A natural layer of ozone in the upper atmosphere protects us from harmful ultraviolet light from the sun.

But ozone close to the ground is pollution. High in the atmosphere or close to the ground, it’s the same stuff, O3 – three molecules of oxygen bonded together.

Whether ozone is safe depends on where it is. That’s because ozone is a toxic gas. It kills organisms. We need it way up in the stratosphere to filter out the sun’s radiation. But down here where people live it’s not a good thing to have too much ozone around.

Ground-level ozone is produced when hot stagnant air, usually associated with high pressure weather systems, trap pollutants such as nitrous oxide and carbon monoxide close to the ground. In the presence of sunlight, these pollutants react to produce ozone. That’s why there are sometimes ozone alerts on hot summer days.