As you hold breath the lung CO2-levels start rising while at the same time you consume your oxygen. The rise of CO2 makes you want to breath again - nasty sensations. These are primal mechanisms, that protect us, so normally we start breathing before you must.
More dangerous is a scenario, where you end up under water without having the time to breath and your body in fullpower mode. The ogygen levels hit the floor just when you are allmost out of it and so you may blackout before you know.
I´ve held my breath for only 30 - 40 s while getting worked.