Improving air quality in cities is one of the most critical challenges for public health and environmental sustainability of our time.
With 99% of the world’s population living in places where urban air pollution levels do not comply with WHO Air Quality Guidelines and more than 4.2 million premature deaths per year attributable to air pollution, cities urgently need effective strategies to improve the air we breathe.
In this article, we will address the main concerns related to poor air quality in cities, exploring the causes and devastating effects of urban air pollution and analysing in depth other aspects such as hidden economic costs, technological solutions for measuring and controlling air quality, and the most effective strategies that can be implemented to tackle it.
What are the key pollutants and main sources of pollution in cities?
The air in urban environments is a complex mixture of gases and particles, many of which, in high concentrations, are harmful to human health and the environment.
The sources of these pollutants are diverse and often interrelated, reflecting the urban lifestyle and production model. Understanding their nature and origins is the first step towards designing effective mitigation strategies.
The key urban air pollutants
As we mentioned in this article on primary and secondary pollutants, urban air pollutants can be classified as primary, if they are emitted directly from a source, or secondary, if they are formed in the atmosphere through chemical reactions. According to the WHO and environmental agencies around the world, the most concerning pollutants in terms of their impact on health are the following:
- Particulate matter PM10, PM2.5 and UFP: what is known as particulate matter is a mixture of solid and liquid particles suspended in the air, composed of sulphates, nitrates, ammonia, black carbon, mineral dust and water. We can group suspended particles according to their origin or nature, but it is the taxonomy of PM particles by size that is the most relevant classification, since their danger lies in their aerodynamic diameter:
- PM10 particles: with a diameter of 10 microns or less, they can penetrate the lungs.
- PM2.5 particles: also called fine particles, with a diameter of 2.5 microns or less, they can cross the lung barrier and become trapped in the alveoli.
- UFP particles: these are particles with a diameter of less than 0.1 micrometres (or 100 nanometres), whose nanometric size gives them a particularly high penetration capacity and toxicity. PM0.1 easily reaches the large surface area of the lung, causing inflammation and damaging distal organs.
- Nitrogen dioxide and nitrogen oxides (NO2 and NOx): Nitrogen dioxide (NO2) is considered the most significant pollutant among nitrogen oxides (NOx) due to its high reactivity and toxicity. These oxides, emitted by vehicles and other high-temperature combustion processes, form a brownish-red gas with a pungent odour that contributes to the formation of smog and acid rain. In terms of health, it causes irritation of the respiratory tract and is a key precursor in the formation of other secondary pollutants, such as ozone and nitrate particles.
- Tropospheric ozone (O3): Unlike stratospheric ozone, which protects us from ultraviolet radiation, ground-level or tropospheric ozone is a harmful secondary pollutant. It is formed by the photochemical reaction between nitrogen oxides (NOx) and Volatile Organic Compounds (VOCs) in the presence of intense sunlight. It is the main component of photochemical smog and a powerful irritant to the respiratory system.
- Sulphur dioxide (SO2): SO2 is a colourless gas that originates mainly from the burning of sulphur-containing fossil fuels, such as coal and oil, in power stations and industrial processes. It is a precursor to acid rain and sulphate particles.
- Carbon monoxide (CO): this colourless, odourless gas is the result of the incomplete combustion of carbon-based fuels. At high concentrations, it is toxic because it reduces the blood’s ability to carry oxygen.
- Volatile organic compounds (VOCs): Volatile organic compounds are a broad group of chemicals that evaporate easily at room temperature, as explained in this other article about What are VOCs?. Their sources are varied, ranging from vehicle exhaust gases, industrial emissions, and the use of solvents and paints to cleaning and personal care products.
- Heavy metals: A critical fraction of urban air pollution is due to heavy metals such as lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As) and nickel (Ni). These come mainly from direct combustion emissions, brake and tyre wear, and even the construction and demolition of old buildings, among many other sources. As they are neither chemically nor biologically degradable, they can remain in the environment for hundreds of years and are linked to the particulate matter we breathe directly.
- Environmental noise: although it often goes unnoticed, noise pollution in cities caused by unwanted external sound from numerous human activities is another environmental factor to be taken into account. Long-term exposure to excessive noise can lead to adverse health effects such as sleep disorders, cardiovascular disease, cognitive impairments, etc. Road traffic noise has been identified as the main source of noise pollution in all cities in the European Union.
