Which of the following is NOT one of the six criteria pollutants established by the Clean Air Act?

The correct choice is arsenic, as it is not one of the six criteria pollutants established by the Clean Air Act. The six criteria pollutants that are specifically identified by the Act include carbon monoxide, lead, ozone, particulate matter, sulfur dioxide, and nitrogen dioxide. These pollutants have been determined to cause significant health issues and environmental effects, prompting regulatory measures to control their emissions in the air.

Arsenic, while a toxic element that can pose health risks, does not fall within the category of these six pollutants as defined by the Clean Air Act, which focuses on more common air quality issues that affect public health on a broader scale. By distinguishing between the criteria pollutants and other harmful substances like arsenic, the Clean Air Act prioritizes regulation of those pollutants that have the most significant impact on air quality and human health.