What is smog, how does it form, and why is it harmful?
· 11 min read
In late summer and autumn, open burning of garden waste, crop residues and other biomass can sharply worsen air quality, while smoke from wildfires can travel hundreds or even thousands of kilometres and affect cities far from the fire itself. Across Europe, episodes of stagnant weather, heatwaves, traffic emissions and wildfire smoke can combine to produce severe smog.
Here is what smog is, how it forms, why climate change is making some types of smog more likely, and what can be done to reduce its health impacts.
Poor air quality has become a familiar problem for people living in large cities, but smog is part of a much broader environmental process. Climate change can increase some of the conditions that favour smog formation, while several components of air pollution — particularly black carbon and ozone — can themselves affect the climate.
Air pollution is also one of Europe's largest environmental health risks. According to the European Environment Agency, exposure to fine particulate matter remains responsible for hundreds of thousands of premature deaths in Europe each year, although mortality has declined significantly over the past two decades as air quality has improved.
Smog is a severe form of air pollution involving a mixture of gases and airborne particles. Depending on its type, it may contain smoke, dust, fine particulate matter, soot, nitrogen oxides, sulphur dioxide, carbon monoxide, volatile organic compounds and ozone.
Among the most important pollutants are PM2.5 and PM10 particles:
PM2.5 particles have a diameter of no more than 2.5 micrometres and can penetrate deep into the lungs and even enter the bloodstream.
PM10 particles are up to 10 micrometres in diameter and primarily affect the respiratory system.
For comparison, a human hair is roughly 50–100 micrometres thick.
The World Health Organization's 2021 air-quality guidelines recommend an annual average PM2.5 concentration of no more than 5 µg/m³, with a 24-hour guideline of 15 µg/m³. During severe pollution episodes and wildfires, concentrations can exceed these levels many times over.
The invention of the term smog is often attributed to physician Henry Antoine des Voeux, who used it publicly in 1905 to describe polluted urban air. However, the word had already appeared in print in the United States in the late 19th century. It combines the English words smoke and fog.
Photochemical smog occurs when pollutants such as nitrogen oxides and volatile organic compounds react in sunlight. These reactions produce ground-level ozone and other secondary pollutants, sometimes creating a brownish haze over cities.
It is often called Los Angeles-type smog, because the combination of heavy road traffic, strong sunlight and stagnant air historically made Los Angeles one of its best-known examples.
However, photochemical smog is also a recurring problem in Europe. Southern European cities and regions are particularly vulnerable during hot, sunny weather, including parts of Spain, Italy, Greece and southern France. Episodes can also occur in Central and Western Europe during prolonged heatwaves.
Unlike the protective ozone layer high in the stratosphere, ground-level ozone is harmful to human health and vegetation.
Winter smog has historically been associated with the combustion of coal and other sulphur-rich fossil fuels for heating and industry. It can contain sulphur dioxide, soot and particulate matter.
The problem becomes particularly severe during a temperature inversion. Normally, air near the ground is warmer and rises, helping disperse pollutants. During an inversion, a warmer layer of air sits above colder surface air, preventing vertical mixing.
Pollution therefore becomes trapped close to the ground.
This type is sometimes called London-type smog because of London's historical experience with coal pollution.
Authorities were already attempting to control coal smoke in medieval England. In 1306, King Edward I issued restrictions on the burning of certain types of coal in London. Nevertheless, coal consumption increased dramatically during industrialisation.
By the 19th and early 20th centuries, London's dense yellow-black fogs were sometimes compared to pea soup.
The most notorious episode occurred during the Great Smog of December 1952. Around 4,000 excess deaths were initially attributed to the event, while later epidemiological estimates suggested that the total death toll may have reached approximately 12,000, with many more people becoming ill.
The disaster helped trigger major changes in British environmental legislation, including the Clean Air Act 1956, which restricted smoke emissions and encouraged the transition towards cleaner heating fuels.
Many European cities subsequently introduced similar measures.
Volcanic smog is sometimes known as vog, a combination of volcanic and smog.
Sulphur dioxide and other substances released by volcanoes react with oxygen, sunlight and atmospheric moisture, producing sulphuric acid aerosols and other irritating compounds.
Although volcanic smog is relatively uncommon globally, large eruptions can have far-reaching atmospheric impacts.
The 1815 eruption of Mount Tambora in Indonesia, for example, injected enormous quantities of aerosols into the atmosphere and contributed to global cooling. The following year, 1816, became known as the "Year Without a Summer", with unusually cold conditions and crop failures affecting parts of Europe and North America.
This event was a much larger climatic phenomenon than ordinary local volcanic smog, but it illustrates how volcanic aerosols can influence atmospheric conditions over very large distances.
A rarer form of pollution can develop in extremely cold regions when temperatures fall below approximately −30°C, humidity is high and winds are weak.
Water vapour freezes into microscopic ice crystals, to which pollutants can attach. The combination of ice particles and emissions from vehicles, heating and industry can produce a dense haze.
Such conditions have historically been observed in Arctic and sub-Arctic cities, including parts of Alaska, Canada and northern Eurasia.
Smog forms when substantial emissions occur under atmospheric conditions that prevent pollutants from dispersing or encourage chemical reactions between them.
Major sources include transportation, industry, power generation, fossil-fuel combustion, construction, agriculture and wildfires.
Burning coal releases particulate matter, sulphur dioxide, nitrogen oxides, carbon dioxide, heavy metals and other pollutants.
Historically, coal was one of the main causes of severe winter smog in European industrial cities.
