Diffusion hypoxia with acute use of nitrous is not a significant factor unless one suffers from an airway obstruction or cardiopulmonary disease. Any problems can be avoided by allowing the patient to breath 100% O2 for 3-5 minutes after discontinuation of nitrous oxide.
http://www.springerlink.com/content/t1077557l2105463/
http://www.ncbi.nlm.nih.gov/pubmed/1809049
Of course most people are breathing 21% room air after using 100% nitrous but the point is to breath oxygen in between nitrous! Wearing a mask or breathing multiple balloons will cause you to either get extremely nauseas and/or pass out. The biggest danger of whippets is when it is done consecutively without oxygen or people pass out and hit their heads.
"Hypoxemia is significant for only a matter of minutes and has been documented only when high concentrations (>70%) have been delivered by full mask or by endotracheal tube"
Eger E.I. Pharmacokinetics. In: Eger E.I, editor. Nitrous Oxide. New York: Elsevier; 1985
Good advice from an archived thread:
Don't EVER:
Use industrial/automotive nitrous, or any other form not intended for human consumption. This contains hydrogen sulphide, which is not something you should inhale.
Try to inhale N20 directly from whippits, tank nozzles, or any other point of escaping gas. The gas WILL give you frost burn. Balloon broke? Try a condom or a plastic bag!
Strap a gas mask to your face. If you pass out, you will not be able to remove the mask and will most likely die of oxygen starvation. Yes, people have died doing this.
Lock yourself in a room, closet, car, or refrigerator with a tank of nitrous and open it. Again, the enclosed space will prevent you from receiving the requisite level of oxygen. Yes, people have died doing this.
Stand up and do it. As mentioned previously, doing this is dangerous because you may rapidly collapse after inhalation. Also, stay away from open windows.
Use a large tank without a regulator or which isn't strapped down.
Use homemade nitrous. Unless you are a chemist, you're likely to get a load of rubbish like NO2, H2NO3, and other yummy toxic things.
Allow yourself to lose sight of moderation. No one likes a bong hoarder, and no one likes a Nitrous Whore.