Perennial allergies (or year-round allergies, as opposed to seasonal allergies) are caused by reactions to indoor particles like dust mite droppings, mold spores, and pet dander.
Mold and dust mites are two of the most pervasive indoor allergens; they’re everywhere! Luckily for us allergy sufferers, these two organisms share a weakness: they cannot survive in low-humidity environments.
You can prevent the growth of mold and dust mites by keeping your relative humidity below 50 percent. Dehumidifiers lower indoor humidity by removing extra water vapor from the air. Most dehumidifiers also contain a built-in hygrometer that displays the relative humidity.
Extremely dry air, on the other hand, can cause problems for allergy sufferers as well. Dry air irritates the skin and nasal mucosa, exacerbating conditions like eczema and sinusitis. For this reason, many people use humidifiers to moisten dry air, especially during the winter since artificial heat decreases the amount of water vapor in the air.
As an allergy sufferer, I try to keep my relative humidity at 45 percent year-round; that makes the air too dry for dust mites and mold, but not dry enough to irritate my sinuses. (During the coldest months of winter, sometimes I increase the humidity a bit to feel more comfortable, but I never raise the relative humidity above 50 percent.)
Humidity control will also lower your power bills. Humid air holds onto heat more efficiently and feels warmer. During the summer, dehumidifying the air will make it feel cooler; during the winter, humidifying the air will make it feel warmer.