Smoke smokestacks, village chimneys, CHP and other furnaces and boilers.
Smoke called products of incomplete combustion of fuel. If the fuel burns completely, the smoke is not produced. But this requirement is responsible except that natural gas and its combustion something all the same "precipitates". Without smoke burn high-quality coal - coke and anthracite, and the lower quality coals - brown, bituminous and burn with the formation of smoke.
By itself, the smoke the substance is not homogeneous. It contains particles of soot, ash, dust, ash, organic fabrics, various chemical compounds.
Suspended fine particles gradually settle under the influence of gravity. The city slowly lays down a layer of dirt. The air is harmful to breath. The foliage of the trees, covered with fallen "dry rain"may be effective to produce oxygen.
Industrial emissions are often a lot of water vapor and gaseous oxides. Particles of smoke are the centers around which gather fumes and gases, forming harmful compounds that fall as rain.
Environmental law severely punishes those who smoke stronger than all, and obliges to put on the pipe, where there is smoke, special filters catching the smallest particles.