Distilled water is a specific type of purified water, made exclusively by boiling and recondensing steam. Purified water is a broader category that includes any water treated to remove contaminants, whether through reverse osmosis, deionization, or other filtration methods. The practical differences between them come down to how they’re made, what’s left in the water, and which applications each one is best suited for.
How Each Type Is Produced
Distillation is one of the oldest purification methods. Water is boiled, the steam is collected in a separate container, and then cooled back into liquid form. Because most contaminants, minerals, and dissolved solids don’t evaporate with the water, they get left behind in the boiling chamber. The result is water stripped of more than 99.9% of its dissolved minerals.
Purified water can be made through several different processes. Reverse osmosis (RO) pushes water through a semi-permeable membrane under pressure, filtering out contaminants at a molecular level. Deionization uses specially charged resins to pull mineral ions out of water. Carbon filtration, UV treatment, and other methods can also be combined to reach purified standards. What matters for the “purified” label isn’t the method but the result: the water must meet a specific purity threshold, typically no more than 10 parts per million of total dissolved solids.
Distillation tends to be slower and more energy-intensive than reverse osmosis. An RO system passes water through a series of filters continuously, while a distiller has to heat water to boiling, wait for steam to rise and condense, then collect the output. This is why countertop distillers produce water at a much slower rate than an under-sink RO system.
What’s Actually in Each Type
Distilled water is about as close to pure H₂O as you can get outside a laboratory. The boiling process removes minerals like calcium, magnesium, sodium, and potassium, along with bacteria, viruses, and most chemical contaminants. It also strips out volatile organic compounds less effectively, since some of those can evaporate and recondense along with the steam, though many distillers include carbon filters to catch those.
Purified water from an RO system removes the vast majority of contaminants and minerals, but it may retain trace amounts depending on the membrane quality and filter condition. Many bottled water brands that sell “purified” water actually add minerals back in after filtration. This remineralization improves the taste (pure water tastes flat to most people) and restores small amounts of calcium and magnesium. So a bottle labeled “purified water” at the store often contains more mineral content than distilled water, by design.
Taste and Drinking Experience
Both distilled and purified water lack the flavor profile of tap or spring water, which gets its taste largely from dissolved minerals. Distilled water, being the most thoroughly stripped, tends to taste the most noticeably flat or “empty.” Some people describe it as slightly metallic or odd, though it’s really the absence of familiar mineral flavors that creates that impression. Purified water that has been remineralized tastes closer to what you’d expect from regular drinking water.
Health Considerations for Long-Term Drinking
Drinking distilled or highly purified water occasionally is perfectly safe. The concern is with long-term, exclusive use as your only water source. Water with very low mineral content can disrupt your body’s electrolyte balance over time. Research published in Environmental Research found that chronic ingestion of demineralized water can alter the composition of fluids inside and outside your cells, potentially lowering levels of sodium, potassium, magnesium, and calcium in the body. These shifts affect blood pressure, cell volume, and the normal electrical signaling your cells rely on.
Children are considered particularly vulnerable because they tend to drink more water relative to their body size and can easily over-hydrate beyond what they need. Over time, consistently low mineral intake from water could create metabolic conditions with downstream health effects. That said, most people get the majority of their essential minerals from food, not water. If your diet is reasonably balanced, occasional or moderate use of distilled or purified water is unlikely to cause problems. The risk is most relevant for people who rely on demineralized water as their primary source and also have limited dietary mineral intake.
Best Uses for Distilled Water
Distilled water is the standard choice for any application where mineral deposits cause problems. CPAP machines are probably the most common household example. Sleep Foundation notes that distilled water is the recommended option for CPAP humidifiers because even small amounts of minerals left in purified or spring water can build up as scale inside the humidifier chamber. That residue harbors bacteria, irritates airways, and shortens the life of the machine’s components. Purified water that has been remineralized, which includes most bottled drinking water, is specifically not recommended for this reason.
Other common uses for distilled water include:
- Steam irons and garment steamers: mineral-free water prevents white residue and clogged steam vents
- Car batteries and cooling systems: minerals in regular water corrode internal components
- Aquariums: distilled water gives fish keepers a blank slate to add precise mineral levels for specific species
- Nebulizers and other medical devices: mineral-free water prevents buildup and reduces infection risk
Best Uses for Purified Water
Purified water (especially RO water) is the more practical choice for everyday drinking, cooking, and making coffee or tea. It removes the chlorine taste and potential contaminants from tap water while still being easy to produce in volume. An under-sink RO system can supply several gallons a day with minimal effort, whereas a distiller might take four to six hours to produce a single gallon.
For laboratory and industrial work, both types serve different roles depending on the purity grade needed. Lab-grade water is classified into types: Type III water can be produced by distillation, reverse osmosis, or ion exchange, while Type I water (the highest purity) typically requires distillation or equivalent processing followed by additional polishing steps. In practice, modern labs often use RO as a first step and then further purify the water rather than relying on distillation alone.
Storage and Shelf Life
Distilled water is highly reactive to its environment precisely because it contains almost nothing. It readily absorbs gases, odors, and trace chemicals from whatever container it sits in and from the air once opened. An unopened jug stored in a cool, dark place lasts indefinitely for most practical purposes. Once opened, the timeline shortens significantly: about one week if you’re using it for medical devices like CPAP machines or nebulizers, two to four weeks for general household use, and several months for automotive or technical applications if stored with the cap on. For drinking, one week after opening is a reasonable guideline.
Purified water follows similar storage principles, though remineralized versions are slightly less aggressive about absorbing environmental contaminants since they’re not as chemically “hungry” as pure distilled water. Either way, keeping the container sealed and stored away from heat and direct sunlight gives you the longest usable life.
Which One Should You Buy
If you need water for a CPAP machine, iron, humidifier, or any device that warns against mineral buildup, buy distilled. It’s the only type that reliably prevents scaling and residue in those applications. If you’re looking for clean drinking water, purified water (whether from a home RO system or a bottle) is the better everyday option. It tastes better, costs less to produce in volume, and when remineralized, provides small amounts of minerals your body can use. You don’t need distilled water for drinking unless you specifically want the purest option and are getting plenty of minerals from your food.