Distilled vs Tap Water in a Humidifier: When It Matters and When It Doesn't
Distilled vs tap water in a humidifier: what EPA says about white dust, which humidifier types need distilled water, USGS hardness levels and other water types.
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Use distilled water in an ultrasonic or impeller humidifier, and tap water is fine in most evaporative and steam units. EPA says ultrasonic and impeller humidifiers are very efficient at dispersing the minerals in tap water into the air, where they can settle as white dust, and that steam vaporizer and evaporative humidifiers are not expected to disperse substantial amounts of minerals. EPA recommends bottled water labeled distilled, because distillation is the most effective way to remove minerals.
Your tap water's hardness decides how much this matters. USGS classes water above 120 mg/L of calcium carbonate as hard and above 180 as very hard. If yours is soft, white dust may never be a problem; if it is hard, an evaporative humidifier such as the Vornado EVDC500 saves buying distilled water all winter.
Distilled vs tap water at a glance
| Distilled water | Tap water | |
|---|---|---|
| Minerals | Distillation is the most effective removal method (EPA) | Varies by area; hard water is high in calcium and magnesium (USGS) |
| White dust from ultrasonic units | Avoided | Likely with hard water (EPA) |
| Scale in the tank | Much less | Builds up; EPA says scale can harbor microorganisms |
| Evaporative wick life | Longer | Shorter in hard water (AIRCARE) |
| Cost and effort | Buying and carrying jugs | None |
Difference 1: white dust
Ultrasonic and impeller units turn water directly into droplets, minerals and all. EPA says they are very efficient at dispersing those minerals, that some people are bothered by the white dust that appears on surfaces, and that the effect of breathing the particles is not fully understood. MedlinePlus says minerals from tap water can be released into the air as white dust that can affect breathing, and recommends distilled water. Because the dust is made of fine particles, it can also register on a particle monitor; see PM2.5 explained.
Evaporative units leave minerals in the wick, and steam units leave them in the heating chamber, which is why EPA does not expect either to disperse substantial amounts.
Difference 2: scale and microbes
Hard water leaves scale on tanks, ultrasonic plates and heating elements. EPA notes that scale can be a breeding ground for microorganisms. Distilled water reduces scale, but it does not replace cleaning: EPA still advises emptying and refilling daily and cleaning every third day. See humidifier mold prevention.
Difference 3: wick life in evaporative units
Tap water is workable in an evaporative humidifier, but the minerals have to go somewhere. AIRCARE's manual explains that as the wick traps minerals its ability to absorb and evaporate water decreases, and that in hard water areas more frequent replacement may be necessary. Honeywell and AIRCARE both give 30 to 60 days as the normal wick interval.
How hard is your water?
| Class | Hardness | Our suggestion for ultrasonic units |
|---|---|---|
| Soft | 0 to 60 mg/L | Tap water may be acceptable; watch for dust |
| Moderately hard | 61 to 120 mg/L | Distilled if you see dust |
| Hard | 121 to 180 mg/L | Distilled |
| Very hard | Above 180 mg/L | Distilled, or choose an evaporative unit |
USGS notes that hard water was prevalent in the east-central and western United States in its national survey, but its map is regional; check your utility's water quality report or test your own water.
Other water types
- Spring, artesian or mineral water: EPA says these have not been treated to remove minerals. They are no better than tap water for this purpose.
- Purified or reverse osmosis water: Levoit's manual recommends purified or distilled water. Water labeled distilled is EPA's stated choice.
- Demineralization cartridges: EPA says they can be used if supplied or recommended for your unit, but their effectiveness may vary widely.
- Hot water: do not use it. AIRCARE and Levoit both warn against filling with hot water.
Which humidifier needs which water
| Humidifier | Type | Water |
|---|---|---|
| Vornado EVDC500 | Evaporative | Tap is fine; wicks wear faster with hard water |
| Honeywell HCM-350 | Evaporative | Tap is fine |
| AIRCARE MA1201 | Evaporative console | Tap is fine |
| Levoit Classic 200 | Ultrasonic | Distilled or purified (Levoit) |
| Pure Enrichment MistAire | Ultrasonic | Distilled if water is hard |
Who should use which
- Distilled in any ultrasonic or impeller humidifier with moderately hard or harder water, and in any unit near electronics or dark furniture.
- Tap in evaporative and steam humidifiers, accepting more frequent wick changes or descaling.
- Switch to evaporative if you have very hard water and do not want to carry jugs all winter. See ultrasonic vs evaporative and the best humidifiers.
Frequently asked questions
Do I have to use distilled water in my humidifier?
In an ultrasonic or impeller unit, EPA recommends it to avoid white mineral dust. In evaporative and steam units, tap water works because minerals stay in the wick or heater.
What happens if I use tap water in an ultrasonic humidifier?
EPA says ultrasonic units are very efficient at dispersing tap-water minerals into the air, where they can settle as white dust. Scale also builds up faster.
Can I use bottled spring water in a humidifier?
EPA says spring, artesian and mineral water have not been treated to remove minerals, so they behave like tap water. Water labeled distilled is EPA's recommendation.
Do humidifier demineralization cartridges work?
EPA says they can be used if supplied or recommended for your unit, but their effectiveness may vary widely.
Is white dust from a humidifier harmful?
EPA says the effects of breathing it are not fully understood, and MedlinePlus says it can affect breathing. Using distilled water or an evaporative humidifier avoids it.
How do I know if my water is hard?
Check your water utility's quality report or test it. USGS classes 121 to 180 mg/L as calcium carbonate as hard and above 180 as very hard.
Sources
- Use and Care of Home Humidifiers, US Environmental Protection Agency
- Hardness of Water, US Geological Survey
- Humidifiers and health, MedlinePlus, US National Library of Medicine
- Console Evaporative Humidifier Model MA1201: Use and Care Guide, AIRCARE (Essick Air)
- Classic 200 Ultrasonic Cool Mist Humidifier user manual, Levoit
- Vaporizer or Humidifier: Which Is Best?, Children's Hospital of Philadelphia
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