now have much cleaner air than they used to.

To tackle air pollution effectively — to focus our efforts on the interventions that will have the biggest impact — we need to understand where it’s coming from.

That’s why we wrote this article.¹

Before we discuss the sources of some of the key air pollutants, we should briefly explain how air pollution affects human health and how each of these pollutants contributes to this.

There are three key pathways by which a pollutant can cause harm³:

• Direct exposure: Some gases are toxic and can have an acute effect on health. These acute impacts are more common for people with existing respiratory conditions such as asthma or chronic obstructive pulmonary disease (COPD). This direct exposure does cost lives, but the total number is relatively small compared to the millions that die from chronic exposure to air pollution.
• Formation of particulate matter: Many of the pollutants we’ll look at contribute to health impacts indirectly by breaking down to form secondary smaller particles. These particles are called “particulate matter”. Typically, the smaller the particles are, the worse they can be for human health because they can enter our lungs and airways — and, in some cases, the bloodstream. Particulate matter can cause respiratory and cardiovascular problems, including cancer, strokes, and heart attacks.
• Formation of ozone: Another indirect way these pollutants can affect our health is by forming a gas called ozone (O₃). Ozone can cause breathing problems and worsen acute conditions like asthma and COPD⁴. However, it also affects our health through chronic exposure by causing inflammation of the lungs, increasing the risk of respiratory diseases, and reducing our cardiovascular health.

As we discuss each pollutant, we’ll briefly explain how it affects health through one or several pathways. While it’s difficult to pinpoint exactly how many deaths each pollutant causes, wherever possible, we’ll also try to give a rough order of magnitude estimate.

To give some sense of scale, here is the Global Burden of Disease’s breakdown of global deaths from air pollution.⁵

In 2021, this totaled around 8 million deaths. Note that there are also natural sources of particulate matter, so not all of these pollution deaths resulted from human emissions. But most did. For more on this, see our colleague Max Roser’s article, which looks at estimates from various sources.

3.1 million came from household air pollution, a combination of direct toxicity and particulate matter. 4.7 million came from outdoor particulate matter, and another half a million from outdoor ozone pollution.⁶

Sulfur dioxide: the source of acid rain

Sulfur dioxide (SO₂) is the main pollutant that causes acid rain. This has been a major environmental problem because acid rain can change the chemistry of rivers and lakes, affecting fish populations, soils, and the extent and quality of forests. You can also see the effects of acid rain on older limestone and marble buildings and statues, where the acidity dissolves parts of the structure.

There are two ways that SO₂ can threaten human health. First, direct inhalation of SO₂ can exacerbate respiratory problems such as asthma and bronchitis. But its main contribution is by breaking down to form small particulate matter. While an exact figure is hard to pin down, given that sulfur dioxide is a substantial contributor to particulate matter and that at least 4 million deaths are linked to these small particles yearly, we would estimate that hundreds of thousands of deaths per year are linked to SO₂.

SO₂ is formed when we burn fuels that contain sulfur.

The charts below show where global emissions come from and how these sectors have changed over time. In 2022, energy production was the biggest contributor by far. This is predominantly due to power from coal, which has sulfur impurities that are released when it’s burned.

The main contribution of the industry is the metal smelting process.⁷ This is because many of the ores that are used to produce metals – such as pyrite – contain large amounts of sulfur, which is released when they are roasted at high temperatures.

Oil also contains sulfur, which is why road transport, shipping, and aviation all contribute. Shipping emissions have received a lot of attention in the last few years because they dropped by more than 70% in 2020 after the introduction of tight regulations on maritime fuels.

You’ll notice that global emissions of SO₂ peaked in 1979 and have almost halved since then, thanks to the introduction of po

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