Particulate matter (PM₂.₅ and PM₁₀)

Particulate matter is the name for the mix of tiny solid bits and liquid droplets floating in the air. The particles are sorted by size. PM10 means particles up to 10 micrometres wide. PM2.5 means the even smaller ones, up to 2.5 micrometres. To picture this, a single human hair is about 30 times wider than the largest PM2.5 particle. Around two-thirds of PM10 is made up of these finer PM2.5 particles.

Particles come from many sources, both human and natural. The biggest human sources are burning fuel and traffic. Burning wood and coal at home is a major source, especially on winter evenings. Traffic produces particles not just from exhausts but from brakes, tyres and dust thrown up from the road, so even electric cars create some. Building work, quarrying and industry add more. Natural sources include sea spray, pollen, wildfires and dust blown in from as far away as the Sahara.

Some particles are released directly, such as the soot from a fire. Many others form in the air when gases react together, including ammonia and the gases given off by burning fuel. Because of this, cutting those gases also cuts particle pollution.

Particulate matter is the pollutant most strongly linked to harm. Breathing it in over both short and long periods is tied to heart and lung illness and to earlier death. Smaller particles reach further into the body: PM2.5 can travel deep into the lungs and pass into the blood. There is no level low enough to be safe for everyone, which is why the aim is to lower how much people breathe in overall, rather than just to meet a fixed limit.

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Nitrogen oxides (NOₓ, including NO₂)

Nitrogen oxides, often shortened to NOx, are a pair of gases: nitric oxide (NO) and nitrogen dioxide (NO2). NO changes quickly into NO2 once it is in the air. Of the two, NO2 is the one that matters most for health, so it is usually the focus.

Almost anything that burns fuel at high temperature makes nitrogen oxides. In the UK the main source is road traffic, followed by power stations and other industry. Because of this, NO2 levels are highest near busy roads and in traffic, which is why it is often the main pollutant of concern in towns and cities.

At high levels NO2 inflames the airways. Longer exposure may reduce how well the lungs work and make breathing problems worse, and it can make sensitive people react more strongly to things they are allergic to. NO2 also helps form other pollutants, including some particles and ground-level ozone. When it lands on soil and plants downwind, it can make them more acidic and add too much nitrogen, which harms wildlife — sometimes far from where the gas was released.

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Ozone (O₃)

Ozone is unusual. High up in the atmosphere it is helpful, forming a layer that shields us from the sun’s harmful rays. But down at ground level, where we breathe, it is a pollutant.

Ground-level ozone is not released by anything directly. It forms when other pollutants — nitrogen oxides and a group of gases called volatile organic compounds — react in strong sunlight. This takes hours or days, so ozone builds up on hot, still, sunny summer days. The pollution that causes it may have travelled a long way, even from other countries.

Ozone irritates the eyes and nose. At higher levels it inflames the airways, makes the lungs work less well, and is linked to more hospital visits and deaths from breathing problems. It also harms crops, trees and other plants. Because ozone spreads over wide areas, one place’s levels can reflect pollution from far away, and no single town can solve it alone.

Sulphur dioxide (SO₂)

Sulphur dioxide is a sharp-smelling gas. It is made mainly by burning fuels that contain sulphur, such as coal and heavy oils.

In the UK it used to come mostly from power stations and refineries. Levels have dropped a great deal in recent decades as coal use has fallen. In some areas, such as parts of Northern Ireland, burning coal at home is still a notable source.

Sulphur dioxide can tighten the airways. People with asthma or other lung conditions feel this most. It also helps form particle pollution, adding to that harm indirectly. In the environment it makes soils and water more acidic and can damage plants, often well downwind of where it started.

Carbon monoxide (CO)

Carbon monoxide is a gas you cannot see or smell. It forms when fuels containing carbon do not burn fully.

Outdoors, road traffic is the largest source, with home and industrial burning adding more. Indoors, faulty appliances and poor ventilation can build up dangerous amounts — which is why carbon monoxide alarms are fitted in homes.

Carbon monoxide lowers the blood’s ability to carry oxygen around the body. People whose heart or brain is already short of oxygen, such as those with heart disease, are most at risk.

Polycyclic aromatic hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons, or PAHs, are a large family of chemicals made by burning. Because there are so many, one of them — benzo[a]pyrene — is often used to stand in for the most harmful ones.

In the UK the most harmful PAHs come mainly from burning coal and wood at home, from fires of all kinds, and from some industry. Road traffic releases the largest amount of PAHs overall, though these tend to be the less harmful types.

Studies of workers exposed to PAHs show higher rates of some cancers, with lung cancer most clearly linked to breathing them in. Different PAHs vary widely in how harmful they are. Several, including benzo[a]pyrene, are judged to probably or possibly cause cancer in people.

Benzene

Benzene is a chemical that evaporates easily into the air and is known to cause cancer in people.

It comes from burning fuel at home and in industry, and from road traffic.

Because benzene damages the genetic material in cells, no level in the air can be called completely safe. Workers exposed to high amounts have shown a raised risk of leukaemia, a cancer of the blood.

1,3-Butadiene

1,3-butadiene is another chemical that evaporates into the air and is known to cause cancer.

It comes mainly from burning petrol, so vehicles and other engines are the main source, with some from industry such as making the synthetic rubber used in tyres.

As with benzene, no level in the air is completely safe. The main concern is cancer of the blood and related tissues, including leukaemia and lymphoma.

Lead (Pb)

Lead is a toxic metal. It used to be a major air pollutant when petrol contained it. Since leaded petrol was phased out, lead levels in the air have fallen sharply.

The lead still found in the air today comes mainly from burning coal and from processing iron, steel and other metals.

Breathing in high amounts of lead can harm the kidneys, gut, joints and reproductive system, and can damage the nervous system. A particular concern is that lead can affect how young children’s brains develop.

Ammonia (NH₃)

Ammonia is a gas containing nitrogen. As other pollutants fall, it is becoming relatively more important.

Almost all of it comes from farming, mainly from animal manure and slurry and from fertilisers. Smaller amounts come from traffic and waste.

Ammonia helps form particle pollution, so it adds indirectly to the large health burden from PM2.5 and PM10. In the environment, it makes soils and water more acidic and adds too much nitrogen, which reduces the variety of wildlife in sensitive places.