What are particles?

Put simply, particles are specks of solid material suspended and transported in the air. There is a wide range of particles in the air that we breathe – from relatively large – those that form visible dust, reducing visibility and soiling buildings, to the very fine particles that can penetrate deep into our lungs and have been found to damage health. Particles are classified by size. However, the composition of particles varies widely, and this makes determining the effects of particle pollution on health particularly difficult.

Pollutant Example Definition
Suspended particle matter sand, soil, salt all airborne particles
Nuisance dust from building ,quarrying particles that soil buildings etc
Aerosol Industrial smoke, seawater particles suspended in gas
PM10 sea salts, mineral dust, pollen Particles less than 10µm in diameter
PM2.5 Vehicle exhausts Particle less than 2.5µm in diameter
Smoke Industry, bonfires Particles less than 15µm in diameter
Black smoke Industry Dark particulate matter
Black carbon Diesel vehicle exhausts, industry, solid fuel heating Carbon based particles which contribute to climate change
Inhalable particles Those larger than 4 µm diameter Particles which may be breathed in
Respirable particles Those smaller than 4 µm diameter Particles which penetrate deep into the lung

Where do particles come from?

There are natural sources of particles as well as those emitted from our activities.

Natural sources: windblown dust, soil, sand, sea salt, ash from volcanoes and forest fires, pollen. Most of this natural dust in our air is inert – it may cause a nuisance, but does not have a serious impact on our health. The particles that we are responsible for are smaller and more reactive, and therefore more likely to cause problems.

Primary particles

These are particles that are emitted or blown directly into the air – there are many sources including emissions from vehicles and road surfaces, chimney stacks, dust from storage areas, spoil heaps, emissions from buildings (ventilation, boilers and solid fuel combustion), materials handling and construction sites.

Secondary sources

Particles may form when substances react in the atmosphere. These are often from the oxidation of sulphur and nitrogen oxides, which form nitrates and ammonium salts. These are usually less than 10 µm diameter, and originate from combustion and natural sources.

Size is important

The behaviour of an atmospheric particle is determined by size. Larger particles will not stay in the air for very long and will be deposited close to their source. Smaller particles can be carried long distances.

  • nucleation mode (< 0.2µm diameter) – particles recently emitted from a process or freshly formed within the atmosphere, present in large numbers in urban air.
  • accumulation mode (> 0.2µm diameter – 2µm diameter) particles which have grown from nucleation mode by coagulation or condensation of vapours. Stable with an atmospheric lifetime of 7-30 days.
  • coarse mode (> 2µm diameter) – mainly formed by weathering processes, includes soil dust, sea spray and industrial dusts. Large size means the atmospheric lifetime is short.

Composition of Particles

In urban air, particles come from many sources. Along with size, the composition of particles in air will determine any health effects they have. Assessing the exact makeup of particles in air is complex, however, as they are monitored either by size or composition.

Health effects

Particles can cause irritation to the eyes, nose and throat. Larger particles reaching the nose or throat will be filtered out by the body’s natural defence system. However, the very tiny particles that reach deep into the lung may be absorbed into the blood stream or cause lung problems. Those between 4µm and 10µm diameter are trapped by mucus in the airways, and those less than 4µm can reach deep into the air sacs in the lungs. Particle pollution <2.5µm diameter comes mainly from vehicles, and is responsible for elevated levels during pollution episodes, and is therefore associated with health effects. The most recent evidence from the Government’s Committee on the Medical Effects of Air Pollution (COMEAP) suggests that some 29,000 deaths per year (4,000 in London alone) are brought forward by exposure to man-made particulate air pollution at current levels.

Both the size and composition of the particles, and length of time of exposure, determine any potential health effect. Exposure to particles can lead to mortality (death), increased admissions to hospital of people suffering from cardiovascular (heart) disease (attacks and strokes) and pulmonary (lung) disease, such as chronic obstructive pulmonary disease (COPD), bronchitis and asthma. Other compounds found on particulates, such as some hydrocarbons or metals, can cause cancer or poisoning. People who are particularly vulnerable to the effects of particulate pollution include the young, the old and those with existing cardiovascular and respiratory conditions, who often find their symptoms worsen during episodes of high pollution levels.

As the composition of particle pollution is so variable an average approach is taken to assessing health effects. The Committee on the Medical Effects of Air Pollution (COMEAP) estimate that long term exposure to a 10µg per m3 increase in PM2.5 concentrations leads to a 6% increase in ‘all cause mortality’, or total deaths. Across Europe particle pollution is believed to reduce the average life expectancy by 8 months.

Environmental effects

Climate Change

Black carbon particles are ‘short lived climate forcers’. These have a similar effect on the climate to carbon dioxide (CO2) but on a much shorter timescale. They can also have a strong regional impact on climate warming. At present most effort to address climate change is focused on CO2, but there is also an opportunity to reduce the impacts through the control of black carbon particulate pollution.

Environmental Protection UK, with ClientEarth, have established the Black Carbon Campaign, which aims to raise awareness of the need to cut global black carbon emissions in order to achieve rapid climate change mitigation and slow the rate of Arctic melting. The Campaign will work at international, national and local levels to raise awareness of both the need to reduce emissions and the practical ways in which emission reductions can be achieved.

The Black Carbon and Climate Change Summit was held in March 2012 at the Royal Society in London in London. It aimed to explore the UK’s options for a policy response in light of the United Nations Environment Programme (UNEP) report ‘Integrated Assessment of Black Carbon and Tropospheric Ozone’ released late in 2011. Further details of this conference are available on the Client Earth website.


Particles in the atmosphere can affect visibility as they absorb and/or scatter light. Particles can also reduce sunshine reaching the earth, especially when the sun is low in the sky. It has been estimated that carbon is responsible for 25 – 45% of reduction in visibility in urban areas. At high humidity levels they also contribute to fog formation.


In 1990/91 a survey estimated that £79 million was spent on stone cleaning every year (QUARG, 1993). Soiling by particulates is most commonly observed on cars, washing, paintwork on houses and internal windowsills. The extent of this is difficult to quantify but can cause serious problems for people living nearby.


Dust can cause nuisance problems, particularly near quarries, construction sites and cement works. Dust from roads is also a frequent source of complaint.


The European Union has set two objectives for PM10. These are an annual mean of no more than 40 µg/m3 and a daily mean of 50 µg/m3. In the latter case the objective is for no more than 35 exceedences of this limit per year. A large proportion of EU member states have reported breaches of the PM objectives, including the UK. The EU is considering infringement proceedings which could results in heavy fines for the UK government.

The EU Thematic Strategy on Air Pollution is currently under review, and Environmental Protection UK are lobbying for the need for policies and legislation with strong limits and timescales to protect human health.

Scotland has set a tougher annual objective of 18 µg/m3, which must be complied with by 2010.

The UK Air Quality Strategy introduced target values for PM 2.5. This includes the concept of exposure reduction – as PM 2.5 is understood to have no safe limit for health it is beneficial to reduce levels at all locations, not just hotspots that break a set limit. The new targets are therefore a 25µg/m3 ‘cap’ for hotspots and a 15% reduction in PM 2.5 levels in all urban locations by 2020.

Scotland again has set a tougher target of 12µg/m3 by 2020.

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