During
the winter months air pollution can get worse due to the different weather conditions. Due to less sunlight and colder weather, the conditions keep pollutants lower down in the atmosphere where they can be breathed in by humans. Air pollution is also higher in winter due to increased use of road vehicles and heating to keep homes warms during the colder months.
On hot, sunny, summer days, when it's not very windy, a gas called ozone is formed by chemical reactions between other pollutants such as nitrogen dioxide. The reactions are helped by sunlight.