It absorbs ultra-violet radiation from the sun and prevents most of it from reaching the Earth's surface. When pollutants from human activity increase these concentrations, ozone can reach harmful levels. Our chapter addresses ground-level ozone, a major air pollutant and a main component of smog.
Ozone is a powerful and irritating pollutant that is colourless and invisible to the naked eye.
Thus, air can appear to be clean and yet still do harm to human health and the natural environment, as well as to buildings and materials. Ozone is not emitted directly. Whether ozone forms depends on many factors - weather conditions, the landscape, prevailing winds, and the amount of NOx and VOC in the air. Because its formation depends on strong sunlight and is accelerated by heat, its daytime levels are much higher in the summer than in the winter. Levels of ozone can also vary at night.