How to diffuse essential oils with a humidifier?

Some humidifiers can diffuse essential oils. There are two ways to do this:

• Add the oils directly into the water, at the bottom of the tank: the device must have a polypropylene base. In 99% of cases, you should not put oils directly into the water because the base of your humidifier is made of ABS plastic. The oils risk eating away at the ABS, which would cause a leak. You can put oils directly into the base of your humidifier if it is made of polypropylene. This information is usually given in the explanatory note.
• Soak the cotton with oil and place it in the compartment provided for this purpose: a PP receptacle placed in the air flow of the fan. Diffusion is done by air. The essential oil diffuses in the air flow. In all these cases, never pour oil into the tank!