What does a humidifier do, and do I need one in Florida?

What a humidifier does, and why people run one

The EPA's consumer factsheet, Use and Care of Home Humidifiers, says humidifiers are commonly used in homes to relieve the physical discomforts of dry nose, throat, lips, and skin. The moisture they add also eases some problems that winter heating causes, such as static electricity, peeling wallpaper, and cracks in paint and furniture. The same factsheet warns that the extra moisture can cause its own problems. Excess moisture can encourage the growth of biological organisms in the home. These organisms include dust mites, which are microscopic animals that produce materials causing allergic reactions to household dust, and molds.

So the benefit and the risk come from the same thing, which is water in the air. A humidifier helps only when the air is actually dry. When the air is already moist, it adds to the very problems the EPA lists. The target number that separates the two cases is covered in the guide to good indoor humidity, and the ceiling section below gives the EPA's own limit.

Why most Tampa Bay homes do not need one

The National Weather Service (NWS) office for Tampa Bay says that if you want to know how dry or humid it will feel outside, you should look at the dew point instead of the relative humidity. The dew point is the temperature at which water vapor turns into liquid, so a higher dew point means more moisture in the air and a muggier feel.

Relative humidity can mislead for a simple reason: it is measured against how much moisture the air could hold at its current temperature. Warm air holds more, so the same amount of water reads as a lower percentage. The NWS gives a pair of examples. Air at 50 degrees with a dew point of 50 degrees reads 100 percent relative humidity and feels fine, while air at 90 degrees with a dew point of 69 degrees reads about 50 percent and feels far more humid.

For Tampa Bay the picture is mostly moist air. The same NWS page describes summer days in West Central Florida when dew points are above 80 degrees. It also says the region has cool winter days with dew points below 30 degrees. That second part matters for whether a humidifier is needed at all.

Our reading, which is not a rule from either agency: in most months the outdoor air already carries plenty of moisture, so a humidifier adds water to air that does not need it. The exception is a dry winter spell, when indoor air can drop low enough to cause the discomforts the EPA lists. Check the indoor reading before you buy, and do not assume the house needs more moisture because the season feels dry.

The indoor ceiling: at or below 50 percent

The EPA's humidifier factsheet gives a clear ceiling. It reads: "Do not humidify to indoor relative humidity levels exceeding 50 percent." Relative humidity is the amount of moisture in the air compared with the most it could hold at that temperature, written as a percent.

The EPA's mold guide, A Brief Guide to Mold, Moisture, and Your Home, gives a slightly wider band for the whole house. It says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent. The two documents agree on the number that matters for a humidifier. Stay at or below 50 percent, and treat anything above 60 percent as a problem to fix.

Measuring it is the only reliable way to know. The factsheet notes that hygrometers, which are small meters that read humidity, are available at local hardware stores and may be used to measure humidity levels. Some humidifiers contain a humidistat, which is the control that switches the unit on and off at a set moisture level. Set that control so the reading stays at or below 50 percent, and do not rely on the setting alone.

The factsheet also gives two warning signs. If water condenses on windows, walls, or pictures, relocate the humidifier, lower its setting, or reduce its use. Do not let the area around the unit become damp. The mold guide adds that condensation on windows, walls or pipes is a sign of high humidity, and that the surface should be dried quickly.

When the house is too humid rather than too dry, the fix runs the other way. The pages on house humidity with the AC running and whole-home dehumidifiers in Florida cover that case.

Which types spread the most into the air, and how to clean them

The EPA groups humidifiers by how they make mist, because the type decides how much gets pushed into the room. The factsheet says the two types that generally appear to produce the greatest dispersions of both microorganisms and minerals are ultrasonic and impeller. Ultrasonic units create a cool mist with sound vibrations. Impeller units, also called cool mist, use a high-speed rotating disk. Evaporative units, which blow air through a wet absorbent pad, and steam vaporizers, which heat water, are said to disperse less, if any, of these pollutants.

Any type with a tank of standing water can grow microorganisms. The EPA's cleaning schedule for portable units is specific. Empty the tank, wipe all surfaces dry, and refill the water every day to reduce growth. Clean portable units every third day with a brush or scrubber to remove scale, deposits, or film. Dry all parts before storing the unit at the end of the season.

Water matters as much as the schedule. The factsheet says ultrasonic and impeller units are very efficient at spreading the minerals in tap water into the air, and that minerals build up as scale, which can hold microorganisms. The EPA's first choice is bottled water labeled distilled, because distillation removes minerals most effectively. Bottled water labeled spring, artesian or mineral has not been treated to remove minerals, so it is the wrong choice.

If you use a cleaning agent, the factsheet says to rinse the tank thoroughly with several changes of tap water so chemicals do not disperse into the air. Where the manufacturer gives no cleaning recommendation, it suggests a 3 percent hydrogen peroxide solution for surfaces that touch water.

Stop using the humidifier and contact your physician if you have respiratory symptoms you believe are linked to using it, even when you follow the cleaning directions. The factsheet says the young, the elderly, and people with lung diseases or respiratory allergies may be particularly susceptible to certain airborne pollutants.

Whole-house humidifiers on the furnace

Some humidifiers are built into the furnace or air handler and humidify the whole house. The EPA's factsheet calls these central humidifiers and says they are built into heating and air conditioning systems. Console units, which are cabinets designed for floor use, and portable units are the other two groups.

Central units follow the same water and cleaning principles, but the factsheet tells owners to follow the manufacturer's directions for console and central units, and not to let water stand in the tank for long periods. It also notes that some consumers see a white dust on surfaces when tap water is used. That is the same mineral problem described above, showing up in the room.

Our reading: a whole-house unit ties into the furnace or air handler, usually through a water supply line and a control wire, and it has to be matched to the home. Those connections, and any work inside the unit, are licensed-contractor work, not a homeowner job. Ask the contractor to confirm the unit's setting against a hygrometer reading before you rely on it. The EPA ceiling of 50 percent applies to a whole-house unit as much as to a portable one.