Sources of air pollution in cities
Although there are many different sources of pollution, there are five that are clearly identified as being primarily responsible for poor air quality in most urban environments:
- Traffic and transport: this is the predominant source of pollution in most cities around the world. Vehicles, especially those with petrol and diesel engines, are responsible for most emissions of nitrogen oxides (NOx), carbon monoxide and PM10 and PM2.5 particles, mainly due to driving at low speeds in traffic jams, which intensifies emissions per kilometre travelled, creating pollution “hot spots” along major roads.
- Domestic and commercial heating: this has a greater influence in cities where fuels such as coal, diesel or biomass are used for heating, being a significant source of PM2.5, especially in winter.
- Energy sector: the energy supply sector is the main source of SO2 emissions, accounting for 44% of the total.
- Manufacturing and extractive industry: this is responsible for 46% of non-methane volatile organic compound emissions.
- Agricultural sector: its influence on pollution is mainly through ammonia (NH3), which in turn contributes to the formation of PM2.5 in the atmosphere.
Illustration of the main pollutants and emission sources in cities. Source: European Court of Auditors
Summary chart of the main pollutants, sources and health effects
| Pollutant | Chemical Symbol | Main Sources | Key Health Effects |
| Fine and Ultrafine Particulate Matter | PM2.5, PM0.1 | Traffic (diesel), industrial processes, biomass burning and fossil fuels (heating, energy). | It enters the bloodstream, increasing the risk of cardiovascular and respiratory diseases and lung cancer. |
| Coarse Particulate Matter | PM10 | Construction dust, brake and tyre wear, industrial processes, soil erosion. | Irritation of the respiratory tract, aggravation of asthma and other lung diseases. |
| Nitrogen Dioxide | NO2 | Combustion in vehicle engines, power stations, industry. | Inflammation of the respiratory tract, reduced lung function, increased susceptibility to respiratory infections. |
| Tropospheric Ozone | O3 | Produced by the reaction of NOx and VOCs with sunlight (secondary pollutant). | Irritation of the eyes and throat, difficulty breathing, lung damage, worsening of asthma and chronic diseases. |
| Sulphur Dioxide | SO2 | Burning of fossil fuels containing sulphur (coal, oil) in industry and power generation. | Affects the respiratory system, eye irritation, worsening of asthma and chronic bronchitis. |
| Carbon Monoxide | CO | Incomplete combustion in vehicles, heating systems and industry. | Reduces the blood’s ability to carry oxygen, causing headaches, dizziness and, at high concentrations, death. |
| Heavy Metals | AS, Cd, Pb, Hg, Ni | Energy sector, manufacturing industry and road traffic. | The accumulation of these metals can manifest itself as acute or chronic toxicity depending on the metal, the concentration and the duration of exposure. |
| Environmental Noise | – | Airports, railway lines, roads and crowds. | Prolonged exposure causes sleep disorders, discomfort, cognitive impairment, cardiovascular problems and mental health problems. |
What are the most effective measures for reducing urban air pollution?
There is no single solution for improving urban air quality. Controlling air pollution in cities requires a comprehensive approach that combines actions at the governmental, industrial and citizen levels. The most effective measures are those that tackle the main sources of emissions in a coordinated manner.
Transforming mobility towards a sustainable model
Since traffic is the main source of pollution, the most effective measures must involve reducing the use of private vehicles. This can be achieved by actively promoting public transport, creating safe infrastructure for pedestrians and cyclists (active mobility) and implementing Low Emission Zones.
What role do Low Emission Zones play in improving air quality?
Low Emission Zones (LEZs) are one of the most powerful and proven regulatory tools for combating urban pollution, especially that caused by road traffic.
Their main function is to improve air quality by restricting the access, circulation and parking of the most polluting vehicles in a defined urban area, generally based on their environmental labels.
Various studies carried out in European cities show that well-designed LEZs can drastically reduce concentrations of pollutants such as nitrogen dioxide (NO2), achieving an average reduction of 20%. In successful cases such as central London and its Ultra Low Emission Zone (ULEZ), reductions have reached up to 46%. Significant decreases in PM10 particles and black carbon have also been recorded.
