Cold air causes narrowing of the airways in some asthmatics. For some asthma patients it is the only trigger. It is believed that the mechanism is airway drying which increases osmolarity and thickens airway mucous. The effect of cold air on asthmatics, especially exercising asthmatics, is dose-dependent. The colder the air, the more dramatic the impact on asthma. This poses a challenge for elite athletes. Even otherwise mild asthmatics may be so severely affected that no medical regimen adequately controls symptoms.
An alternative is to protect the airway from drying by use of a heat-exchange device which uses the warmth and moisture of exhaled air to condition inspired air. This has been shown in clinical trials (Beuther and Martin) to significantly reduce asthma symptoms, even more so than a bronchodilator.
Beuther DA and Martin RJ. Efficacy of a Heat Exchanger Mask in Cold Exercise-Induced Asthma. CHEST May 2006 vol. 129 no. 5 1188-1193