Can I freely use the fire exit symbol and similar ISO symbols?

Apparently, Wikimedia Commons considers it freely usable for all purposes, i.e. effectively in the public domain.

Specifically, they base this on the wording on the Japanese Foundation for Promoting Personal Mobility and Ecological Transportation website (the “green man running through the door” sign was originally designed in Japan, by one Yukio Ota, before it became an international standard), which says (emphasis mine):

“The symbols presented in these guidelines can be freely used by anyone and can be accessed at the Web site (http//www.ecomo.or.jp) of the Eco-Mo Foundation. However, registering these symbols as your trademark or design can be a copyright infringement. If you have any questions, please contact us.”

To be safe, I would suggest basing your sign on the specific design available from the Eco-Mo Foundation website, just to avoid any possibility of anyone else claiming copyright, however tenuous, on any variant design. Also, as noted on the website, you should not pass off the sign as your own design nor use it in a trademark. Of course, if you want to be sure, you could always e-mail the Eco-Mo Foundation and ask.

Moved from comments below:
The same answer presumably applies to all the other symbols found on the Eco-Mo website. Beyond that, I’d be hesitant to generalize in the absence of evidence. I haven’t been able to find a blanket statement by, say, the ISO saying that all their standard symbols would be free to use (for any purpose, or at least their intended one), even if the fact that they are standardized symbols does strongly suggest it.

Furthermore, many standard symbols may have local laws (or even international treaties) regulating their use independently of copyright, which can further complicate things.
For example, the Red Cross symbol is specifically protected by the Geneva Conventions and implementing national legislation, despite it being essentially ineligible for copyright due to its simplicity, and the various Red Cross societies have been known to occasionally sue misusers of the symbol.