How is the air polluted

There are various factories in the [b] City, as well as the large and small boiler rooms, standing a stack of smokestacks, full of smoke in the day and night, the cooking stoves of every household and a large amount of fumes, the cooking stoves of each household, and the discharge of a large amount of waste gas. Motorcycle exhausts exhaust at a moment. These soot and exhaust gases enter the atmosphere, making the air above the city dirty. If the weather is good, the smoke will dissipate slowly as the air flows. If we catch up with the special weather, the smoke and dust in the air can not diffuse, and it will accumulate in the atmosphere close to the ground above the city, causing serious air pollution.
air can not be seen or touched, whether the atmosphere is polluted or not, and the degree of pollution must be detected by instruments. Not by contrast can also be found: people living in the countryside can see the stars in the dark sky at night; to the city, only a few stars are visible and dark. The lighting of the night sky is one of the reasons, and the air is dirty and the transparency is small, which also affects the observation of the stars.
there are many harmful substances in the polluted atmosphere, which can enter the human body through the respiratory tract, cause respiratory diseases, and can also enter the blood and deliver to all organs of the body, causing greater harm to the human body. [/b]