Scientists know smog as ground level ozone.
Ozone is a natural constituent of the atmosphere and plays a vital function in the stratosphere by protecting life on earth from the sun's harmful ultraviolet rays. But in the lower troposphere and in the presence of sunlight, ozone reacts with nitrogen oxides and volatile organic compounds and becomes a pollutant. These two precursor gases are produced by natural sources such as forest fires and by motor vehicle exhaust and industrial emissions. As well as harming vegetation, ground level ozone damages human health. It can inflame breathing passages, decreasing the lung's working capacity.