The human activity that causes the most air pollution is the burning of

Fossil fuels

Explanation:

The human society utilizes the thermal energy produced after burning fossil fuels like coal, petroleum and natural gas. This energy is used for the purpose of cooking food, running their vehicles and in industrial processes. The burning of the fossil fuels liberates greenhouse gases like carbon dioxide, carbon monoxide, methane, sulphur dioxide into the atmosphere. These gases are poisonous if present in air in high concentration they are irrespirable. The increase in concentration of these gases has resulted in increase in global temperatures as these gases traps heat energy from sunlight.

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