Nitrogen dioxide (NO2) is a gas that occurs naturally in our atmosphere. NO2 plays an important role in the formation of ozone in the air we breathe. Ozone high in the atmosphere helps us. It is like sunscreen, and it protects us from harmful ultraviolet (UV) rays from the Sun. Near the ground though, ozone is a pollutant. It damages our lungs and harms plants, including the plants we eat. Ozone occurs naturally in the air we breathe, but there's not enough of it to hurt us. Unhealthy levels of ozone form when there is a lot of NO2 in the air. NO2—and ozone—concentrations are usually highest in cities, since NO2 is released into the atmosphere when we burn gas in our cars or coal in our power plants, both things that happen more in cities. Ozone pollution is worse in summer. NO2 is also unhealthy to breathe in high concentrations, such as on busy streets and highways where there are lots of cars and trucks. When driving, it is typically a good idea to keep the car windows rolled up and the car's ventilation set to “recirculate” so as to keep pollution out of the interior of the car. It is also important to reduce outdoor activities like playing or jogging if government officials warn you that air quality will be bad on a certain day.