This is an excerpt from a media article posted here on FFN:
"three others treated for dehydration "
The article is about wildfire, but it could also apply to structural fires. Dehydration is serious, think of where it can lead to.
We must always bear in mind that prevention of dehydration is easy. It must be on our minds at all times at any incident, very much so at a wildfire. It's a subject that is hammered here all the time - with the need to be aware of our crew mates as well as ourselves. The need to keep up our fluid intake. We stllk have people who don't do that one simple thing - even with good crew leaders mentionig it frequently. So be aware of the problem and don't become a casualty.
I'm a trainer. One of the things I say to new members is that if you're at a wildfire and you're not urinating regularly then you are already dehydrate. If the RH is very low, you may not even realise that your perspiration level is low, so not urinating might be the only telltale sign.