Its importance has declined sharply in many parts of Western Europe as coal heating has been phased out and electricity systems have become cleaner. However, coal remains significant in some regions of Europe and continues to be an important source of air pollution globally.
Reducing coal combustion therefore produces both climate and public-health benefits.
Internal-combustion vehicles emit nitrogen oxides, carbon monoxide, hydrocarbons and particulate matter.
Nitrogen oxides and volatile organic compounds can undergo photochemical reactions in sunlight, producing ground-level ozone — one of the principal components of summer smog.
Diesel engines have historically been an especially significant source of urban nitrogen dioxide and black-carbon emissions.
Electric vehicles eliminate tailpipe emissions, although they do not completely eliminate particulate pollution because tyre, road and brake wear also generate particles.
Large construction projects can release substantial quantities of dust.
Diesel-powered excavators, generators, trucks and other machinery can additionally emit nitrogen oxides and particulate matter.
Construction activity can therefore worsen particulate pollution during both summer and winter, particularly where dust-control measures are inadequate.
Farmers in some regions burn crop residues after harvesting to clear fields quickly and prepare them for the next planting season.
Burning biomass releases smoke containing PM2.5, carbon monoxide, volatile organic compounds and black carbon.
Open burning of leaves and garden waste can produce similar local effects.
Even where the practice is restricted or prohibited, illegal burning can temporarily cause very high particulate concentrations in nearby communities.
Wildfires are increasingly important sources of episodic air pollution.
Wildfire smoke contains large quantities of PM2.5 as well as carbon monoxide, nitrogen oxides and numerous organic compounds.
Smoke can travel across national borders. During major fire seasons in southern Europe, smoke from Portugal, Spain, France, Italy, Greece or the Balkans can be transported hundreds or thousands of kilometres and influence air quality elsewhere in Europe.
Satellite observations have also repeatedly recorded smoke crossing entire continents and even oceans.
Trees and other vegetation naturally release biogenic volatile organic compounds, or BVOCs, including substances such as isoprene and terpenes.
Under certain atmospheric conditions, particularly in the presence of nitrogen oxides and strong sunlight, these compounds can participate in reactions that produce ozone and secondary organic aerosols.
This does not mean that urban trees generally make air quality worse. Urban vegetation can provide major benefits by reducing heat, capturing some pollutants and improving resilience to heatwaves.
However, tree species differ considerably in their BVOC emissions. Urban planners can therefore improve air-quality outcomes by selecting appropriate species and considering street geometry, ventilation and local emission sources rather than simply maximising the number of trees.
The relationship between smog and climate change works in both directions.
Higher temperatures can accelerate the chemical reactions that produce ground-level ozone.
Heatwaves are frequently associated with:
These factors can combine to produce severe air-pollution episodes.
Climate change is also increasing the likelihood of extreme fire-weather conditions in many regions. The Mediterranean is particularly vulnerable because hotter temperatures, drought and dry vegetation can create favourable conditions for large wildfires.
Not every individual fire is caused by climate change, but rising temperatures can increase the probability and severity of the conditions in which fires spread.
Some air pollutants influence Earth's energy balance.
One of the most important is black carbon, a component of soot produced by incomplete combustion in diesel engines, biomass burning and wildfires.
Black-carbon particles absorb solar radiation and warm the atmosphere.
If they are transported to snow- or ice-covered regions and deposited on the surface, they darken the snow and reduce its albedo, or ability to reflect sunlight.
The darker surface absorbs more solar energy and melts faster.
This mechanism is particularly important in Arctic and high-mountain regions.
Ground-level ozone is also a greenhouse gas, meaning that reducing emissions of its precursor pollutants can deliver both air-quality and climate benefits.
However, the overall relationship between aerosols and climate is complex. Some aerosol particles reflect sunlight and temporarily produce a cooling effect, while others, particularly black carbon, contribute to warming.
Smog affects multiple organs, not only the lungs.
Fine particles such as PM2.5 can penetrate deep into the respiratory system and enter the bloodstream.
Exposure is associated with increased risks of:
Research has also identified associations between long-term air-pollution exposure and neurological conditions, including cognitive decline and dementia, although the biological mechanisms continue to be studied.
Children, older adults, pregnant people and people with cardiovascular or respiratory conditions are particularly vulnerable.
Air pollution is not only dangerous during spectacular visible smog events. PM2.5 and nitrogen dioxide can reach harmful concentrations even when the air appears relatively clear.
During periods of severe air pollution:
There is no single solution. Effective smog control requires reducing emissions across multiple sectors.
Preventing wildfires and uncontrolled burning is one element. Individuals should avoid burning waste, extinguish campfires properly and follow fire restrictions. Governments can improve fire prevention through monitoring, early-warning systems, vegetation management and rapid-response capacity.
Cities can reduce chronic pollution through measures such as:
Europe provides several examples of these policies. London operates an Ultra Low Emission Zone, many German cities have environmental zones, and a growing number of European cities are introducing increasingly strict restrictions on high-emitting vehicles.
At EU level, air-quality standards are also being tightened with the objective of moving European pollution limits closer to World Health Organization recommendations.
The long-term solution to smog therefore overlaps significantly with climate policy: cleaner electricity, low-emission transport, energy-efficient buildings, cleaner industry and reduced fossil-fuel combustion can simultaneously cut greenhouse-gas emissions and improve the air people breathe.
Smog is consequently not simply an unpleasant haze over a city. It is a visible manifestation of interconnected problems involving energy, transportation, urban planning, land management, public health and climate change.
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