My first thought was “why is nitrogen dangerous?” but I was thinking about it at room temperature or around 20C.

I know about decompression sickness (the bends) but I wouldn’t expect that to be a problem at 1 atmosphere. Then I stumbled upon isobaric counterdiffusion and I wondered if that could happen from pumping any pure gas into the rectum at atmospheric pressure, since it’d be at a higher partial pressure than any gas in the tissue.