Which one is not a pollutant normally?

Carbon-dioxide is not a pollutant normally.
Carbon dioxide is not considered an air pollutant, as it occurs naturally in the air. However, the CO₂ concentration in the atmosphere has significantly increased due to human activity. Carbon dioxide is a greenhouse gas; alongside nitrous oxide (N₂O), and methane (CH₄).
Carbon dioxide in the atmosphere warms the planet, causing climate change. Human activities have raised the atmosphere's carbon dioxide content by 50% in less than 200 years.