The 6 major pollutants that the EPA monitors closely to protect health in urban areas are ozone, carbon monoxide, nitrogen dioxide, particulate matter, sulfur dioxide and lead.  Ozone provides protection from the sun's harmful rays up in the stratosphere, however ground level ozone is harmful to breathe.  Sources of ozone include vehicles, factories, landfills, industrial solvents, gas stations and farm and lawn equipment.  Carbon monoxide  is emitted in incomplete fossil fuel combustion. 

The sources for carbon monoxide include automobiles, buses, trucks, and small engines.  Nitrogen Dioxide is the result of burning fuels in utilities, industrial boilers, cars and trucks.  Particulate matter is derived from industrial processes such as smelters, automobiles, burning industrial fuels, woodsmoke, and dust from paved and unpaved roads.  Sulfur Dioxide is emitted from industrial, institutional, utility, and apartment or house furnaces and boilers, in addition to petroleum refineries, smelters, paper mills and chemical plants.  Lead is released from coal combustion, smelters, car battery plants, and the combustion of waste containing lead products.  

These 6 pollutants are only a drop in the bucket so to speak when it comes to pollution.  The consequences of these pollutants range from smog to acid rain, and environmentally damage the planet.  The effect on human beings is still being determined.