Two separate problems show up with tap water in humidifiers, and they have different causes and different solutions.

Understanding which problem you’re actually dealing with makes the water choice much simpler.

Problem One: White Mineral Dust (Ultrasonic Humidifiers)

Ultrasonic humidifiers use a vibrating plate to create a fine mist of actual water droplets. This is different from evaporative humidifiers, which blow air through a wet wick. Ultrasonic mist contains real water, and real water from the tap contains dissolved minerals.

When those tiny droplets land on your furniture, electronics, floors, and walls, the water evaporates and leaves behind the minerals it was carrying. Calcium carbonate is the primary offender in hard water areas. The result is the white powder you find on surfaces near the unit.

The EPA published consumer guidance specifically on this issue and recommends distilled or demineralized water in ultrasonic humidifiers to prevent it.

Distilled water solves this because distillation removes dissolved solids to near-zero TDS. No minerals in the water means no minerals deposited on surfaces.

Problem Two: Bacterial and Mold Growth

Standing water in a humidifier reservoir is an ideal environment for bacteria and mold. Warm, still water. All the more so in a warm-mist unit, where the surrounding temperature is elevated.

This problem is not primarily about water mineral content. It’s about cleaning frequency.

Bacteria grow in distilled water just as they do in tap water, just slightly more slowly because dissolved minerals support biofilm formation. Neither distilled nor filtered water eliminates the need to clean your humidifier on a regular schedule.

The EPA recommends draining the reservoir daily, drying the tank after draining, and cleaning with diluted hydrogen peroxide or white vinegar at least once a week. Some sources recommend twice weekly during heavy-use months.

Legionella bacteria have been documented in humidifier outbreaks, though these are rare in properly maintained household units. The risk is highest in units left running with stagnant water for extended periods.

What Filtered Water Does

RO-filtered water has a TDS of roughly 10-40 mg/L compared to 200-500 mg/L in typical tap water. In an ultrasonic humidifier, this produces significantly less white dust than tap water. You’ll still see some, but far less.

Pitcher-filtered water (Brita, PUR, ZeroWater) is a different story. Standard carbon pitcher filters address taste, chlorine, and some heavy metals. They don’t meaningfully reduce dissolved minerals or hardness. The TDS coming out of a Brita pitcher is nearly identical to the TDS going in. It won’t reduce white dust from an ultrasonic humidifier.

ZeroWater’s five-stage filter does remove dissolved solids (it comes with a TDS meter for a reason). ZeroWater output approaches near-zero TDS in practice, which makes it a functional substitute for distilled water in humidifiers.

What no filtered water eliminates: the need for cleaning. Bacteria growth is a maintenance issue, not a water quality issue.

Why Distilled Is the Standard Recommendation

Distilled water is essentially mineral-free. The distillation process boils water, captures the steam, and condenses it. Dissolved solids, minerals, and most contaminants are left behind. Output TDS is typically under 5 mg/L.

No minerals means no white dust. Fewer minerals means less biofilm support. It’s also fully compatible with any humidifier manufacturer’s guidance without voiding warranties, which some manufacturers tie to water type.

Getting distilled water for a humidifier is straightforward. Grocery stores carry 1-gallon jugs for $1-2. If you run a humidifier heavily (a whole-home unit, for example), a countertop distiller costing $100-200 produces distilled water at $0.05-0.10 per gallon.

CPAP Humidifier Chambers

The same logic applies to CPAP heated humidifier chambers. Distilled is the universal recommendation from CPAP manufacturers, sleep clinics, and ResMed/Philips documentation. Scale in a CPAP chamber damages the heating element and creates a cleaning problem in the water path that feeds your airway all night.

If you’re using a CPAP, use distilled water in the chamber. RO water is an acceptable substitute if you already have an RO system. Tap water accelerates scale and requires more frequent chamber replacement.

The Practical Bottom Line

For any ultrasonic humidifier, use distilled water. The white dust problem is real, the EPA guidance is clear, and distilled water is inexpensive.

For warm-mist humidifiers in soft water areas, tap water is workable if you clean consistently. In hard water areas, distilled or RO water reduces mineral buildup on the heating element and keeps the unit working better for longer.

Regardless of what water you use, clean the reservoir and tank weekly. No water choice substitutes for maintenance.

For more on hard water and how mineral content affects household appliances, the hard water contaminant guide covers TDS, hardness testing, and treatment options including water softeners and salt-free conditioners.