Air pollution is caused by multiple factors, we can divide them in two groups, natural and human caused.
From the natural, the once that cause the biggest air pollution are the volcanoes with their release of ashes and toxic gasses into the atmosphere, the methane released from the melting permafrost around the polar regions and Siberia, and the cattle with the release of methane gas...
From the human caused pollution of the air we have the burning of the fossil fuels, dust from the building activities, poisonous gasses that releases the industry, burning of wood in the households (and all sorts of other stuff alongside it)...
