Outdoor air pollution is one of the most pressing environmental and public health challenges of the modern era. It refers to the presence of harmful substances in the atmosphere, including gases, particulate matter, and biological molecules, that can negatively impact human health, ecosystems, and the climate. Sources of air pollution range from natural phenomena such as volcanic eruptions and wildfires, to human activities such as industrial emissions, transportation, and agriculture. 

Fine particulate matter (PM2.5), measuring less than 2.5μm in diameter, remains a major concern for policy makers and the public due to its widespread presence and harmful effects on human health. According to the WHO air quality guidelines (AQG), fine particulate matter (PM2.5) is small enough to infiltrate the lungs and enter the bloodstream, posing risks to all major organs. Exposure to PM2.5 can lead to severe health issues, affecting both the cardiovascular and respiratory systems. This includes conditions such as stroke, lung cancer, and chronic obstructive pulmonary disease (COPD). Emerging research has also revealed a concerning link between prenatal exposure to elevated levels of air pollution and developmental delays by age three. Furthermore, it suggests potential long-term psychological and behavioral challenges, including symptoms of attention deficit hyperactivity disorder (ADHD), anxiety, and depression.

This blog post explores the WHO air quality guidelines and the European Parliament directive on ambient (outdoor) air quality, with a focus on particulate matter concentration limits. Both frameworks are essential in establishing air quality standards aimed at safeguarding public health. The World Health Organisation’s (WHO) air quality guidelines (AQG) establish a global benchmark for national, regional, and municipal authorities striving to enhance public health by curbing air pollution. Rather than imposing legally-binding mandates, these guidelines offer evidence-based recommendations to steer policy makers toward more stringent air quality standards.  In contrast, the directive of the European Parliament and of the Council enforces legally-binding air quality regulations for its member states. These directives set definitive pollutant concentration limits, including thresholds for particulate matter (PM10 and PM2.5), ensuring uniform compliance across Europe.

WHO’s Air Quality Guidelines

First introduced in 1987, the WHO air quality guidelines (AQG) have undergone multiple revisions to reflect the latest scientific findings. The guidelines were first updated in 2005, with periodic reviews to ensure ongoing relevance. The 2021 revision of the WHO guidelines was driven by growing scientific evidence of air pollution’s severe and persistent impact on public health, reinforcing the urgent need for stricter air quality standards. The WHO air quality guidelines set recommended limits for various common air pollutants, including particulate matter (PM), which is one of the most critical pollutants affecting public health.

Table 1. WHO Air Quality Guidelines

The latest WHO AQG have introduced stricter limits for particulate matter (PM2.5 and PM10) to better protect public health. The recommended annual concentration level for PM2.5 was reduced by half, and is now 5µg/m³. The 24-hour concentration of PM2.5 was reduced from 25µg/m³ to 15µg/m³. PM10 limits were also revised downwards in terms of the annual average, from 20µg/m³ to 15µg/m³, and for the 24-hour from 50µg/m³ to 45 µg/m³. The 24-hour concentration levels of PM2.5 and PM10 may exceed 15 µg/m³ and 45 µg/m³, respectively, on 3 to 4 days per year.

The EU Ambient Air Quality Directives

Directive 2008/50/EC & Directive (EU) 2024/2881 of the European Parliament and of the Council on particulate matter are shown in Table 2.

Table 2. The EU ambient air quality directives specifically relating to particulate matter

The new EU ambient air quality directive limit and target values are to be met by the member states by 1 January 2030. According to this directive, the daily limit of PM2.5 and PM10 should not be exceeded more than 18 times per calendar year. In the previous directive, the limits were set to not be exceeded more than 35 times a calendar year. The latest directive has revised the limits further, as shown in Table 2, even though the recommended level in the latest WHO air quality guidelines (Table 1) are still not met; only the daily PM10 concentration limit is aligned with the WHO AQG recommendations.

Facts about particulate matter

According to a study [1] on estimates of the global mortality burden associated with short-term exposure to fine particulate matter (PM2.5), from 2000 to 2019, short-term exposure to PM2.5 contributed to approximately 1 million premature deaths annually, accounting for 2.08% of total global deaths. Each year, approximately 230,000 deaths due to short-term PM2.5 exposure occur in urban areas, making up 22.74% of all global deaths attributed to this cause and 2.30% of total urban mortality worldwide.

The European Environment Agency reported that although air quality has been improving across Europe, current EU standards remain unmet in many areas. In 2022, 96% of the European urban population was exposed to fine particulate matter (PM2.5) exceeding the WHO air quality guidelines, while for less than 1% the EU standards were exceeded. For PM10 particles, 86% of the urban population was exposed to levels above the WHO guidelines, whereas for 9% the EU limits were exceeded.

Call to action

For policy makers:

Reducing the difference in PM limits between the EU Ambient Air Quality Directives and the WHO’s recommendations will ensure better protection of public health.

Stricter regulations should be implemented and enforced effectively, ensuring all member states meet lower PM concentration levels.

Governments should invest in cleaner energy, public transportation, and pollution control technologies to reduce PM emissions at the source.

For individuals and communities:

Raise awareness of the health risks of air pollution and demand stricter air quality laws from policy makers. For better air quality, individuals should have information about their environment, and the air quality in the indoor spaces which people spend 90% of their time. In indoor environments, cooking and smoking in the home are the main sources of particle matter.  The findings of a study [2] on the mitigation of exposure to airborne particles in improving indoor air quality in residential buildings show that of 100 participants, 76% had never looked for information on the IAQ in homes before because they were not aware of IAQ-related problems (68%), and secondly because they did not consider it an important issue (32%).  

Reducing personal contributions to air pollution starts with choosing cleaner transportation, such as public transit, cycling, or walking, to cut vehicle emissions. Limiting wood-burning stoves and fireplaces helps reduce harmful pollutants such as PM2.5 and carbon monoxide. Supporting clean energy by opting for renewable sources, energy-efficient appliances, and sustainable policies further minimises air pollution. Small lifestyle changes can collectively improve air quality and protect public health.

Join environmental groups, support air quality campaigns, and advocate for clean air policies at local and national levels, and worldwide.

Bibliography:

[1]. Yu, W., Xu, R., Ye, T., Abramson, M. J., Morawska, L., Jalaludin, B., Johnston, F. H., Henderson, S. B., Knibbs, L. D., Morgan, G. G., Lavigne, E., Heyworth, J., Hales, S., Marks, G. B., Woodward, A., Bell, M. L., Samet, J. M., Song, J., Li, S., & Guo, Y. (2024). Estimates of global mortality burden associated with short-term exposure to fine particulate matter (PM2·5). The Lancet Planetary Health, 8(3), e146–e155. https://doi.org/10.1016/s2542-5196(24)00003-2
[2]. Caracci, E., Canale, L., Buonanno, G., & Stabile, L. (2022). Effectiveness of eco-feedback in improving the indoor air quality in residential buildings: Mitigation of the exposure to airborne particles. Building and Environment, 226, 109706. https://doi.org/10.1016/j.buildenv.2022.109706
[3]. World Health Organization air quality guidelines. https://www.who.int/news-room/feature-stories/detail/what-are-the-who-air-quality-guidelines 
[4]. Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe. http://data.europa.eu/eli/dir/2008/50/oj
[5]. Directive (EU) 2024/2881 of the European Parliament and of the Council of 23 October 2024 on ambient air quality and cleaner air for Europe (recast) PE/88/2024/REV/1 OJ L, 2024/2881, 20.11.2024, ELI: http://data.europa.eu/eli/dir/2024/2881/oj