Snorkeling is a very physical activity that requires people to use their leg, arm and back muscles extensively. Doing this for long periods of time means breathing often becomes difficult because the body’s need for oxygen exceeds the amount being supplied to it. The muscular exercise also causes perspiration on the chest and chin, adding to one's discomfort. Furthermore, carbon dioxide builds up in the blood more quickly when people are snorkeling and this makes it harder for the body to get rid of it. All of these factors contribute to why breathing can be so difficult when snorkeling.