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Smog is a type of air pollution visible as a murky haze over urban skylines. The term "smog" originated from a blend of the words "smoke" and "fog," signifying its primary components. Traditionally, smog was prevalent in areas where burning coal was common, with particles from combustion mixing with water droplets in the air. This phenomenon is more pronounced during warm and sunny weather, which discourages air circulation and traps pollutants near the ground.
Smog reduces air quality, affects respiratory health, and decreases visibility. Though modern regulations have reduced coal-based activities, smog still occurs in various forms due to urban and industrial pollution. It's crucial to recognize smog's impact on health and the environment to implement effective preventive measures.

Photochemical smog is a more specific form of smog that involves chemical reactions in the atmosphere. Known as "Los Angeles smog" due to its prevalence in that city, this type of smog is a brownish haze caused by sunlight acting on pollutants. The primary ingredients are nitrogen oxides (NOx) and volatile organic compounds (VOCs).
When these compounds interact with ultraviolet radiation from the sun, they undergo chemical reactions, forming a complex mixture of secondary pollutants including ozone. Photochemical smog is most common in cities with abundant sunlight and heavy traffic, as vehicle exhaust releases significant quantities of NOx and VOCs. This type of smog is particularly hazardous, triggering health problems and degrading air quality.

Nitrogen oxides (NOx) are key contributors to air pollution, including both smog and photochemical smog. These compounds consist primarily of nitric oxide (NO) and nitrogen dioxide (NO2), commonly emitted from automobiles, power plants, and industrial activities. These gases are harmful in both direct and indirect ways.
Directly, exposure to nitrogen oxides can irritate the lungs and decrease immunity to lung infections. Indirectly, they play a pivotal role in forming photochemical smog by reacting with volatile organic compounds (VOCs) under sunlight to produce ozone and other pollutants. The widespread release of NOx raises concerns for urban air quality, calling for regulations and technological innovations to reduce emissions.

Volatile organic compounds (VOCs) are carbon-based chemicals that easily vaporize at room temperature, contributing significantly to air pollution and smog formation. These compounds can originate from a variety of sources, including vehicle emissions, industrial processes, and use of solvents and paints.
In the presence of sunlight, VOCs react with nitrogen oxides (NOx) to form ozone and other components of photochemical smog. This reaction process intensifies air pollution, posing health risks such as respiratory problems and eye irritation. Understanding the sources and impacts of VOCs is essential for developing strategies to control emissions and mitigate smog-related health and environmental issues.