Ozone is a gas. It can be good or bad, depending on where it is. "Good" ozone occurs naturally about 10 to 30 miles above the Earth's surface. It shields us from the sun's ultraviolet rays. Part of the good ozone layer is gone - destroyed by man-made chemicals. Without enough good ozone, people may get too much ultraviolet radiation. This may increase the risk of skin cancer, cataracts and immune system problems. "Bad" ozone is at ground level. It forms when pollutants from cars, factories and other sources react chemically with sunlight. It is the main ingredient in smog . It is usually worst in the summer. Breathing bad ozone can be harmful, causing coughing, throat irritation, worsening of asthma , bronchitis and emphysema , and even permanent lung damage, if you are regularly exposed to it.