Healthy Home Tips: The Ups and Downs of Humidification

If your child has a cold or asthma you may be tempted to operate a humidifier in his or her bedroom. But humidifiers can cause mold and indoor air quality problems.

There are four common types of portable humidifiers: ultrasonic, evaporative pad (cool mist), steam, and warm mist.

(image of a furnace humidifier with actinomycetes – (c) J. May 2008)

Ultrasonic units use ultrasonic sound energy to directly convert water into microscopic droplets. The process aerosolizes whatever is in the water, including minerals, bacteria and algae – thus distributing dust and allergens in the fog these units emit.

Evaporative pad humidifiers – those that contain a cellulose (paper mesh) pad sitting in a reservoir of water– are prone to biological contamination because the cellulose pad itself is a nutrient, and also because the pad acts as a filter, capturing house dust that contains biodegradable skin scales, lint and cornstarch from body powder.

In one family I worked with, a mother and her daughter had been suffering respiratory problems for most of that winter. The worse their symptoms became, the longer they operated their evaporative humidifier. I found high levels of actinomycetes (filamentous bacteria) in the indoor air and in the humidifier pad. The family stopped using the humidifier and gave the house a thorough cleaning. Within two weeks, the mother and daughter were nearly symptom-free. I sometimes even find Stachybotrys chartarum mold (the feared “toxic black mold”) growing on the paper pad in cool-mist humidifiers. These humidifiers are the most popular type, but because they are so prone to biological contamination, I believe that they should be banned from the marketplace.

The only portable humidifier I recommend is a warm-mist unit, which boils water and thus emits only water vapor. Minerals and other contaminants then remain behind in the water reservoir. Use only the kind of humidifier that has a humidistat, which shuts the unit off when the relative humidity in the room reaches a set point. (Old-fashioned steam humidifiers lack that kind of control and keep making steam until they run out of water, sometimes excessively elevating the relative humidity.) Lastly, place the warm-mist humidifier on a surface your children can’t reach, because when operating, the unit contains a small amount of boiling water.

Are central humidification systems preferable? In almost every furnace humidifier I’ve seen, the water tray was full of biological growth and its by-products: chemicals that mold and bacteria can create and excrete. When the water film breaks on the humidifier sponge, water droplets containing allergens are ejected from the rotating drum and become airborne. These droplets are then taken up by the air flowing through the humidifier and into the furnace supply system. When these droplets evaporate, the allergens that were within the droplets remain suspended in the airflow as particulates and move with the air through ducts into the habitable areas, where they can be inhaled. When the particulates settle out of the air onto surfaces, they can be re-aerosolized when surface dust is disturbed and again be inhaled. These allergen-containing particulates are small enough to be highly respirable (breathed deeply into the lung).
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I only recommend two types of central humidifiers: a steam-type, which boils water and injects steam into the supply airflow; or a trickle-type, which uses water flowing down a metal mesh pad and has no water reservoir to support biological growth. Any central humidifier should be checked monthly, to be sure there are no leaks and that water is not soaking into the fiberglass lining material inside the ducts or furnace, which can lead to mold growth within the system.

I do have a concern about all humidifiers, however: they introduce moisture into the air. This may feel good on your skin, but it can lead to elevated relative humidity. A number of molds commonly found indoors, including some of the most allergenic and even toxic, can flourish when the relative humidity is in excess of 75% to 80%. So whether you have central humidification or use a warm-mist portable humidifier, it’s important to measure the relative humidity with a hygrometer, available in many hardware and building supply stores. In the winter, reduce the amount of humidification if the relative humidity is in excess of 35% or if there is condensation on the walls (and the wall paper is peeling off!).

If your skin feels dry, use skin lotion – preferably one without fragrance, but that’s a topic for another article for Clever Parents.