I just heard recently that they ban X-C skiing competitions in Finland at temperatures below -50C as, from their experience, the combination of temperature and excessive physical exertion begins then to hurt the body. It may be useful to keep this in mind as a reference.
Yah, but -50c is like... SO cold. Riding around at -30C does add a new level of bitter crispness to the air. I don't want to try at -50C, well at least for very long.