Ok, have been searching again, have found this..
http://oacu.od.nih.gov/ARAC/EuthCO2.pdf
which seems to be the 'proper'way,
It mentions about stress in too quick exposure to C02, and has to be levelled slowly so that they pass out before getting stressed...
Pinkies up to 10 days are immune to C02?? or similar gasses....have a look!