I just checked the amazon page again. It says 99.5% purity.
Maybe I also messed up somehow. I basically filled a large balloon with helium and went underwater in the bathtub, then kept inhaling from the balloon and exhaling into the water, expecting to pass out within a few breaths and then drown, but when the balloon was empty I was still conscious, so I held my breath while going up and quickly refilling the balloon, then diving down again. Repeated this a few times until the 250 litres of helium were empty. By the end I found it extremely hard to hold my breath while refilling as my body had noticed the lack of oxygen and was urging me to breathe fast.
If I managed to pass out underwater I would have been dead, so maybe with a larger balloon and different helium it would work better, but I'd be hesitant to do it with just the gas after this experience.