Ground-level ozone is a tricky pollutant that can be both good and bad. Up high in the sky, ozone protects us from harmful UV rays. But down on the ground, it can harm our health, crops, and materials. This bad ozone forms when pollutants from cars and factories mix with sunlight. Unfortunately, we've messed things up by decreasing good ozone in the sky and increasing bad ozone on the ground. Human-made chemicals are to blame for ozone holes in the poles and increased ozone in cities and rural areas. We've created a problem where we have the worst of both worlds.