What does the freshness of the air in a room primarily determine?

The freshness of the air in a room is largely determined by the levels of carbon dioxide present. Fresh air typically contains lower concentrations of carbon dioxide, as it is a byproduct of respiration and various forms of combustion. High levels of carbon dioxide can indicate poor ventilation or a buildup from the occupants' activities, which diminishes air freshness and can lead to discomfort and impaired cognitive function. Therefore, when assessing the freshness of air, lower carbon dioxide levels directly correlate with a more refreshing and healthier environment.

While other factors—such as the percentage of carbon monoxide, oxygen, and odorous substances—can also influence the perceived quality of air, they do not predominantly define its freshness. For example, carbon monoxide is usually present in very low concentrations in a well-ventilated space, and fresh air would have an adequate amount of oxygen; however, it is primarily the buildup of carbon dioxide that serves as a key indicator of air quality in enclosed environments.