Ozone is a special form of oxygen with a sharp, clean smell you might notice after a lightning storm. While regular oxygen molecules have two atoms bonded together, ozone molecules have three. This extra atom makes ozone both useful and dangerous.

High in the atmosphere, a layer of ozone protects Earth like an invisible shield, blocking harmful ultraviolet rays from the sun. Without this ozone layer, those rays would damage living things, making it hard for plants and animals to survive. When scientists discovered that certain chemicals were destroying parts of the ozone layer, countries worked together to limit those chemicals, and the ozone layer began recovering.

Closer to the ground, ozone becomes a problem. On hot, sunny days, pollution from cars and factories can create ozone at ground level, making the air hazy and harder to breathe. That's why weather forecasters sometimes warn about high ozone days.

Ozone has a distinctive scent. Some people describe it as smelling like chlorine or electrical sparks. You might notice this smell near photocopiers or after using certain electronic equipment, since they can produce small amounts of ozone.