Humidity refers to the amount of water vapor in the air, and it plays a role in both the development and treatment of allergies. Allergic rhinitis, for example, often manifests as nasal congestion, irritation, and inflammation of the delicate, moist tissues of the nasal mucosa. Some symptoms, such as dry mouth, may result from treatment with various drugs for symptomatic control of allergies. Breathing higher-humidity air is one way to relieve the discomfort and unpleasant symptoms of allergies and drug treatment side effects. 

On the other hand, one extremely common allergenic culprit, house dust mites, can only thrive at humidity levels that exceed a certain minimum. For this reason, maintaining a certain ideal humidity level in one’s home and work environment may be worth the trouble for people suffering from perennial allergies and allergy-induced asthma.

It’s a bit of a catch-22: lower humidity levels help ensure that dust mites and mold cannot thrive, and is linked to reduced indoor air pollution, but higher humidity is far more comfortable for the tissues of the throat and nasal passages. The trick is to strike a Goldilocks-like balance—maintaining indoor air that is neither too damp, nor too dry.

Modern homes and office buildings tend to be tightly sealed spaces with indoor air that is heated in winter and cooled in summer. Humidity is removed from warm, summer air, which tends to be very high in relative humidity. But in winter, furnaces may dry already dry air.

Unless a humidification system is incorporated into your home heating and ventilation system, indoor air in winter is likely to be dry enough to cause minor discomfort, such as dry, itchy skin and scratchy, dry nasal passages.

Understanding Humidity

Humidity (commonly called relative humidity) is a measure of the amount of water vapor present in air. While it can theoretically range from zero to more than 100 percent, the relative humidity in indoor environments usually ranges from about 20 percent to 70 percent.

Personal comfort is partially dependent on humidity, since air of a particular temperature feels warmer when humidity is higher. Conversely, air at the same temperature feels cooler if humidity is low. For this reason, avoiding desert-dry air conditions (30 percent humidity or lower) in winter enables you to feel warmer at a given temperature.

House dust mites, near-invisible creatures that “recycle” dead skin cells, live wherever humans live. Technically, it’s the enzymes in their waste that become airborne and are eventually inhaled, provoking allergies.

These arthropods thrive in higher temperatures (75 to 80 degrees Fahrenheit) and high humidity in the 70 to 80 percent range. While it’s virtually impossible to eliminate house dust mites and their allergenic droppings, it is possible to control their numbers by controlling humidity levels. The mites cannot thrive at humidity levels below 50 percent.

Modern humidifiers typically feature a built-in hygrometer, or humidity-measuring device. Simply choose a setting between 40 and 48 percent humidity and you will strike a balance between dust mite control and comfortable, reasonably moist air your skin will love.

Types of Humidifiers

Assuming you do not have a whole-house humidifier, your best option is a point-of-use humidifier. These household appliances are usually portable and capable of humidifying the air in one to several rooms. They feature a water tank, which must be refilled regularly. 

The most common type is an evaporative humidifier. Also called a “cool mist” humidifier, this type of appliance uses a reservoir, wick, and filter to disperse room temperature water vapor into the air. As noted above, some are programmable and feature an integrated hygrometer to maintain an optimal humidity level. Filters must be changed regularly to avoid mold overgrowth and saturation with minerals from tap water.

Other types of home humidifier include vaporizers (which produce steam), impeller humidifiers, and ultrasonic humidifiers. Because they do not use filters, impeller and ultrasonic models may deposit minerals as a fine dust on adjacent furniture, and thus may not be the best choice. Vaporizers, while effective, use considerably more energy than simple evaporative humidifiers.