Which of the following is usually not an air-pollutant?

The six air pollutants are carbon monoxide, lead, ground-level ozone, nitrogen dioxide, particulate matter, and sulfur dioxide.The combustion of gasoline and other hydrocarbon fuels in automobiles, trucks, and jet airplanes produces several primary air pollutants. Nitrous oxide (N₂O), commonly known as laughing gas, is used in surgery and dentistry for its anaesthetic and analgesic effects.