Air pollution has harmful effects on the health of humans, wildlife and our environment.

It’s caused by a variety of sources - in the UK, the biggest threat to clean air is traffic emissions. Other contributors include power plants, industrial facilities, agricultural processes and domestic burning, including wood burning stoves. But our air quality can be improved. Discover the major role trees have to play, and how we can all take action to reduce air pollution.

What is air pollution?

Air pollution is the presence of substances in the air that have harmful or poisonous effects for people, wildlife and the environment. 

Air pollution: some key terms 

Particulate matter 

A mix of solids and liquids suspended in the air. Some are visible to the naked eye, for example as soot or smoke. The smaller ones, PM2.5, are more damaging as they can be inhaled deep into the lungs. These particles have a diameter of 2.5 microns - over 100 times thinner than a human hair. The chemical make up varies, but can include oxides of nitrogen and ammonia. 

Reactive nitrogen   

Nitrogen is a stable element which makes up 70% of Earth's atmosphere, but its other forms, including ammonia (NH3) and oxides of nitrogen (NOx), are constantly in flux in the environment. These are referred to as reactive nitrogen. 

  • Ammonia (NH3) 

Coming from sources including animal waste, fertiliser use and vehicle exhausts, ammonia can combine with other compounds in the atmosphere to create PM2.5. It's one of the main forms of nitrogen pollution.

  • Oxides of nitrogen (NOx)  

Nitrogen oxides (NOx) form when nitrogen and oxygen react in the air during combustion processes, including in vehicles and power stations. They are also released from agricultural soils when fertilisers and manures are added.  


of people on Earth live with poor or dangerous air quality

according to a World Health Organisation report.

Air pollution in the UK

Air quality in the UK has improved in recent years, but many UK cities continue to exceed safe limits for harmful particulates and oxides of nitrogen (NOx).

The World Health Organisation recommended limit for levels of PM2.5 particles in the air is 10 micrograms/m3. But dozens of UK towns and cities are over this limit. Data collected in 2016 shows the worst places in the UK for particulate pollution were:

  • Stanford-Le-Hope, Essex - 13 micrograms/m3
  • Storrington, West Sussex - 13 micrograms/m3
  • Swansea, South Wales - 13 micrograms/m3
  • London - 12 micrograms/m3

The best air quality tends to be in more open areas and coastal areas, where the air changes more frequently. But even the countryside isn't free from air pollution. Emissions of ammonia (NH3) continue to increase - in 2016, agriculture accounted for 88% of all UK ammonia emissions. The largest contributions were from livestock farming, especially cattle and the expanding pig and poultry industries. Ammonia pollution can devastate plants and animals, but much of these agricultural emissions are unregulated.

Did you know?

The UK has one of the highest rates of childhood asthma in the world.

The impact of poor air quality on human health

Poor air quality is linked to many health conditions, including cancer, asthma, stroke, heart disease, diabetes and obesity. In addition to the personal cost of mortality and ill health, the impacts of air pollution also have a high cost to society. Air pollution isn't distributed evenly, and often the poorest neighbourhoods suffer the worst air quality.

A 2018 study in Bradford showed that around 38% of childhood asthmas were attributable to poor air quality, specifically levels of NOx, and that much of this was from traffic related pollution.

Up to 36,000

UK deaths per year

can be attributed to human-made air pollution.


estimated cost to UK society

of impacts of air pollution every year.

Air pollution and the climate crisis

Air quality is a local problem. The climate crisis is a global problem. But the two are closely linked in several ways.

Many of the drivers of local air pollution are also sources of greenhouse gas emissions which cause climate change. For example, burning fossil fuels for transport, industry and in our homes releases locally harmful particulates and the greenhouse gas carbon dioxide.

The sorts of health problems made worse by air pollution, like heart or respiratory failure, are also made worse by prolonged high temperatures - especially among the elderly, very young or chronically ill. We're already seeing extreme heatwaves due to climate change and they'll become more severe in the coming years. The combination of soaring temperatures and poor air quality can be fatal.

Did you know?

Reactive nitrogen in the environment is one of the most significant threats to global biodiversity according to the 2019 global biodiversity assessment by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services.


Air pollution and the biodiversity crisis

Air pollution doesn't just harm our health and climate - it impacts ecosystem health too.

Nature is in crisis in the UK. Only 7% of our woods are in favourable ecological condition. Irreplaceable ancient woodlands continue to be damaged and destroyed by development. But a more insidious threat facing UK woods and trees is unseen reactive nitrogen air pollution, mainly in the form of ammonia emissions from agriculture. 

