The human body can survive the hard vacuum of space unprotected, despite contrary depictions in much popular science fiction. Human flesh expands to about twice its size in such conditions, giving the visual effect of a body builder rather than an overfilled balloon. Consciousness is retained for up to 15 seconds as the effects of oxygen starvation set in. No snap freeze effect occurs because all heat must be lost through thermal radiation. The greatest danger is attempting to hold one's breath before exposure, as the subsequent explosive decompression can damage the lungs. The effects of vacuum on the human body are known through various experiences (often accidents) in very high altitude conditions and training vacuum chambers.