It all depends on the type of humidifier. Cold air humidifiers work similar to diffusers and you may be able to add essential oils to it. However, essential oils can cause damage to your diffuser. The oils can collect in the humidifier and cause the plastic to break down. Some humidifiers include a spot for essential oils but always read the directions of your humidifier. If you really need to add essential oils to your diffuser, put a few drops on a cotton ball over the mister outlet.