<P> Smog can form in almost any climate where industries or cities release large amounts of air pollution, such as smoke or gases . However, it is worse during periods of warmer, sunnier weather when the upper air is warm enough to inhibit vertical circulation . It is especially prevalent in geologic basins encircled by hills or mountains . It often stays for an extended period of time over densely populated cities or urban areas, and can build up to dangerous levels . </P> <P> According to the Canadian Science Smog Assessment published in 2012, smog is responsible for detrimental effects on human and ecosystem health, as well as socioeconomic well - being across the country . It was estimated that the province of Ontario sustains $201 million in damages annually for selected crops, and an estimated tourism revenue degradation of $7.5 million in Vancouver and $1.32 million in The Fraser Valley due to decreased visibility . Air pollution in British Columbia is of particular concern, especially in the Fraser Valley, because of a meteorological effect called inversion which decreases air dispersion and leads to smog concentration . </P> <P> For the past few years, cities in northern India have been covered in a thick layer of winter smog . The situation has turned quite drastic in the National Capital, Delhi . This smog is caused by the collection of Particulate Matter (a very fine type of dust and toxic gases) in the air due to stagnant movement of air during winters . </P> <P> Delhi is the most polluted city in the world and according to one estimate, air pollution causes the death of about 10,500 people in Delhi every year . During 2013--14, peak levels of fine particulate matter (PM) in Delhi increased by about 44%, primarily due to high vehicular and industrial emissions, construction work and crop burning in adjoining states . Delhi has the highest level of the airborne particulate matter, PM2. 5 considered most harmful to health, with 153 micrograms . Rising air pollution level has significantly increased lung - related ailments (especially asthma and lung cancer) among Delhi's children and women . The dense smog in Delhi during winter season results in major air and rail traffic disruptions every year . According to Indian meteorologists, the average maximum temperature in Delhi during winters has declined notably since 1998 due to rising air pollution . </P>

The smog in cities in india mainly consist of