Health concerns about humidifiers have led several manufacturers to refine older technologies to try to reduce the bacteria and mineral dust they discharge.
The newest design, known as a "wicking" humidifier, updates a process in use for at least 20 years: the evaporation of water to produce a vapor. "Warm mist" humidifiers are being refined, too; they first boil water and then mix the steam with cool air to create a mist.
In recent years, Government studies have found that harmful bacteria and molds multiply in the water tanks of humidifiers and are spewed into the air. The studies have raised the possibility that some humidifiers also emit toxic substances like lead and asbestos if they are present in the water that goes into the tank.
Ultrasonic humidifiers, whose popularity peaked in 1987, became a particular focus of public-health concerns, and their sales have suffered. The newer humidifiers represent efforts by manufacturers to restore consumer confidence.
"I expect that the warm-mist market will grow, and the ultrasonic market will diminish," said Brett Gannon, the vice president of finance at Bionaire, a company in Lachine, Quebec, that makes humidifiers and other appliances. Mr. Gannon said Bionaire expects sales of its warm-mist units to surpass those of its ultrasonic models this year. Sound, Mist and Heat
There are three kinds of humidifiers besides evaporative and warm-mist units: ultrasonic, impeller or cool-mist, and steam. Ultrasonic devices use high-frequency sound to break up water into mist. Cool-mist units do the same thing mechanically. And steam units, as their name implies, make steam by boiling water.
A study completed in 1989 at the Oak Ridge (Tenn.) National Laboratories found that ultrasonic and cool-mist units released the most bacteria, while warm-mist units released insignificant amounts. The study also found that cool-mist units released significant levels of mold.
Some scientists maintain that a strain of bacteria that can be released by humidifiers can cause a respiratory disease similar to pneumonia and that molds from humidifiers can produce allergic reactions.
The new wicking humidifiers are supposed to reduce bacteria, molds and mineral dust, their makers say. In older evaporative models, a belt of sponges passes through water; air is blown across the sponges to dry them and release water vapor. In the newer models, water is drawn up through a paper wick that is supposed to trap bacteria and other contaminants. This wick can be replaced.
"The big problem with the older units was that they were difficult to clean," said Michael Berger, a national sales manager for the Bemis Manufacturing Company in Sheboygan Falls, Wis. Bemis sells wicking humidifiers, as do Sears and the Emerson Electric Company.
The suggested retail prices for Bemis table-top units are $39.95 to $59.95, with larger console-size units costing $79 to $119, Mr. Berger said. Gordon Jones, a Sears spokeman, said console models retail for $89 to $149. Bionaire said its warm-mist humidifers retail for $100 to $165.
Eva Lehman, a toxicologist at the Federal Consumer Product Safety Commission, said her agency had not studied wicking humidifiers. But she said that evaporative units generally discharge insignificant levels of bacteria. "They emit a fine mist which does not seem to pick up a lot of microbials," she said. Cutting Mineral Dust
Mineral dust is not believed to reach dangerous levels in vapor from steam, evaporative or warm-mist humidifiers, several health experts said. To reduce mineral-dust emissions from ultrasonic humidifiers, several makers sell demineralization cartridges with their units or offer them as accessories.
There is still some scientific debate over the medical value of humidifiers. But regardless of a humidifier's design, all health authorities urge that it be kept scrupulously clean. And those who maintain that humidifiers can be of value emphasize that they must be properly maintained.
"If there is mold growing in a reservoir, an asthmatic will do himself more harm than good," said Dr. Norman H. Edelman, a professor of pulmonary medicine at the Robert Wood Johnson Medical School in New Brunswick, N.J.
In winter, the relative humidity in a home should be kept at 30 to 50 percent. A level of about 60 percent will lead to indoor condensation and contribute to the growth of mold and bacteria. Hygrometers, which measure indoor humidity, can be bought at many hardware stores. 1. SCRUB IT 2. SCRUB IT
Don't use a humidifier unless you are prepared to take care of it. To reduce the chance of health hazards, the Federal Consumer Product Safety Commission recommends some precautions.
Change the water daily if possible, and clean the water tank vigorously, following the manufacturer's instructions. Consumer Reports recommends using a cleaning solution of an ounce of chlorine bleach to a pint of water; rinse thoroughly with fresh water.
Use distilled or demineralized water in the humidifier to reduce deposits and the release of mineral dust. Use demineralization cartridges or filters if supplied or recommended for your unit.
Drain and clean the tank before storing a humidifier, and dust the unit when taking it out of storage.
Graph: "Shifting Sales," tracks sales of ultrasonic and other types of humidifiers, 1985-1989 (Source: Appliance magazine) Diagram: How a 'Wicking' humidifier works.
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