It depends on the species, but for a cockatoo (U2) I have read as close to 60 as possible, without going over (mold happens between 60-65).
I use a Safety First humidifier that I got at Walmart in the baby aisle a long time ago--It has a ceramic cylinder that heats and I have used it for a long time without any issues, even though, at the time, I didn't call to verify that it was Teflon free---bad bad bad me-- seriously...(you might still call and check, since it does matter). It goes through a lot of water when running on the highest setting, but it shuts off when it runs low, so that is nice. The water is also never hot and the heating implement shuts off the second the water level gets low (before the hot parts are ever exposed)...assuming it never fails. It is meant for babies, so the lawsuit potential would be pretty huge if it didn't. My bird has had a thousand tests run etc and she has perfectly healthy lungs and organs lol (just saying, I should have called--if you don't and I am wrong, it will kill me)...I researched the materials at the time and I was confident enough to proceed, but again, please call, as things can and do change.
It has no filter and so you need to use distilled water in order to prevent minerals/chemicals from going airborne. There are some people who say you can leave tap water out for a period of time and slowly pour from the top (allowing minerals to settle), but I don't feel comfortable doing that without a filter.
I wash it every 3 days or so with f10 sc and then rinse it out before refilling. I probably should clean it more, but I try to keep it pretty sterile in between fillings (and F10 SC is a very solid cleaner).