It is safe to add scented oils to a humidifier. However, this needs to be done properly so that the humidifier is not damaged. If the correct precautions are taken, there should be no problems to be concerned with.
Dilution
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When adding scented oil to a humidifier, always be sure to dilute it well. Add no more than three to nine drops of oil to the water chamber of the humidifier. Then fill the chamber with water.
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Using Cotton Balls
A safer way to use scented oils in a humidifier is by first placing drops of the oil on cotton balls. Drop the cotton into the medicine well of the humidifier. The steam will cause the scent to vaporize out into the air without having the oil come into direct contact with the inside parts of the humidifier.
Cleaning
It is not common, but the alcohol in the oils is strong enough to eat away at the plastic parts inside a humidifier. Always be sure to clean the humidifier thoroughly each time after using the essential oils. This will remove all possibly harmful residue from inside the humidifier.