Is the kind in a tank as pure? I'd think it would be more dangerous to use the stuff they stick in your engine than the medical grade. I don't know. Like I said, I just did it for the first time a few months ago and don't plan on making a habit of it.
Theres two kinds you can get in a tank, medical grade and industrial grade. Med is for dentists and such, industrial is obviously for use in machinery, like people use in cars to boost their ride. You need a license to get the med grade, they have a standard mix which I donno off the top of my head, 40% nitrous and 60 oxygen, something like that. At that point in the dentists chair, the dentist adds additional oxygen to keep you from getting too spaced out, or cuts back to make you spaced.
The industrial isn't filtered and contains all sorts of shit like sulphur. There is a distillation process whereby you can filter the industrial grade through water and charcoal (I believe its charcoal), the apparatus looks like grain silos and bubbles until pure, you catch the filtered gas in garbage bags.