Overview

Smog is a form of air pollutant characterized by reduced visibility and a harmful mix of gases and particles. The word combines smoke and fog, reflecting two principal ways smog can form: as a sulphurous, smoke-laden haze from combustion or as a photochemical product from sunlight acting on vehicle and industrial emissions. Both varieties can persist under particular weather conditions and pose risks to human health, ecosystems and infrastructure.

Composition and types

Broadly, smog falls into two categories. Classic or “industrial” smog arises when smoke—often from burning coal and other solid fuels—and gases mix with fog and cool air. It is produced by the burning of carbonaceous fuels and results from a mixture that often includes soot, ash and gases such as sulfur dioxide. Photochemical smog, by contrast, forms when nitrogen oxides and volatile organic compounds react in sunlight to create ozone and secondary particles.

History and notable events

Urban smogs became prominently documented during industrialisation. One of the best known events was a severe episode in London, which highlighted how coal smoke combined with stagnant air to create deadly conditions and led to cleaner-air legislation. As fuel sources and transport evolved, other cities faced different smog patterns: Los Angeles became associated with photochemical smog driven by vehicle exhaust, while modern growth and seasonal factors have contributed to hazardous episodes in cities such as Delhi and Beijing.

Health, environment and economic impacts

Smog contains fine particulate matter (PM2.5 and PM10), ozone and other irritants that can trigger respiratory and cardiovascular problems, aggravate asthma and increase hospital admissions. It also reduces light levels, corrodes materials and damages crops. The health burden is greatest for children, older adults and people with preexisting conditions.

Measurement, regulation and mitigation

  • Monitoring: networks measure PM, ozone, NOx and SO2 and issue air-quality indices to guide public health advice.
  • Regulation: controls on fuel quality, industrial emissions and vehicle standards have reduced many forms of smog in developed regions.
  • Mitigation: practical measures include switching away from high-sulfur fuels, adopting cleaner transport and energy, using low-emission zones, restricting burning during inversion events and urban planning that improves ventilation.

Distinctions and notable facts

Not all hazes are identical: ‘‘smoke + fog’’ type events typically involve sulfur and soot, while photochemical smog is defined by ozone and secondary organic aerosols formed in sunlight. Local climate, topography and policy all shape how often and how severely smog appears. Effective reduction combines technology, regulation and public behaviour to lower emission sources and reduce population exposure.