I found that from April to mid-June there's an increase in the ground level ozone which creates smog (smog is created by the emissions from vehicles and smokestacks when they come in direct with sunlight.)
Nitrogen Dioxide (NO2) It comes factories, old cars from the 1900s, etc. it is a poisonous gas that comes from old engines can when breathed into the human body it can cause loss of fatigue, restlessness, anxiety, etc.. When it is released it gets stuck into the atmosphere but still gets stuck into the air but a small amount so little it can't harm large number of humans or animals.
