Humidifiers are prone to hard water deposits at the water tank base and around the heating port. I tried to scrub this area with all sorts of materials and could never really get it clean, until I found that sprinkling citric acid works wonders to remove that build-up with minimal scrubbing.
Boil six to eight cups of water, depending on the size of your humidifier, and grab a box of powdered citric acid. Remove the water tank and pour citric acid into the base enough to cover the surface area, then pour in the boiling water. Add citric acid as needed to cover the affected area. Let it sit for five minutes, then scrape away the deposits. Repeat this process as many times as needed, but it shouldn’t take many.