What concentration defines an oxygen-rich atmosphere?

An oxygen-rich atmosphere is defined as having a concentration of oxygen that exceeds the normal ambient level found in the earth's atmosphere, which is approximately 21%. Specifically, an atmosphere with greater than 23.5% oxygen content is considered significantly enriched with oxygen. This increased concentration can enhance combustion processes and elevate risks of fire in certain environments.

In contrast, levels below 19.5% are classified as oxygen deficient, which can pose health risks and limit the ability to sustain normal physiological functions. At 20.5%, the atmosphere is still within the normal range for oxygen, posing no significant risks associated with increased fire hazards or respiratory issues. Therefore, the concentration that clearly indicates an oxygen-rich situation is anything above 23.5%, making the correct choice that option.