The layer of ozone in the upper atmosphere helps protect the earth from the sun's harmful ultraviolet rays and is the "good" ozone. Ground-level ozone, however, is unhealthy to breathe. It can narrow a person's airways, causing the lungs to work harder to provide oxygen to the body. Individuals most susceptible to the effects of ozone exposure include those with a pre-existing or chronic respiratory disease, children, and adults who actively exercise or work outdoors.