A collapsed lung, also known as a pneumothorax, occurs when air escapes out of a lung and becomes trapped inside the space between the chest and lung cavities. This can be caused by air blisters in the lungs that break open, sudden air pressure changes, or trauma to the chest or ribcage.
The IRS interpreted this provision as applying only to full-time students. In a 1993 technical advice memorandum, the IRS provided a safe-harbor definition of a full-time undergraduate student as one who is taking 12 credit hours and working no more than 20 hours per week.
Well, the tax law definition is more liberal than you might imagine. A full-time student is one enrolled for some part of five calendar months (whether or not consecutive) in a given year for the number of hours or courses considered full-time attendance by the school that the student is attending.