Seriously i am still trying to grasp the physics or mechanics behind that. It isnt air-tight. Water is at the neck area and i can put my hand inside, so i am still lost. It can last for as long as you want really. That was like 20 to 30 minutes. I think air just keeps the water at bay. That head gear is like 35 kg on land and like 1 to 2 kg in water. My sister felt like 1kg