I first heard about this (i.e., water poisoning) after somebody died during a brutally hot Boston marathon a few years ago.

I've often wondered -- how much water do you need to drink to cause the imbalance to start reaching "dangerous" levels? I've gone through water bottles at an alarming rate on long rides during hot days and I still felt more dehydrated than anything (i.e., kept drinking after the ride in order to feel "normal"). How fast do you need to be gulping water in order for this to become an issue? It seems like sweat and the need to urinate would do a pretty effective job of eliminating excess water from your system.