Besides, by limiting the circulation of older, more polluting vehicles, LEZs encourage citizens and businesses to switch to cleaner vehicles, including electric and hybrid ones.
As part of a comprehensive strategy, these zones promote the use of cleaner alternatives such as public transport, cycling and walking, contributing to a more sustainable and less congested city.
Green infrastructure and urban planning
The way our cities are designed has a direct impact on air quality.
Smart urban planning can create environments that not only emit fewer pollutants, but also actively help to clean them up. The key concept is green infrastructure: a strategically planned network of natural and semi-natural areas that provide ecosystem services. This goes far beyond traditional parks and includes green roofs, vertical gardens, ecological corridors, urban forests and permeable pavements.
The benefits of green infrastructure are manifold: plants filter pollutants from the air, trees sequester carbon and provide shade, reducing the ‘Urban heat island’ effect. In addition, it improves rainwater management, increases biodiversity and promotes the physical and mental well-being of citizens.
Image of the “Madrid Nuevo Norte” Project
Predictive and dynamic air quality management
Traditionally, anti-pollution measures were activated after air quality monitoring stations recorded dangerous levels. Today, Big Data, fed by a vast network of sources—air quality sensors, real-time traffic data, weather forecasts, satellite imagery, and land use data—provides all the ingredients for creating sophisticated predictive models.
Advanced predictive models such as CoNOAir, a machine learning model based on a neural operator called the Complex Neural Operator for Air Quality, have already been developed. This model can effectively forecast carbon monoxide (CO) concentrations in both the short term (1 hour) and long term (72 hours), with forecasts for the following hour having R2 values greater than 0.95 for all cities considered.
The implementation of such a model can greatly assist government agencies in providing early warnings, planning intervention strategies, and developing effective strategies considering various hypothetical scenarios.
What urban policies help to prevent pollution episodes?
To prevent pollution levels from reaching dangerous levels for health, cities implement Short-Term Action Plans or Protocols for High Pollution Episodes. These plans establish a series of measures that are activated progressively as pollutant levels—mainly PM2.5, PM10, nitrogen dioxide (NO2), ozone (O3) and sulphur dioxide (SO2) particles—exceed certain warning, advisory or alert thresholds.
The policies and measures included in these protocols are usually organised into several levels or scenarios for action:
- Information measures, to alert the population about pollution levels, health risks and recommendations. Vulnerable groups (children, the elderly, people with respiratory diseases) are advised to avoid prolonged exposure to the outdoors and intense exercise.
- Promotion of public transport in order to encourage its use, in some cases offering it free of charge.
- Traffic restrictions. These are the most forceful measures and are applied in the most serious scenarios. They include:
- Speed reduction, limiting the maximum speed on main access roads and ring roads to reduce emissions.
- Parking restrictions, prohibiting the most polluting vehicles from parking in regulated areas.
- Traffic restrictions. At the highest alert levels, a percentage of vehicles (e.g. based on odd or even number plates) or those without a ZERO or ECO environmental label may be prohibited from driving in specific areas or throughout the city.
How is the effectiveness of municipal air quality policies evaluated?
Evaluating the effectiveness of air quality policies is a complex but essential process, both to ensure that the measures implemented are achieving their objectives and to justify future investments.
The evaluation is based on a multidimensional approach that combines quantitative and qualitative data. Key methods and tools for this evaluation include:
- Air Quality Monitoring. The most direct indicator is the measurement of key pollutant concentrations before, during, and after the implementation of a policy or action plan. Annual air quality assessment reports, which compare measured levels with legal limit values and WHO guidelines, are an essential tool.
- Emissions inventories. Inventories are carried out to estimate the total amount of pollutants emitted by different sources (traffic, industry, heating, etc.). Comparing inventories over time makes it possible to verify whether policies are succeeding in reducing emissions at source.
- Impact and monitoring indicators. Policies are designed with specific objectives that are measured using indicators. For example, for a Low Emission Zone, monitoring indicators could include the number of ZERO and ECO label vehicles on the road, the number of electric charging points installed or the kilometres of new cycle lanes built. Impact indicators would directly measure the reduction in pollutant concentrations.
- Public health assessment. Health data is analysed to identify trends in pollution-related diseases, such as hospital admissions for asthma or cardiovascular disease.
