At what concentration of Carbon Dioxide does a person experience marked headaches and collapse?

A concentration of 4.5% to 5% Carbon Dioxide in the air represents a critical level at which individuals can start to experience significant physiological effects, such as marked headaches and a risk of collapse. At this concentration, the normal function of the respiratory system is impaired, leading to increased levels of carbon dioxide in the blood (hypercapnia). This condition can result in symptoms including headache, dizziness, shortness of breath, and ultimately unconsciousness if exposure continues without proper ventilation or removal from the environment.

Lower concentrations, such as 0.5% and 2%, do not typically lead to these severe symptoms in healthy individuals. Although they can induce mild discomfort or fatigue over prolonged exposure, the physiological effects remain manageable and usually do not lead to collapse. A concentration of 10%, while significantly higher than what is found in typical environments, poses a more extreme risk causing immediate loss of consciousness or even death, but it is typically faced in dangerous industrial accidents and not in common scenarios of exposure. Therefore, the 4.5% to 5% range is recognized as a critical threshold for experiencing marked headaches and the possibility of collapse, making this the correct choice.