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During the winter, dry indoor air is often the cause of chapped lips, dry skin and irritated sinus passages. The moisture from a humidifier can soothe dry sinus passages. However, if you have indoor allergies, dust and mold from the humidifier may cause more harm than good.
The number one indoor allergen is the dust mite. Dust mites grow best where there is moisture. Moisturizing the air with a humidifier creates the perfect home for dust mites to live and prosper. Keep the humidity level in your house between 40-50%. You can monitor the levels with a hygrometer.
Mold spores can also be an issue for people with mold allergies. It is important to clean and change the filter in the humidifier on a regular basis so mold does not grow in the unit and blow into the home. Read the manufacturer instructions for tips on cleaning your humidifier.
If possible, use distilled or demineralized water in your humidifier. The higher level of minerals in tap water can increase bacteria growth, resulting in a white dust and additional irritation to your sinuses.
If you have indoor allergies, check with your allergist / immunologist to see if using a humidifier can help you and your sinuses survive winter.
Citation: https://www.aaaai.org/conditions-and-treatments/library/allergy-library/humidifiers-and-indoor-allergies