Which pollutant is released primarily from motor vehicles?

Carbon monoxide is primarily released from motor vehicles due to the incomplete combustion of carbon-containing fuels, such as gasoline and diesel. When an engine burns fuel, it ideally combines carbon with oxygen to produce carbon dioxide. However, in many cases—especially in engines that are not running efficiently or under certain conditions—carbon combines with only one molecule of oxygen, resulting in carbon monoxide, a colorless, odorless gas that can be harmful to human health at higher concentrations.

Motor vehicles are one of the main sources of carbon monoxide emissions in urban areas, particularly in heavy traffic conditions or congested street environments. In contrast, while sulfur dioxide and lead may also come from vehicles, they are not as significant in comparison to carbon monoxide. Sulfur dioxide is primarily generated from the burning of fossil fuels containing sulfur, while lead emissions have drastically decreased in recent decades due to regulations on leaded gasoline. Particulate matter, while also emitted from vehicles, includes a variety of sources such as industrial processes and dust, making it less specific to motor vehicles alone than carbon monoxide.