Hydrocarbons, carbon monoxide, and sulphur dioxide are widely recognized as air pollutants due to their direct harmful effects on human health and the environment (e.g., toxicity, smog formation, acid rain). Carbon dioxide is a natural component of the atmosphere and essential for plant life. While its increased concentration due to human activities is a major driver of climate change and in this context is often referred to as a greenhouse gas pollutant, it is not normally classified as a direct toxic pollutant like the others listed.