Suffocating is defined as "the quality of impairing the breathing", but what is it about suffocating air that makes it hard to breathe? Is it the humidity? What would you replace suffocating with to make it clear on how the air is? What are the qualities of air that makes the air suffocating?
Literally, suffocating air is air which in not poisonous, but does not support life - generally because it contains insufficient oxygen. Air in a closed chamber becomes suffocating as the oxygen is used up by breathing. Equally, air can become suffocating if the oxygen is displaced, for example in a mine, or due to volcanic activity.
Be aware that "suffocating" is also used by extension to describe other situations where an individual's life feels restricted, usually in a social context. For example:
“Women who live their lives according to the suffocating religious rules created by men represent one of the saddest tragedies of the whole human history!” ― Mehmet Murat ildan
It is also sometimes used as a more general expression of discomfort: "The summer heat here, is suffocating"