What is the matter with Particulate Matter?

16.12.2021 7 min read

Particulate matter, (PM), refers to airborne particles of any material. This pollutant is usually split into two groups of PM10, which is particles less than 10 micrometres (μm) across, and PM2.5, which is particles less than 2.5 micrometres across. To put that into perspective, human hair is between 50-70 micrometres across, and 2.5 micrometres is approximately one 400th of a millimetre.

At sizes of less than 2.5μm across, particles can be indistinguishable to the naked eye, and the smaller the particle the more harmful they can be to our health. Whilst PM10 is usually caught in the tiny hairs and mucous in our noses and throats, PM2.5 is small enough to bypass our bodily defences and enter our bloodstream through our lungs. If these particles enter our bloodstream, they can end up in our brain, causing blood clots, or in our hearts causing heart attacks and heart failure. It is these reasons that mean that there is no safe level of PM2.5 in the atmosphere, and when exposed to high levels of ambient PM2.5 in our day to day lives we are put at much more risk of serious cardiovascular and respiratory health implications.

One problem with trying to put restrictions and limit values on the particulate matter is that there are almost unlimited sources of PM2.5, as the term refers to any particle in the size fraction of fewer than 2.5 microns across. As a car travels along a road, PM2.5 comes out in the exhaust fumes, can come off the brake pads as the car slows down and can even come off both the tyres and the road through abrasion. Almost any mechanical activity involving two components touching or rubbing together can cause particles to break off and become airborne. Also, another major source of this pollutant is the burning of solid fuels, such as wood in fireplaces and for cooking. This pollutant can either be a natural substance, for example, sea salt and mud as it dries and becomes dust when agitated, or man-made such as exhaust fumes. However, global communities are becoming increasingly aware of the threats this pollutant poses and greater emphasis is now being put on monitoring, restricting and putting limit values on concentrations in the environment.

In the UK, the Air Quality Standards Regulations 2010 set an objective level for PM2.5 to be met by 2020. The objective was an annual mean concentration of 25 micrograms per cubic metre of air (µg/m3), which most parts of the country meet, with a stricter limit of 10µg/m3 adopted across Scotland. But this objective does not go far enough in protecting public health from the potential impacts of PM2.5, and the government’s Committee on the Medical Effects of Air Pollution (COMEAP) has estimated that 29,000 premature deaths in the UK every year can be attributed to exposure to PM2.5. On average, 1 in 20 deaths in people aged over 30 is due to PM2.5. These statistics outline just how big an impact PM2.5 has on the population and highlights the fact that something more drastic needs to be done. The World Health Organization (WHO) recently revised its guideline limit for annual average ambient PM2.5 concentrations to 5µg/m3, from 10µg/m3. Although this is not a legally binding objective, it sets a precedent for the level of PM2.5 in the atmosphere that should be aimed to improve the long and short-term health impacts of this pollutant. In London, the Mayor had already announced that the city will be working towards achieving the WHOs previous limit of 10µg/m3 by 2030, but it is yet to be announced whether the new target will be adopted. In the Environment Act 2021, the UK government has committed to setting a new PM2.5 target but offers no real insight into how this will be done or what the target is going to be. It has been mentioned that legislation outlining the new target is to be put before parliament before the end of October 2022, so only time will tell just how far the UK’s new target goes.

As PM2.5 is generated by different processes across many industries, attempting to reduce concentrations globally will not be an easy task. It will require a range of measures implemented at various levels of governance to tackle region-specific issues. The WHO estimate that 2.6 billion people around the world cook and heat their homes with solid fuels and this is primarily in lower-income countries that bear the brunt of the health impacts of environmental risks. Whilst measures to improve the indoor air pollution of these societies will have the knock-on impact of improving general air pollution in that area, with indoor air pollution accounting for up to 50% of outdoor air pollution in some parts of the world (WHO, 2021), in more developed countries these measures may not be as effective. In cities like London, where cooking and heating with solid fuel is far less common, tackling indoor air pollution will not have as drastic an impact on outdoor air pollution.

To achieve the WHOs new PM2.5 objective, a major societal change will need to occur in almost all built-up areas of the developed world. Furthermore, this societal change will need to occur in bordering regions and nations simultaneously due to the cross-boundary impacts of transient PM2.5. In the UK, the south, east and south-eastern parts of the country are exposed to pollution sources from mainland Europe. To achieve the new objective, countries will have to work together with their bordering neighbours and other communities further afield. One of COMEAP’s findings was that there is no safe level of PM2.5 in the environment and studies on the effects of the pollutant at lower concentrations has often been overlooked. But there is not a level below which there is no harm and aiming for as low a target as possible is how we will reduce public health impacts of PM2.5.

Research undertaken by the Taskforce for Lung Health found that background concentrations of PM2.5 in the UK in 2018 ranged from 5.1µg/m3 on the Isles of Scilly to 13.5µg/m3 in the London Borough of Newham (BLF, 2021). Whilst this shows how PM2.5 levels vary greatly across the country, it also shows that even in the Isles of Scilly where background concentrations are the lowest in the country, in 2018 the ambient levels were still higher than the WHOs recommended target of 5µg/m3. This further highlights how hard it would be to meet the 5µg/m3 limit in urbanised areas such as London. There will have to be major infrastructural changes to enable more walking and cycling and higher provisions for public transport, in what is currently one of the most interconnected cities in terms of access to, and availability of, public transport. London has just expanded the Ultra Low Emission Zone (ULEZ) to include all roads within the North and South Circular Roads, not including the North or South Circular, to combat air pollution from older or more polluting vehicles. But PM2.5 is generated by zero-emission vehicles and sources not related to transport, so much tighter restrictions and more innovative measures will be required to reduce PM2.5 to anywhere near the WHOs guideline objective.

According to IQAir, the most polluted city in terms of PM2.5 concentrations in 2020 was Hotan, China, which had an annual average PM2.5 concentration of 110.2µg/m3, with the highest monthly concentration being 264.4µg/m3 in March. Additionally, in the top ten most polluted cities in the world in 2020, in terms of PM2.5 concentrations, ranks 2 to 10 are all Indian cities with annual average PM2.5 levels above 84µg/m3 (IQAir, 2021). Whilst trying to achieve the 5µg/m3 objective in the UK, or even the 10µg/m3 objective, is a major challenge that will require very specific and bespoke measures or even the implementation of radical new legislation, trying to achieve this target in cities with much higher concentrations will require a different approach and will take a lot longer to work towards. But doing so is imperative to reducing levels of PM2.5 in and around these highly polluted cities.

Currently, London is working towards a PM2.5 target of 10µg/m3 to be met by 2030 (GLA, 2019), but this may become tighter following the announcement of the WHOs revised limit as London tends to stay ahead of the rest of the UK when it comes to environmental targets and objectives. When the new national target is proposed, it will be interesting to see what measures have been recommended or if any plan is put in place as to how to achieve the target, or if Local Authorities will be asked to come up with and implement ways in which they will reduce PM2.5 emissions in their jurisdiction. For now, we wait patiently to see how closely aligned with the WHOs recommendations the UK Governments PM2.5 target will be, and just how much it differs from the current 25µg/m3 target.

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Key Contacts

Dr Xiangyu Sheng Director - Air Quality, Climate & Carbon