What are the advantages/disadvantages of breathing with one's nose while running? Is it a good idea to develop this habit?
I'm referring to relatively slow, long runs, and for experienced runners.
You should definitely breathe the way that feels most comfortable.
At slow paces, breathing through your nose will help filter out dust particles and other impurities in the air that might irritate your lungs. Your nose also helps humidify and heat up the air before it reaches your lungs. Cold and dry air can further irritate your airways, especially on people with asthma or bronchitis.
At moderate paces, it starts to feel uncomfortable to breathe only through your nose. Most people recommend inhaling through the nose and exhaling through your mouth. Personally, I don't find that it helps at all, so I do it the other way around. Using both your nose and your mouth at the same time will definitely help increase the airflow, but some people find it hard to get used to.
At fast paces it becomes impossible to use only your nose. Breathing at a fast rate through your mouth significantly increases the risk of getting a sore throat or other respiratory problem, but there's no way around it.