What is the EPA limit for CO expressed as a percentage?

The correct response highlights that the Environmental Protection Agency (EPA) has established limits for carbon monoxide (CO) in ambient air, which reflects the health risks associated with this pollutant. The EPA's standard for CO is 0.0009%, which translates to 9 parts per million (ppm). This limit is designed to protect the public from the harmful effects of CO exposure, particularly for vulnerable populations such as children, the elderly, and those with pre-existing health conditions.

Understanding the significance of this limit is essential, as even low concentrations of CO can cause adverse health effects over time, particularly due to its ability to interfere with the blood's ability to carry oxygen. This makes proper awareness of environmental standards crucial for public health and safety.

Other options do not represent the EPA's established limits for CO, making them incorrect in the context of this question.