A skier was skiing down the mountain and impacted a tree. A branch puntured his left chest wall and now he is laying on the ground with difficulty breathing. Upon examination you see a 1cm puncture wound that is bleeding but also appears to have air entering the chest wall as the patient inhales. No air is seen to be exiting the chest wall upon exhalation? Why would this be a medical emergency? Where is the air likely entering in the chest cavity?