Excess nitrogen, including ammonia, has many impacts on the natural world. Our State of the UK’s Woods and Trees report identifies nitrogen air pollution as one of the most widespread and significant threats to woodland ecosystems in the UK.

Nitrogen air pollution directly affects many plants and fungi. It strips trees of their protective lichens and causes a fertiliser effect where grasses out-compete more delicate woodland flowers. This disrupts woodland ecosystems in ways we are only beginning to understand.

Most UK woodland habitats are exceeding the level of nitrogen deposition at which the ecosystem is considered to deteriorate.

How trees reduce air pollution

The links between local air quality, climate change and the biodiversity crisis mean that acting on air pollution can offer a ‘win-win-win’ strategy for the climate, people and nature. Trees can play several important roles in this scenario.

Credit: John MacPherson / WTML

Capturing pollutants

Trees and other vegetation planted in the right places can help improve urban air quality on a local scale by forming a barrier between people and pollutants. They also remove some particulate pollution from the air by catching the tiny particles on their leaf surfaces. Research has found significantly lower asthma rates among children aged 4-5 in areas with more street trees. As well as reducing air pollution, trees take carbon dioxide from the air, helping in the fight to limit further climate change.

Credit: Keith Morris News / Alamy Stock Photo

Beating the heat

Trees will be essential in helping us adapt our cities and landscapes to the climate impacts which are now inevitable. Green spaces with trees in cities provide shade and reduce the ambient temperature through the cooling effect of evaporation of water from the soil and through leaves – crucial during frequent severe heatwaves.

Credit: Tony Cox / WTML

Havens for wildlife

Planting and protecting native trees will also provide vital homes for wildlife, supporting biodiversity. They provide food and shelter for a myriad of species, from mammals and reptiles to birds and butterflies.

Credit: Philip Formby / WTML

Protecting irreplaceable habitats

Strategically placed new native woodland creation can help provide a buffer around precious ancient woodlands. This will protect these fragile ecosystems from the worst of the nitrogen pollution and buy time while we fight to cut the emissions at source.

Time for change

There are real costs to health, climate and nature associated with poor air quality. Tackling air pollution in all its forms will make all our lives better, right now and in the future.

The key to improving air quality overall is to reduce pollutants at source as well as implementing measures to limit exposure to the pollutants.

The Government has promised us 50 million more trees each year until 2050 to achieve net zero carbon. Improving air quality should be identified as an important objective and given careful consideration as part of woodland creation and greening projects in areas including:

  • urban streets
  • school playgrounds
  • hospital grounds
  • car parks
  • city parks and green space
  • around industrial sites
  • around intensive livestock units
  • around sensitive habitats where there is a pollution source in the vicinity.

Credit: Phil Formby / WTML

In our countryside

Decades of pollution have shifted the public’s image of what a woodland should look like. Grass, brambles and nettles have replaced a diversity of wildflowers. Bare trunks devoid of lichen is the new normal. Historically woodland would have been covered in a rich array of lichens – adding biodiversity value and beauty. Let’s stop accepting degraded ecosystems as the best version of nature we can expect to experience. Let’s work to restore whole, healthy, and functioning biodiversity. 

Credit: / william87

In our towns and cities

We should live in green places and move around through green spaces. Instead of walking alongside busy roads, inhaling the fumes and detritus of urban traffic, let’s make places where we can walk or cycle to school, work or leisure through parks and green corridors with trees and plants – places where we can breathe. It is critical that we maintain existing urban trees and expand tree cover. This requires vision from those responsible for design and planning in our towns and cities.

What can I do about air pollution?

We can all do our bit to make a difference. Here are some ideas:

  • Consider the best way to travel - instead of driving, could you walk, cycle, car share or use public transport?
  • Use energy efficient appliances and light bulbs, and turn them off when you're not using them.
  • Reduce how much you burn at home. If you have a wood burning stove, follow guidance on correct installation, maintenance and fuel.
  • Plant trees - get involved in planting activity, or organise your own.
  • Stand up for trees and woods - join us in defending the trees we already have, from street trees to ancient woods.
  • Visit woods - if you can, reset your baseline expectations and visit one of our least polluted forests. Breathe in the pure air and remind yourself what we're fighting for.
  • Speak up - ask your local council how it's addressing air pollution and let them know the role trees can play. Or let your MP know you will support bold action on reducing ammonia pollution from agriculture.

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