Which statement about primary air pollutants is true?

• A. They are emitted directly into the air
• B. They include only natural sources
• C. They are always harmless
• D. They do not include carbon monoxide

Answer: (A)

Primary air pollutants are substances released directly into the atmosphere from identifiable sources, rather than forming through reactions in the air. This direct emission is what makes them “primary.” Examples include carbon monoxide from vehicle exhaust, sulfur dioxide and nitrogen oxides from combustion, and particulate matter from various industrial and combustion processes. Because they are emitted straight into the air, the true statement is that they are emitted directly into the air. They can come from both human activities and natural events, so saying they come only from natural sources isn’t accurate. They are not always harmless; many primary pollutants harm human health and the environment at elevated levels. Carbon monoxide is indeed a primary pollutant, so a claim that primary pollutants do not include CO would be incorrect.