Ozone is an unstable and highly reactive molecule comprised of three oxygen atoms. It's an essential part of the atmosphere, but surface-level, tropospheric ozone, better known as smog, causes pollution. Combustion from cars and power plants release nitrogen oxide gases (NOx) and volatile organic compounds (VOC's) that combine chemically with oxygen to create ozone. Ozone inflames the lungs, and breathing too much of it can lead to conditions like asthma and chronic bronchitis.