I ve checked this site before...
From what I have read for formaldeyhde I concluded that all this stuff containing f/de is harmfull for those who use it on a daily basis, thus hairdressers are in great risk. The lady who makes the treatment once in 3 months, is within the limits of acceptable exposure.

Check wikipedia for formaldehyde, it states the exact limits for exposure in f/de, when in gaseous form.