Smog
Visible air pollution, primarily composed of ground-level ozone and fine particles, that reduces visibility and harms health.
Detailed Explanation
Smog is a term originally coined by combining "smoke" and "fog" to describe the visible, thick air pollution that blanketed industrial cities like London in the early 20th century. Today, the term most commonly refers to photochemical smog — a brownish haze formed when sunlight drives reactions between nitrogen oxides and volatile organic compounds to produce ground-level ozone, along with secondary particulate matter. Unlike the coal-smoke smog of the industrial era, modern photochemical smog is worst in sunny, warm climates and is primarily driven by vehicle emissions and industrial VOC sources. Los Angeles became infamous for smog in the mid-20th century, and while conditions have improved dramatically due to regulation, the city and broader Southern California remain among the most ozone-polluted regions in the country. Smog reduces visibility — on heavy smog days, landmarks and distant terrain disappear behind the haze. More importantly, smog causes respiratory irritation, aggravates asthma, and reduces lung function. The visibility impairment caused by smog is also why the Clean Air Act includes secondary NAAQS standards aimed at protecting visibility, particularly in national parks and wilderness areas through the Regional Haze Rule. AirHistory's ozone trend data for each city is essentially a proxy for smog conditions, as ozone is the primary chemical component of photochemical smog.
Related Terms
Ozone (O3)
A reactive gas formed at ground level when sunlight triggers chemical reactions between nitrogen oxides and volatile organic compounds.
Volatile Organic Compounds (VOCs)
Carbon-containing chemicals that easily evaporate at room temperature and contribute to ozone formation and direct health effects.
Nitrogen Dioxide (NO2)
A reddish-brown gas produced primarily by vehicle engines and power plants that contributes to smog and respiratory problems.
PM2.5 (Fine Particulate Matter)
Airborne particles smaller than 2.5 micrometers in diameter — small enough to penetrate deep into the lungs and enter the bloodstream.
Frequently Asked Questions
Visible air pollution, primarily composed of ground-level ozone and fine particles, that reduces visibility and harms health.
Smog is a term originally coined by combining "smoke" and "fog" to describe the visible, thick air pollution that blanketed industrial cities like London in the early 20th century. Today, the term most commonly refers to photochemical smog — a brownish haze formed when sunlight drives reactions between nitrogen oxides and volatile organic compounds to produce ground-level ozone, along with secondary particulate matter. Unlike the coal-smoke smog of the industrial era, modern photochemical smog is worst in sunny, warm climates and is primarily driven by vehicle emissions and industrial VOC sources.