Cold air feels irritating compared to warm air not because of temperature but because of relative humidity. Colder air has less ability to hold moisture and is drier. Therefore, when someone breathes cold air, the lack of moisture irritates the lining of the trachea and lungs.
The respiratory system's primary function is to take in and transport oxygen to the blood and expel carbon dioxide. To do this, both gases must be saturated with liquid for efficient diffusion. The cells in the lungs and trachea cannot transfer air if it is moistened and warmed. However, contrary to popular belief, there is no inherent danger in breathing cold air versus breathing warmer air aside from irritation in the bronchial tubes.
It's possible to alleviate this discomfort by staying hydrated in cold weather, especially when exercising. Individuals can do this by drinking water or a non-carbonated sports drink; carbonated beverages are more likely to cause dehydration. Tea and coffee contain caffeine, which also has a dehydrating effect. Another way to reduce discomfort when exercising in cold weather is to take deep, slow breaths. Each time a person inhales in winter weather he gets a lungful of cold, dry air that causes irritation to the lungs.