Nose or mouth when you train? Both pieces of standard advice are half right, because the best route depends on how hard you are working. At easy effort the nose delivers enough air and filters, warms and humidifies it. As intensity climbs the nose becomes a bottleneck, and forcing it to the finish line means fighting your own airway. Exercise to try: breathe through your nose for the entire warm-up and every easy session, and only open your mouth once you pass the point where you can still hold a conversation.