What factor is a primary cause of suffocation?

Insufficient air supply is a primary cause of suffocation because it directly impacts the body's ability to receive the oxygen necessary for survival. When there is a lack of sufficient air, the body cannot adequately exchange oxygen and carbon dioxide, leading to hypoxia and eventually asphyxiation if the condition persists. This can occur in various scenarios, such as drowning, choking, or being in an environment with very low oxygen levels.

The other options, while related to respiratory issues, do not serve as the primary cause of suffocation in the straightforward manner that insufficient air supply does. Excessive exposure to smoke can lead to respiratory distress and toxic inhalation, high altitude environments can contribute to decreased oxygen availability, and low humidity can exacerbate respiratory conditions but are not the direct causes of suffocation in the same immediate sense.