THC is totally right. The heat from HID lights builds up really really fast. I have a big fan in my grow tent (4x4x8) and I have a 5" inline fan cooling my tube. Even though my tent is located in a room with a pretty low ambient temp, I have to always cool my tube so that my tent doesn't overheat.
I can vouch for this, my temps go from 21c (70f) to 26c (77f) in 2 minutes after switching the lights (600w HPS) on and this is with a an oscalating fan blowing on the plants and my extraction clearing the grow room every 30 secs