PM2.5 and PM10 particles: what they are, limits and air quality

PM2.5 and PM10 particles are suspended particulate matter, an air pollutant made up of tiny solid fragments and liquid droplets that float in the air. The number refers to their maximum diameter in microns: PM10 have a diameter of 10 micrometres or less, and PM2.5 of 2.5 micrometres or less. For perspective, a human hair is about 70 micrometres thick, so these particles are between seven and thirty times finer.

The smaller the particle, the deeper it penetrates the body. PM10 stay mostly in the upper airways, while PM2.5 reach the lung alveoli and even the bloodstream. That is why fine particles are one of the pollutants with the greatest health impact and a priority for European regulation.

In 2021 the World Health Organization (WHO) tightened its air quality guidelines, and in 2024 the European Union adopted a new directive that substantially lowers the limit values with a 2030 horizon. For companies, measuring and reducing the emissions linked to combustion, traffic and industrial processes is increasingly relevant, both for compliance and reputation.

What suspended particles are

Suspended particles, also called particulate matter or PM, are a heterogeneous mixture of solid or liquid substances dispersed in the air. Their composition varies with their origin: they may contain carbon, sulfates, nitrates, heavy metals, mineral dust, pollen or organic compounds.

They are classified by size, because size drives both their behaviour in the atmosphere and their health effect:

• PM10 (coarse particles): aerodynamic diameter of 10 micrometres or less. They largely come from mechanical processes such as tyre and brake abrasion, construction work or dust resuspension.
• PM2.5 (fine particles): diameter of 2.5 micrometres or less. They usually originate in combustion processes (diesel engines, boilers, biomass burning) and in chemical reactions in the atmosphere.
• Ultrafine particles (PM0.1): diameter below 0.1 micrometres, still without a regulated limit value but under growing scientific attention.

Sources of PM2.5 and PM10 particles

Particles can be primary, emitted directly into the atmosphere, or secondary, formed in the air from precursor gases such as nitrogen oxides, sulphur dioxide or ammonia. The main sources are:

• Combustion and traffic: vehicle exhausts, especially diesel, together with brake and tyre wear, are a leading source in cities.
• Industry and energy: thermal power plants, cement works, steelmaking, refineries and processes involving combustion or handling of dusty materials.
• Heating and the residential sector: oil boilers and, very especially, wood and biomass burning release large amounts of PM2.5.
• Agriculture: livestock and fertiliser use emit ammonia, a precursor of secondary particles; tilling generates dust.
• Natural sources: Saharan dust intrusions, wildfires and sea spray, which can raise concentrations temporarily.

Health effects of particles

Exposure to particles is linked to respiratory and cardiovascular disease, worsening asthma, lung cancer and premature death. The WHO considers air pollution one of the greatest environmental health risks worldwide, and PM2.5 is the pollutant with the highest attributed disease burden.

The effect is not only respiratory. By reaching the blood, fine particles contribute to inflammatory processes that affect the heart and blood vessels. The most vulnerable groups are children, older people and those with existing chronic conditions. Reducing exposure brings clear health benefits, one of the reasons that drive climate change mitigation and clean air policies, which often share solutions.

WHO 2021 guidelines and EU limit values

Two references need to be distinguished. The WHO 2021 guidelines are evidence-based health recommendations with no binding legal force. The European Union limit values are legally binding. The new Directive (EU) 2024/2881 on ambient air quality, adopted on 23 October 2024, recasts the previous rules and brings the legal limits closer to the WHO guidelines, with a 2030 application horizon. It entered into force on 10 December 2024 and Member States must transpose it by 11 December 2026 at the latest.

The directive's daily limit values allow up to 18 exceedances a year. In Spain, the reference framework is still Royal Decree 102/2011 on improving air quality, which transposed the earlier European directives and will need to be adapted to the 2024 Directive. Reducing particles is closely tied to reducing other greenhouse gases, because they share sources such as the burning of fossil fuels.

How particles are measured

Official measurement is carried out through monitoring stations that make up the air quality networks of regional and local authorities. The reference gravimetric method captures air through a filter and weighs the mass of retained particles, expressed in micrograms per cubic metre (µg/m³). Estimating emissions at source, meanwhile, relies on activity data and emission factors. Automatic equipment providing near real-time data is also used. In recent years, low-cost sensors have been added that, without replacing official stations, help densify the information and raise public awareness.

What companies can do

Organisations influence particle emissions directly, through their processes and vehicles, and indirectly, through their energy and supply chain. Some lines of action:

• Renew the fleet towards lower-emission vehicles and optimise routes to cut mileage, in line with sustainable transport.
• Improve energy efficiency and replace combustion boilers with electric or renewable solutions.
• Install filtering and dust-capture systems in industrial processes and control fugitive emissions in construction and material storage.
• Promote staff mobility plans, remote work and collective transport, measures that reduce both particles and carbon emissions.
• Measure the environmental impact of the activity to prioritise where to act and demonstrate progress with data.

Mobility regulation reinforces this trend. You can go deeper into how the regulatory framework affects transport in the article on how the Sustainable Mobility Law is changing the transport sector.

Frequently asked questions

What is the difference between PM2.5 and PM10?

The difference is size. PM10 have a diameter of 10 micrometres or less and PM2.5 of 2.5 micrometres or less. Being finer, PM2.5 penetrate deeper into the lungs and the blood, so they are linked to more serious health effects.

Are the WHO limits mandatory?

No. The WHO 2021 guidelines are health recommendations with no legal force. The binding values in the European Union are set by Directive (EU) 2024/2881, which lowers the limits with a 2030 horizon and must be transposed into national law before 11 December 2026.

How is particle concentration measured?

Through air quality monitoring stations that capture air via filters and determine the mass of particles per cubic metre (µg/m³), using reference gravimetric methods or automatic equipment. Complementary low-cost sensors also exist.

Measure and reduce your air quality impact with Manglai

Acting on particles starts with knowing where your emissions come from. With Manglai you can calculate and manage your organisation's carbon footprint, identify the combustion and mobility sources that pollute the most and plan reductions with reliable data. It is the basis for improving both your climate performance and your contribution to cleaner air.