If "largest" refers to the total mass or volume emitted into the atmosphere by human activities, Carbon dioxide (CO2) is by far the largest pollutant. Although often discussed primarily as a greenhouse gas rather than a direct air pollutant impacting human health at ambient levels, its sheer volume of emission from the combustion of fossil fuels dwarfs that of other major air pollutants like carbon monoxide, sulfur oxides, and hydrocarbons.