Acute or chronic cold exposure elicits several effects on the respiratory system. Pulmonary mechanics are compromised by bronchoconstriction, airway congestion, secretions and decreased mucociliary clearance. These responses are active in cold- or exercise-induced asthma, and are possibly responsible for decreased immune function and protection against airborne pollutants. The primary ventilatory effect of cold air is to decrease baseline ventilation and respiratory chemosensitivity. Although these responses provide significant protection against heat loss in many animals, the effect in humans is minimal. Cold exposure also elicits an increase in pulmonary vascular resistance. This stimulus is synergistic with hypoxia and may mediate pulmonary hypertension and edema at altitude. Chronic exposure to cold environments results in morphological changes such as increased numbers of goblet cells and mucous glands, hypertrophy of airway muscular fascicles and increased muscle layers of terminal arteries and arterioles. These latter two factors may play a role in the symptoms of chronic obstructive pulmonary disease and bronchitis, high altitude pulmonary hypertension and edema, and right heart hypertrophy.