It is dangerous. By extending the length of the snorkel, you are increasing the amount of air that you have to pull in before you get fresh stuff. So each breath has more CO2 than you should expect. (Eventually if the hose is long enough you are never getting any fresh air down to your lungs, and it is essentially the same as breathing into a paper bag.)
Add to this the fact that the increased water pressure will mean that you won't be able to breathe as deeply as you would normally (and that you need to do to get past the increased hose length).
The risk is then that due to increased CO2 content you start to hyperventilate to compensate, shortening your breath further, resulting eventually in asphyxiation and blackout..... (obviously blackout underwater is a bad idea).
You're probably on the edge of OK since you're still alive. Definitely wouldn't lengthen the hose at all though - and ultimately not doing it at all is a safer idea.
Last edited by BritAbroad; 09-04-2007 at 08:41 PM.