What are the six primary air pollutants?

The six primary air pollutants, known as the Criteria Air Pollutants, include carbon monoxide, lead, nitrogen dioxide, ozone, sulfur dioxide, and particulate matter. The correct choice identifies four of these pollutants, but it misses two crucial ones—sulfur dioxide and particulate matter.

In particular, carbon monoxide is a colorless, odorless gas produced by burning fossil fuels, which can pose serious health risks. Lead was historically widely used in fuels and paints and is highly toxic, even at low levels of exposure. Nitrogen dioxide is a result of combustion processes and is a significant contributor to smog and respiratory problems. Ozone at ground level is produced from reactions between volatile organic compounds and nitrogen oxides in the presence of sunlight, and it can lead to serious health issues.

While other choices include some recognized pollutants, they either lack key ones or include substances that do not fall under the six primary air pollutants. For example, carbon dioxide, while a significant greenhouse gas, is not classified as a primary air pollutant in this context. Therefore, the recognition of the pollutants mentioned in the selected answer does align with the definitions, but it doesn't fully encompass all six required pollutants.