Which gas is indicated by levels exceeding 1000 ppm, suggesting potential health risks?

The correct answer is carbon dioxide. Exceeding 1000 parts per million (ppm) of carbon dioxide indicates a concerning level because it can lead to adverse health effects. Carbon dioxide at high concentrations can displace oxygen in the air, leading to headaches, dizziness, shortness of breath, and even loss of consciousness in extreme cases. The threshold of 1000 ppm is significant because it marks the point where symptoms may begin to be noted, especially in enclosed or poorly ventilated spaces.

While other gases listed can also pose health risks, their thresholds and implications differ. Methane in practical settings generally does not have direct toxic effects at such levels, as it is primarily an asphyxiant in very high concentrations. Carbon monoxide is a dangerous gas, often at much lower levels, due to its ability to interfere with oxygen transport in the blood; however, significant health effects can occur below 1000 ppm. Nitrogen dioxide is harmful at lower levels due to its respiratory effects and can be problematic, but its typical concern arises at lower concentrations than those indicated for carbon dioxide.