PM2.5 PollutionPM2.5 pollution (particulate pollution the size of 2.5 microns) is a complex mixture of solid, airborne particles and liquid droplets. Particulate pollution is one of two types of air pollution – the other being gaseous pollution; which includes: ozone, nitrogen oxides and sulphur dioxide. Particulate pollution is classed according to size.

So what is PM2.5? It is particles of diameters 10 micrometres (mm), or less, that are known as PM10s, and smaller particles, of diameter 2.5mm or less, are known as PM2.5s - or ‘fine’ particles. ‘Ultrafine particles’ are particulates the size of 0.3 microns or smaller. The major components of PM pollution are sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust and water.

To put the size of PM2.5 pollution into context, a grain of sand would be around 90mm and a human hair around 50mm in diameter. People used to challenge the notion that PM2.5 exists down to the fact that PM2.5 is invisible to the naked eye and cannot even be seen with a regular light microscope. Due to PM2.5 pollution being so small, their presence in the air is a cause of concern. Pollution at that size can be inhaled deep into the lungs where they cause inflammation, which is known to trigger a range of long term and short term health problems.

Sources of PM2.5 pollution

PM2.5 pollution can come directly from a specific source, or be formed as a result of chemical reactions between other atmospheric pollutants. Sources can be natural or human-made. PM2.5 pollution is produced from all forms of combustion (burning) of solid or liquid fuels. Direct sources of PM2.5 pollution include petrol combustion in motor vehicles, fuel combustion in power plants, wood burning, agricultural burning, forest fires, bonfires and certain industrial processes.

In UK towns and cities, emissions from traffic are a significant source of PM2.5 pollution. Diesel emits the most PM2.5 pollution (although newer vehicles should be fitted with filters to help reduce emissions). However, indoor air may also be polluted by PM2.5 pollution. Indeed, a recent report from Defra says that domestic wood burning accounts for 2.4 times more PM2.5 emissions than all emissions from traffic.

Due to PM2.5 pollution being so small and light, it can travel over great distances. So, under certain meteorological conditions, PM2.5 pollution may travel from continental Europe to the UK and combine with local emissions to create a short-term episode of increased air pollution.

The PM2.5 pollution can also be formed from reactions between sulphur dioxide and nitrogen dioxides. These are known as ‘secondary’ particles.

Health effects of PM2.5s

The European Environment Agency estimates that PM2.5 pollution caused 37,800 premature deaths in the UK in 2012 (compared with 14,100 premature deaths from nitrogen dioxide pollution). Exposure to PM2.5 particles can cause a number of health effects. Research has shown increased hospital admissions and visits to A&E, as well as deaths from heart and lung disease, are linked to higher PM 2.5 levels.

Long-term exposure to particulate pollution is associated with: reduced lung function, chronic bronchitis, lung cancer and even premature death. The Committee on the Medical Effects of Air Pollution produced a report in 2009 that estimates a 6% increase in death rates per 10mg/m3 PM2.5 concentration.

Short-term exposure can exacerbate lung disease, including asthma and bronchitis, and may also increase vulnerability to respiratory infections. Symptoms of exposure include sore eyes, nose and throat irritation, coughing, chest tightness and shortness of breath.

Some groups of people are particularly susceptible to particulate pollution, including PM2.5 pollution. Those who have heart or lung disease (coronary heart disease, congestive heart failure, asthma or chronic obstructive pulmonary disease) are at increased risk of exacerbation of their condition.

Older adults are also more at risk – in some cases because they have undiagnosed heart or lung disease. Studies show that when PM levels are high, older adults are more likely to be admitted to hospitals than other adults (younger). Children are also more at risk because their lungs are still developing and they are more likely to be physically active. This makes them breathe faster and more deeply; breathing in more particles.

What is not completely clear is how the composition and source of the PM2.5, impacts upon health. Are some types of PM2.5 more or less harmful than others? Since there is still a lot of knowledge lacking, we do not know. The World Health Organization has recommended that we treat all PM2.5s the same when it comes to recommending safe limits.


Monitoring and legislation

European air quality legislation states that the annual average concentration of PM2.5 pollution should not exceed 25mg/m3 (WHO guideline values – to be achieved in the longer term – are lower than this, at 10mg/m3).  PM2.5 levels are one of the five air pollutant levels measured in the UK to generate Defra’s Daily Air Quality Index (DAQI). Levels of 0-11 mg/m3 are classed as low and the range extends to 71 or more - which is classed as very high.

Hourly data on PM2.5 pollution levels are generated by the Automatic Urban and Rural Network (AURN) of monitoring stations throughout the UK, which exists for compliance reporting to European air pollution directives and to give out information to the public. Of course, this data relates to outdoor PM2.5 pollution (as the stations are situated outside). As stated above, PM2.5 pollution levels indoors may also be significant. The World Health Organisation has highlighted the risk of fine particle pollution from household fuel combustion and has set safe levels of PM2.5s at 0.23mg/minute for unvented systems -  0.80mg/minute for vented systems.

Reducing your exposure to PM2.5s

If you are out and about, especially in an urban area, keep an eye on the DAQI so you know when PM2.5 pollution levels are high. When indoors, consider carefully before you buy a trendy wood burning stove, as it may seriously impact the quality of the air you breathe. You may want to invest in an air purifier that filters out PM pollution - especially if you live near a busy road; as outdoor pollution may enter your home when windows are open or via gaps in windows or walls.


Further information

COMEAP 2009. The mortality effects of long-term exposure to particulate air pollution in the United Kingdom

Friends of the Earth.

Clean Air in London.

Daily Air Quality Index, Defra UK.

Robinson DL (2015). 2.4 times more PM2.5 pollution from domestic wood burning than traffic. BMJ 2015; 350 h3757