- Modelling and simulation. Computer models are used to simulate the impact a policy would have before it is implemented in order to isolate its effect from other factors, such as weather conditions, once it is in place.
This comprehensive approach, which combines environmental monitoring, tracking of specific indicators and health impact assessment, enables public administrations to make evidence-based decisions, adjust strategies and communicate results to the public in a transparent manner.
Technologies for monitoring, controlling and improving urban air quality
To effectively manage urban air quality, it must first be accurately measured. Technology has evolved dramatically in recent years, enabling the creation of extensive air quality monitoring networks with different types of interconnected devices.
AAQM stations
Ambient Air Quality Monitoring stations or fixed monitoring stations are the backbone of air quality monitoring networks. These facilities use high-precision instrumentation and standardised analysis methods to measure concentrations of atmospheric pollutants with a high degree of reliability and legal validity.
For example, they use methods such as chemiluminescence to measure NOx, ultraviolet fluorescence for SO2, or gravimetric methods for PM particles.
However, their main limitations are their cost and size (for installation in city centres), which restricts their number and creates limited spatial coverage, potentially masking significant variations in air quality within the same city.
Low-cost sensors
There is now a new generation of smaller, more affordable and easier to deploy air quality sensors. These devices, based on technologies such as electrochemical sensors for gases or laser scattering for particles, allow for the creation of much denser monitoring networks.
This capacity for hyperlocal monitoring is essential for identifying pollution ‘hot spots’ that reference stations cannot detect.
Despite these advantages, their limitations must be recognised, namely that their accuracy and selectivity are inferior to those of reference equipment and they require calibration and maintenance over time to ensure data reliability.
On-board monitoring and IoT
One of the most promising innovations is on-board air quality monitoring. This involves installing compact sensors in fleets of vehicles that routinely travel around the city, such as buses, taxis or municipal service vehicles.
By taking advantage of their routes, it is possible to generate dynamic, real-time pollution maps of the entire urban area instead of relying on measurements at fixed points.
These systems, connected via the Internet of Things (IoT), send their data to a central platform where it is analysed and visualised. This dynamic, high-resolution view of air quality is transforming environmental management, enabling authorities to move from a reactive to a proactive, data-driven approach.
How can air pollution be monitored on a continuous and reliable basis?
Local councils have a range of tools and strategies at their disposal to establish a robust and reliable pollution monitoring system that combines technology, regulation and citizen participation.
The steps to achieve this would be as follows:
- Establish a clear regulatory framework: local governments must implement and enforce regulations such as Low Emission Zones and other emission standards for industrial and commercial sources.
- Official monitoring networks: the basis of monitoring is fixed monitoring stations, which use high-precision equipment to measure regulated pollutants with legal validity. Although costly, they are the indispensable benchmark.
- Complement with low-cost sensors to overcome the spatial limitations of fixed stations. These devices enable hyperlocal monitoring, identifying pollution hotspots in specific neighbourhoods or streets that would otherwise go unnoticed. This allows local councils to deploy dense air quality monitoring networks using more economical sensors. In small towns where budgets are more limited, it is possible to start with this network of low-cost sensors, provided that they are calibrated against the fixed reference stations of the official regional network.
- On-board (mobile) monitoring. One of the most effective innovations is to install sensors in municipal vehicle fleets which, as they travel around the city, generate dynamic, real-time pollution maps of the entire urban area, providing a much more comprehensive view than fixed points.
- Inspection and control. A final fundamental step that ensures compliance with regulations and the effectiveness of established air quality policies.
The combination of these technologies and strategies will enable local authorities not only to fulfil their legal monitoring obligations, but also to obtain an accurate and continuous picture of urban air quality.
References:
https://www.who.int/es/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health
https://www.nature.com/articles/s12276-020-0403-3
https://www.eca.europa.eu/ECAPublications/SR-2025-02/SR-2025-02_ES.pdf
https://www.20minutos.es/imagenes/salud/las-ciudades-mas-contaminadas-union-europea-5221810/
https://edumasterplus.com/descubre-como-el-big-data-puede-mejorar-la-calidad-del-aire/
https://climatetech.es/2025/09/25/inteligencia-artificial-contra-la-contaminacion/
https://arxiv.org/abs/2501.06007
https://www.zaragoza.es/sede/portal/medioambiente/calidad-aire/episodios-no2/
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