Book 25, Question 110
What is smog?
The word smog comes from smoke and fog.
Today it mostly means a haze of ozone and tiny particles. Ground-level ozone forms when sunlight reacts with gases from vehicles and factories. Smog is thickest on hot, sunny, still days and can make it harder to breathe. Cleaner cars, better fuels, and rules for factories have helped reduce it in many cities.