Do not put hydrogen peroxide into a cool mist humidifier and then turn it on. These operate by very rapidly vibrating a metal ring submerged in liquid. This physically atomizes whatever liquid is in there. You don’t want to breathe hydrogen peroxide.

You’re supposed to wipe these with vinegar daily and then rinse them and put them back into service.

Always check what the manual says to do. If you don’t have it it’s almost certainly uploaded somewhere.

I wouldn’t want to breathe H2O2 vapor.

But H2O2, over time, dissolves into H2O + O2, so washing/rinsing with H2O2 is fine, just let it air out for a few days afterwards to be safe.

For thorough cleaning, is running it with H2O2 outside an option?

Disclaimer: I’m not a chemist. At least mot to you. I’m a random dork on the internet who could be trying to kill you for the fun of ut. Anything I say should be treated as such.

Cleaning the humidifier with H2O2 is relatively benign. It’ll probably take care of the mold/algae issue. I’d run it outside or in the garage as a precaution, but the risk is low as consumer grade H2O2 is very diluted.

I don’t know if the H2O2 is going to cause any problems, but if you have one of those ultrasonic type humidifiers and you do manage to dissolve the black goop, turning it on will just toss that goop onto all the surrounding surfaces.

Feel free to crosspost to !homeimprovement@lemmy.world for additional advice