Smog is polluted air containing a mixture of gases, particles and atmospheric reaction products that produces degraded air quality and, in many circumstances, reduced visibility. The word smog historically combines smoke and fog, reflecting early episodes in which smoke from coal combustion mixed with moisture and fog in heavily industrialised cities. Modern use encompasses more than the original smoke and fog mixture and commonly distinguishes between traditional sulfurous smog and photochemical smog.
Sulfurous smog was strongly associated with coal combustion and high concentrations of sulfur dioxide, smoke and particulate matter under stagnant, humid conditions. Photochemical smog develops when sunlight drives chemical reactions involving nitrogen oxides and volatile organic compounds, producing secondary pollutants including ground level ozone and other oxidants. Smog is therefore not a single chemical substance but a complex atmospheric mixture whose composition depends on sources, weather, sunlight and atmospheric chemistry.
Photochemical smog is especially associated with urban and industrial regions where nitrogen oxides and volatile organic compounds are emitted by vehicles, industrial processes, fuel use, solvents and other activities. Sunlight initiates chemical reactions involving nitrogen dioxide and other compounds, producing highly reactive radicals that drive further atmospheric chemistry. Ground level ozone is a major product, although it is not emitted directly in the same way as primary pollutants.
Other secondary products include aldehydes, organic aerosol and a range of oxidised organic compounds. Photochemical smog can be most intense during warm, sunny and stagnant weather because strong sunlight accelerates reactions and weak ventilation limits pollutant dispersion. The chemistry is nonlinear, meaning that reducing one precursor does not always produce a proportionally simple change in ozone because ozone formation depends on the balance between nitrogen oxides and reactive organic compounds. Air quality management therefore requires understanding atmospheric chemistry rather than treating each pollutant independently.
Ground level ozone is one of the most important components of photochemical smog. Ozone has the same chemical formula as stratospheric ozone but behaves very differently depending on where it occurs. In the stratosphere, ozone absorbs ultraviolet radiation and provides essential protection from biologically damaging radiation. Near the Earth's surface, ozone is a strong oxidant and air pollutant. It can irritate the respiratory system and damage vegetation.
Ozone enters plant leaves through stomata and can interfere with photosynthetic processes and plant growth. Human exposure can cause coughing, throat irritation and breathing difficulties, particularly during periods of elevated concentration. Ozone formation involves nitrogen oxides, volatile organic compounds and sunlight, but atmospheric conditions determine the actual concentration reached at a particular location. Consequently, an area with large emissions of ozone precursors does not necessarily have the highest ozone concentration at the exact emission source because ozone can form and accumulate downwind.
Traditional sulfurous smog is associated particularly with the historical use of coal and other sulfur containing fuels in densely populated industrial cities. Combustion produced smoke, soot, sulfur dioxide and other pollutants, while cool, humid and stagnant weather could prevent dispersion. Fog droplets provided surfaces for chemical reactions and the resulting mixture could become extremely polluted. Historic episodes demonstrated the severe consequences of concentrated air pollution, particularly where large populations were exposed.
The classic example is the severe London smog episode of 1952, during which coal smoke and sulfur pollution accumulated under stagnant atmospheric conditions and was associated with a major public health crisis. Such historical events contributed to stronger air pollution controls and changes in fuel use. Modern air pollution in many cities differs chemically because coal smoke has been reduced substantially in many places, while traffic related nitrogen oxides and photochemical pollutants have become more prominent.
Smog formation depends heavily on meteorology. Wind determines how quickly pollutants are transported away from their sources, while stagnant conditions allow concentrations to build. Temperature inversions can restrict vertical mixing by placing warmer air above cooler air near the surface. Sunlight influences photochemical reactions, making summer or otherwise sunny periods particularly favourable for ozone formation in many regions. Humidity and cloud cover affect atmospheric chemistry and visibility.
Topography also matters because valleys and basins can restrict air movement under certain conditions. Pollution can therefore become concentrated during specific weather episodes even when emissions have not changed dramatically. Forecasting smog requires combining emission inventories with weather predictions and atmospheric chemistry models. Air quality warnings often depend on these combined assessments because measured concentrations represent the outcome of emissions, transport, transformation and removal rather than emissions alone.
Smog can affect human health, vegetation, materials and visibility. Particulate matter can penetrate the respiratory system and contribute to respiratory and cardiovascular effects, while ozone can irritate airways and reduce lung function during exposure. Nitrogen dioxide and other pollutants can add further effects. Vulnerability varies with age, health status, exposure duration and concentration. Vegetation can be damaged by ozone and other oxidants, potentially reducing photosynthesis and growth. Particles can scatter and absorb light, producing haze and reducing visibility.
Some pollutants can contribute to material corrosion or surface soiling. The environmental effects depend on the chemical composition of the smog episode, so the word itself does not identify a single toxicity profile. A smoke dominated event, a sulfur rich episode and a photochemical ozone episode may have substantially different pollutant mixtures and therefore require different control measures.
Smog control focuses on reducing precursor emissions and improving atmospheric dispersion and urban exposure conditions. Vehicle emission controls can reduce nitrogen oxides and volatile organic compounds, while cleaner industrial processes and fuels can reduce emissions from stationary sources. Solvent and industrial chemical management can reduce volatile organic compound releases. Public transport, vehicle efficiency, electrification and changes in travel behaviour can reduce some combustion emissions, although their effectiveness depends on the electricity system and other factors.
Industrial emission controls can remove particulate matter, sulfur dioxide and other pollutants before exhaust reaches the atmosphere. Forecasting allows temporary measures to be introduced during expected pollution episodes. Urban planning can also influence exposure by considering traffic, building design and ventilation. Because ozone formation is nonlinear, successful management may require carefully targeted reductions in particular precursor combinations rather than simply reducing all pollutants by the same proportion.
Smog illustrates the difference between primary emissions and secondary atmospheric pollution. A vehicle may emit nitrogen oxides and volatile organic compounds, but it does not directly emit the entire mixture that later constitutes photochemical smog. Sunlight and atmospheric chemistry transform precursor gases into ozone and other secondary products, while wind transports and mixes them. This explains why smog can occur far from the sources that emitted its precursor chemicals and why pollutant concentrations can change rapidly during weather transitions.
The term smog is also broader and less chemically precise than naming individual pollutants such as PM2.5, nitrogen dioxide or ozone. For scientific analysis, researchers therefore measure specific pollutants and atmospheric variables rather than treating smog as one substance. Nevertheless, smog remains a useful general term for episodes of complex atmospheric pollution, particularly in urban environments where emissions, weather and chemical reactions interact to produce degraded air quality.
