Runners ventilate a lot of air. In winter, this air is very cold and dry and should therefore be warmed up and moistened before it enters the lungs. Otherwise, it's a big strain on the lungs. During normal physical activity, the nose and oral cavity have the capacity to moisten and warm up the air. But during strenuous athletic activity, more air needs to be pumped into and out of the body. The available capacity is no longer sufficient, and the air reaches the lungs cold and dry. The cold air overstrains the lungs' mucous membrane and can be damaging if the training is too intense or if the airways aren't protected.
It's difficult to give general recommendations about when exercising in the cold makes sense. As a basic principle, one has to distinguish between athletes who already have problems with their lungs and those who don't. I'm mainly thinking of people with known exercise-induced asthma. For them, difficulties already start at temperatures below 10 °C if the physical strain is too high. With lungs that are already damaged, cold weather training is counterproductive and can cause further damage.
Scandinavian cross-country skiers, for example, often train in low minus temperatures. Many of them develop a sort of exercise-induced asthma. It's believed that this condition derives from the lungs mucous membrane being damaged by cold air.
Protect your airways and don't overdo your training
"Lung-healthy" individuals can exercise in temperatures below zero without any problems as long as they protect themselves and don't overdo it. Airways can, for instance, be protected with a scarf or something similar that covers nose and mouth. The resulting reservoir warms up the air and moistens the lungs.
Not overdoing it means, for example, that personal limits shouldn't be pushed in very cold temperatures, and that the decision on how hard you can and want to exercise should always be based on common sense.
Of course, temperatures below -10 °C are an enormous burden for our bodies. But a passionate swimmer or rower, for instance, won't let that hold him back from outdoor training and they are probably also more used to it. But even these "cold trained" individuals should always listen to their bodies. If something is bothering your airways, do not force yourself to exercise.
The following tips will help you to make it through winter without overstraining your body
Let reason prevail
Adapt your training to your personal fitness level, your health and the temperatures outside. A fit, lung-healthy athlete can train better in cold temperatures than an unfit individual with health problems. In general, the colder the air, the less you should push.
The right clothes
Wear good clothing. Choose items that stop you getting cold and wet. Breathable materials worn directly on the skin ensure that you don't get wet from your own sweat. The top layers should make sure you don't get cold. Depending on the weather, the topmost layer should be windproof and waterproof. Likewise, gloves and a hat make sense with temperatures below zero. The ears are particularly sensitive to the cold.
Protect your airways
A scarf, bandana or facemask covering your mouth and nose will warm and moisten the air before it enters the lungs. This reduces the impact on the lungs.
The right breathing technique as far as possible
Inhaled through the nose, air gets warmed up, thus moistening and filtering when you breathe through your mouth. This sounds good in theory but in real life it's often hard for runners to inhale enough air through the nose during periods of intense strain. Still, it is better to breathe through your nose when temperatures are low. This works during the warm-ups or relaxed jogging. It doesn't matter if you exhale through your nose or mouth as by then the air is dry.
Winter air is dry. That's why you should drink enough before and after exercising. Your drink should neither be hot nor ice cold.
Perform your stretching exercises after running but make sure to go inside!
Don't overstretch a sick body
It doesn't matter if you're suffering from a chronic lung disease or only a common cold, don't overstrain your body, especially outside with cold temperatures.
If you want to train regardless of for example a slight cold, be reasonable. Opt for indoor exercises, for instance light power or stabilization training. Don't push the limit and be easy on your body. Coughs and sneezes don't just stress the lungs and their mucous membrane, they also stress the muscles, including the heart muscle. When you have a severe cold or fever, rest is recommended.
Listen to warning signs from your body!
Listen to your body. If you're running and breathing is difficult, reduce intensity. A burn in the chest, a tickle in the throat and surfacing mucus or bloody spit are clear signs that you've overdone your winter training. Consult your doctor if these conditions don't recede.
Build up gradually
It doesn't matter if you're new to endurance training or coming back from a long training break due of an injury - take your time and gradually build up your condition.
Mind the weather
In contrast to summer - when the ideal time to exercise is more dependent on the time of day (early in the morning when it's not too hot and the air pollution level is low) - you should mind the weather in winter. When it's foggy, the various air layers stop the pollution from rising, and so the polluted air gets stuck in the bottom layer. That's why it's better to train outside when the weather is clear with lower levels of dirt and dust particles.
And last but not least - enjoy!
Enjoy your training! There's a reason why you're exercising outdoors and not on the treadmill. Don't just go all in, enjoy the landscape too!
On this note, I wish you an enjoyable and healthy work-out in the great outdoors!