Jack was fully ready to bargain with John, threaten violence, anything to get him to release the Doctor. However, when the Doctor spoke, Jack paled. He was right, and Jack hadn't even stopped to consider the fact that there might have been radiation leaks through the hallways. Either that, or it was a clever ploy by a very clever man. In any case, he still looked absolutely horrified.

"John... John we need to go back to the TARDIS. You might have been exposed... and if you were, we need to get you treated right away. Please, don't be stupid, and just listen!"