Cold, dry air dries and cools the airway lining during hard breathing, and irritation coughing follows.
For some runners it goes further: the NHS lists cold air and exercise as asthma triggers, and exercise-induced bronchoconstriction can produce cough, wheeze, and chest tightness even in people without day-to-day asthma. Warming the incoming air with a buff or scarf, warming up gradually, and easing into pace all help. Wheeze, chest tightness, breathlessness beyond your effort level, or a cough on every cold run is the get-checked pattern, since a pre-exercise inhaler often fixes it outright.
What to do
Common questions
Why do I cough after running in the cold?
Cold air is dry air, and when you run you pull huge volumes of it past airways that prefer things warm and moist. The airway lining dries and cools, gets irritated, and you cough, sometimes for half an hour after the run. For most people it is just irritation. For a significant minority it is more: the NHS lists cold air and exercise among the triggers that set off asthma symptoms, and exercise in cold, dry air is a classic trigger for exercise-induced bronchoconstriction, where the airways temporarily narrow and cause coughing, wheezing, or chest tightness. You can have this pattern without having everyday asthma. The practical fixes: warm up properly, breathe through a buff, scarf, or mask to warm and humidify the air, and ease into pace rather than sprinting from the door. See a GP if the cough comes with wheeze, chest tightness, or breathlessness out of proportion to your effort, if it happens on every cold run, or if it is joining you indoors too, because a reliever inhaler used before exercise often solves it completely and is worth having assessed properly.