Smog is a mix of smoke and fog, though modern smog is mostly chemical. It forms when pollutants react in sunlight. Nitrogen oxides and volatile organic compounds from vehicles and industry combine to make ground-level ozone. That ozone irritates lungs and eyes. It also damages crops and trees.

Two types exist. Classic smog, from coal burning, is a mix of sulfur dioxide and particulates. It killed thousands in London in 1952. Photochemical smog, first identified in Los Angeles, comes from vehicle exhaust and sunlight. It produces a brown haze and high ozone levels. Both types harm health and reduce visibility.

Smog is worse in cities with lots of traffic and sunlight, surrounded by mountains that trap air. Los Angeles, Mexico City, Beijing, and Delhi all suffer. Solutions include cleaner fuels, catalytic converters, electric vehicles, and public transit. Weather plays a role. Temperature inversions trap pollution near the ground. When the wind blows, the air clears. When it does not, the haze builds.

Common components

• Ground-level ozone
• Particulate matter
• Nitrogen oxides
• Sulfur dioxide
• Volatile organic compounds