Which pollutant was not included in the 1990 amendments to the Clean Air Act?

• A. Carbon dioxide
• B. Nitrogen oxides
• C. Particulate matter
• D. Ozone

Answer: (A)

The question tests knowledge of which substances were targeted by the 1990 amendments to the Clean Air Act. Those amendments focused on the six principal “criteria pollutants” and programs like the acid rain reduction. Nitrogen oxides, ozone, and particulate matter are all part of that framework, with nitrous oxides and ozone influencing air quality and particulate matter being a core pollutant category. Carbon dioxide, however, is not one of the criteria pollutants defined under the Clean Air Act, and it was not added to the Act’s regulated pollutant list in the 1990 amendments. The regulation of carbon dioxide under this Act came later through other authorities and climate change rules. So the pollutant not included in the 1990 amendments is carbon dioxide.