Relative humidity of 30 to 50% is recommended for good health. This is challenging in extreme conditions such as dry heat or too much moisture. A humidifier is used to increase the level of humidity in the air and a dehumidifier reduces the humidity level of the air. A hygrometer can be used to measure the humidity of a particular area to decide whether a humidifier or dehumidifier is required.
Comparison chart
 | Dehumidifier | Humidifier |
|---|---|---|
| Purpose | To reduce the moisture content in the surrounding area. | To increase the moisture content in the surrounding area. |
| Usage | During warm/humid climate in either a single room or basement or the entire house. | During winter or when the air is cold and dry in either a single room or the entire house. |
| Application | Recommended to alleviate allergy by eliminating mold, dust mites and mildew from the air. | Suitable to moisten dry skin and nasal passages that dry up due to common cold. Humidifier works best in children’s room. |
| Humidity levels | Used where humidity is greater than 50% | Used where humidity is less than 35% |
| Types | Mechanical /refrigerative, Air conditioners, Adsorption/desiccant, Electronic, Ionic membrane, Makeshift | Warm mist and cool mist |
What is a Humidifier?
Dry, itchy skin and chapped lips are common in areas with low humidity. Humidity levels over 50% result in the growth of mold spores, bacteria, and dust mites. 40% humidity is required for musical instruments to prevent finishes from crazing or cracking.
In the following video, Dr. Ari Brown explains the uses of a humidifier in a baby’s room.
What is a Dehumidifier?
A place with higher temperature and high relative humidity needs moisture to be pulled out of air. Moist air helps the growth of mold, dust mites and spores which aggravate allergies and asthma. A dehumidifier removes moisture from air and is suited for this climate.
Humidifier or Dehumidifier?
The relative humidity of a particular place is the deciding factor in selecting either a humidifier for dehumidifier. A healthy relative humidity level is 35 to 50%. Based on the humidity levels of a room, dehumidifiers and humidifiers help obtain these levels.
Optmimum humidity level
The flu virus thrives best when humidity is either over 98% or between zero and 50%. Because of this, Dr. Linsey Marr, a researcher at Virginia Tech, recommends keeping humidity levels around 50% to give the flu virus the least chance of survival.[1]
Types
Types of Dehumidifiers
- Mechanical/Refrigerative: This most common type of dehumidifier works by using a small fan to pull moist air over a refrigerated coil. Condensation collects on the coil and drips into a collecting space. These dehumidifiers work best when air temperature is above 65 degrees Fahrenheit (about 19 degrees Celsius) and relative humidity is above 45%.
- Absorption/Dessicant: Dessicants are materials that absorb moisture easily from the air. In this type of dehumidifier, the dessicant is rotated through the area, usually on a belt, with one end of the system heated to draw the moisture out of the material. These dehumidifiers work well in areas with high humidity and low temperatures. This type can reduce humidity levels well under 35%, making them useful for industrial processes.
- Electronic: This type of dehumidifier uses a Peltier heat pump to create a cool surface where condensation can extract water vapor from the air. Although this type is quiet (it lacks moving parts), it also has a low efficiency rating. The electronic type is best for small appliances and restricted areas (under 300 cubic feet).
- Ionic membrane: At a molecular level, water molecules can be removed from the environment, whether liquid or gas. Specialized ionic membranes can help "pump" water molecules out of (dehumidifying) or into (humidifying) a sealed environment, although no water might be visibly produced. This type of dehumidifier is used for chemical engineering, fuel cells and water filtering.
- Makeshift: Air conditioners, both stand-up and window units, operate much like mechanical/refrigerative dehumidifiers, especially if they release the accumulated condensation outside the room while adding heat to it.
How to Choose a Dehumidifier
This video by Consumer Reports offers tips on how to select a dehumidifier.
Types of Humidifiers
- Warm Mist Humidifiers
- Steam humidifer: This common type simply boils water, releasing steam into the room. Substances such as medical inhalants can be added to the water, along with aromatic or antibacterial compounds.
- Cool Mist Humidifiers
- Wick/Evaporative humidifier: This type uses a wick, usually a type of cloth, that absorbs water from a storage container. A fan then blows over the surface area of the wick, evaporating the water. If the water is cold, it can also serve to lower ambient temperature more quickly, essentially a makeshift air conditioner.
- Impeller humidifier: Large area misters are most often of this type, creating a fine foggy mist by using a rotating disc to throw water onto a diffuser. The diffusing element smashes the water into tiny droplets and sprays them into the air.
- Ultrasound humidifier: These humidifiers use a diaphragm, membranous or solid, vibrating at ultrasonic range, to break down water into tiny droplets. A fan then blows the mist out as a fog. The ultrasonic vibrations are outside the range of human hearing, so this type of humidifier is essentially quiet.
How to Choose a Humidifier
Here is a buying guide for various types of humidifiers that outlines the symptoms, size and benefits:
Maintenance
Humidifiers and dehumidifiers require regular maintenance.
Maintaining a Humidifier
- Use distilled or demineralized water only in humidifiers. This cuts down on bacteria, molds and build-up in their reservoirs, tanks and filters.
- Clean the water tank and filter on a regular basis. Water should not be left in the water tank or reservoir for more than a day as this might foster bacteria or molds.
- After cleaning the water tank or reservoir, make sure to rinse it thoroughly to avoid releasing harmful chemicals.
- Keep the area near the humidifier dry, as this helps pinpoint possible leaks and cuts down on the potential for fostering bacteria or molds near the unit.
Maintaining a Dehumidifier
- Clean the coil and water tank (bucket or reservoir) on a regular basis. Do not let water stand in the tank for more than a day, as this can allow bacteria or molds to grow.
- In cold weather, check the coil for frost build-up. Frost reduces the efficiency of the dehumidifier.
Capacity Rating
The output capacity of a humidifier is measured as gallons of moisture per day. A unit with higher tank capacity needs less frequent refilling. The capacity of dehumidifiers is measured as number of pints of water removed in a 24-hour period.
The energy consumption of a humidifier is lesser than that of a dehumidifier. Irrespective of energy efficiency, both these appliances are used under mutually exclusive conditions depending on the requirements.
Cool-mist 
Air Purifier 
Humidifier 
Upright 
Hard Water 
Mildew
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