Purified water is water that has been processed to remove chemicals, dissolved solids, and other contaminants down to very low levels. In the United States, the FDA allows bottled water to carry the “purified water” label only if it has been treated through distillation, deionization, reverse osmosis, or a similar process and meets the standards set by the United States Pharmacopeia. The result is water with far fewer minerals and impurities than ordinary tap or spring water.
How Purified Water Is Defined
The key measure is total dissolved solids, or TDS, which counts the minerals, salts, and metals dissolved in water. The EPA sets a secondary standard for drinking water at 500 milligrams per liter. Purified water goes well beyond that threshold. To qualify under the U.S. Pharmacopeia definition referenced by FDA labeling rules, purified water must have a TDS level of no more than 10 milligrams per liter, roughly 50 times lower than what comes out of most taps.
The source doesn’t matter much. Purified water can start as municipal tap water, well water, or any other supply. What earns it the label is the processing it undergoes afterward.
Common Purification Methods
Reverse Osmosis
Reverse osmosis is the most widely used method in both home systems and commercial bottling plants. Water is pushed under high pressure through a semipermeable membrane with pores small enough to let water molecules through while blocking dissolved salts, heavy metals like lead and mercury, fluoride, arsenic, and even microplastics. A typical home system runs water through a sediment prefilter, then the RO membrane itself, then a carbon post-filter before it reaches your glass.
Research at Duke University’s Nicholas School of the Environment found that under-sink reverse osmosis systems achieved near-complete removal of PFAS chemicals (the persistent “forever chemicals” found in many water supplies), reducing levels by 94% or more. That’s significantly better than standard activated carbon filters, which removed 73% on average and varied widely from unit to unit.
One tradeoff: residential RO systems produce waste. For every gallon of purified water, three to five gallons of concentrated brine get flushed down the drain. Industrial systems can cut that ratio to 1:1, but the equipment is more complex and expensive.
Distillation
Distillation takes a different approach. Water is heated until it turns to steam, leaving behind inorganic compounds, metals, nitrate, and other dissolved solids that can’t evaporate. The steam is then cooled and collected as liquid water. Because most contaminants don’t travel with steam, distilled water is extremely pure. The downside is that it also strips out dissolved oxygen and trace minerals, and the process is slow and energy-intensive compared to membrane filtration.
Deionization
Deionization removes dissolved salts through a chemical swap. Water passes through two types of resin beads. One type exchanges hydrogen ions for positively charged contaminants like calcium, magnesium, and sodium. The other exchanges hydroxide ions for negatively charged contaminants like chloride and sulfate. The hydrogen and hydroxide ions combine to form pure water. This process is especially common in laboratories and industrial settings where even trace minerals would interfere with equipment or experiments.
What Purification Actually Removes
The specific contaminants removed depend on the method, but in general, purification targets dissolved minerals and salts (sodium, calcium, magnesium), heavy metals (lead, mercury, arsenic), chemical contaminants (PFAS, pesticides, herbicides, volatile organic compounds), chlorine and chloramine used in municipal treatment, sediment and particulate matter, and microplastics. Distillation is particularly effective against inorganic compounds but can miss some volatile organic chemicals that evaporate at lower temperatures than water. Reverse osmosis handles a broader range in a single pass. Many commercial purification setups combine multiple methods for exactly this reason.
The pH of Purified Water
Pure water in theory has a neutral pH of 7.0, but in practice, purified water typically falls between 5.5 and 7.5. The reason is carbon dioxide. When purified water is exposed to air, it absorbs CO2, which forms a weak acid and nudges the pH downward. Because purified water has almost no dissolved minerals, it has very little buffering capacity, meaning even tiny amounts of CO2 can shift its pH noticeably. This is why a freshly opened bottle of purified water may test slightly acidic even though nothing is “wrong” with it.
Purified vs. Distilled vs. Spring Water
These labels often cause confusion because they overlap. Distilled water is a type of purified water. It meets the same low-TDS standard but is specifically made through distillation. Under FDA rules, if your purified water was produced by distillation, it can be labeled “distilled water” or “purified water,” your choice. The same goes for “deionized water” and “reverse osmosis water.” They’re all subsets of the purified category, distinguished by method.
Spring water is a completely different product. It comes from an underground source and flows naturally to the surface. It retains its natural mineral content, which varies depending on the geology of the spring. Spring water is filtered for safety, but it isn’t stripped of minerals the way purified water is. That mineral content is actually the selling point for many spring water brands, giving the water a distinct taste that purified water lacks.
Tap water in countries with modern treatment infrastructure goes through coagulation, sedimentation, sand and charcoal filtration, and chemical disinfection (usually chlorine). It’s safe to drink in most U.S. municipalities, but it contains significantly more dissolved minerals and residual treatment chemicals than purified water. You can taste the difference, and depending on your local water supply, the TDS gap can be substantial.
Potential Downsides
Purification removes beneficial minerals along with harmful ones. Calcium, magnesium, and potassium are all stripped out during the process. For most people eating a reasonably varied diet, this isn’t a nutritional concern because food provides the bulk of these minerals. But if your diet is limited or you rely heavily on water as a mineral source, purified water won’t contribute anything on that front.
The lack of minerals also makes purified water taste flat or “empty” to some people. Spring water and mineral water have a mouthfeel and flavor profile that comes directly from their dissolved mineral content, something purified water simply doesn’t have.
There’s also the environmental cost. Bottled purified water requires energy for the purification process itself, plastic for packaging, and fuel for transport. Home reverse osmosis systems avoid the plastic but still produce that 4:1 waste-to-pure-water ratio, which adds up on your water bill and in regions facing water scarcity.
Common Uses Beyond Drinking
Purified water shows up in places you might not expect. Medical facilities use it for sterilizing instruments and mixing medications because even small mineral traces can interfere with drug formulations. Laboratories rely on deionized or distilled water to prevent mineral contamination in experiments. CPAP machines and steam irons call for purified or distilled water because minerals in tap water leave crusty deposits over time. Car batteries and aquariums also benefit from water with predictable, near-zero mineral content.
If you’re using purified water at home, the most practical reason is usually taste preference or concern about specific contaminants in your local water supply. Checking your municipality’s annual water quality report can help you decide whether the investment in a home purification system is worth it for your situation.