Question: why can I only hold my breath under water for 30 seconds?

Lindy Heath answered on 21 Jun 2012:

When you hold your breath you are not giving your body any oxygen. Your body needs oxygen so that it can react the oxygen with glucose to form energy. This reaction also produces carbon dioxide as a waste product which you need to breathe out. So when you hold your breath your body has too much carbon dioxide and not enough oxygen. Your body can only survive a small amount of time without oxygen.