Purified water is water that has been processed to remove chemicals, contaminants, and dissolved solids down to extremely low levels. In the United States, the FDA recognizes “purified water” as a specific labeling category for bottled water, covering water treated by distillation, deionization, or reverse osmosis. The key distinction from regular tap water isn’t that tap water is unsafe, but that purified water has undergone additional processing to strip out nearly everything except the water molecules themselves.
How Purified Water Is Defined
The term “purified” isn’t just marketing. The FDA regulates bottled water labeling and groups purified water alongside distilled, demineralized, deionized, and reverse osmosis water as recognized product types. Producers must follow Current Good Manufacturing Practices that require sanitary processing, source water protection, quality control testing, and contaminant sampling of both the source and final product.
For context, the EPA sets a secondary standard of 500 milligrams per liter for total dissolved solids (TDS) in public drinking water. Purified water typically falls far below that threshold. In pharmaceutical settings, the United States Pharmacopeia sets even stricter requirements for purified water used in drug manufacturing, mandating specific limits on conductivity and total organic carbon to ensure exceptionally low levels of dissolved substances.
Three Main Purification Methods
Reverse Osmosis
Reverse osmosis forces water through a semi-permeable membrane under pressure. The membrane allows water molecules through while blocking dissolved salts, metals, bacteria, and other contaminants. The process produces two streams: treated water (called permeate) and a concentrated reject stream containing the filtered-out substances. This is the most common method used in both home filtration systems and bottled water production.
Distillation
Distillation works by boiling water and collecting the steam. When water evaporates, it leaves behind anything with a higher boiling point: minerals, heavy metals, salts, and most bacteria. The steam is then cooled and condensed back into liquid form. This collected liquid, called distillate, is extremely pure. Distillation typically removes around 99.9% of all minerals found in tap water, making it the gold standard for medical and laboratory settings.
Deionization
Deionization uses specially designed resin beads to swap dissolved minerals for hydrogen and hydroxide ions. Water passes through two types of resin. One captures positively charged minerals like calcium, magnesium, sodium, and iron, replacing them with hydrogen. The other captures negatively charged substances like chloride, nitrate, and sulfate, replacing them with hydroxide. The hydrogen and hydroxide then combine to form pure water. The result is water with very low conductivity, meaning almost no dissolved ions remain.
What Purification Removes
Purification targets a broad range of substances: bacteria, algae, fungi, parasites, metals like copper and lead, and chemical pollutants. Depending on the method, it also strips out beneficial minerals like calcium and magnesium along with the harmful contaminants. Some purification systems also remove fluoride, which is worth knowing if you rely on fluoridated water for dental health.
In most Western countries, public drinking water already goes through purification processes to meet safety standards. The water coming out of your tap has been treated. Bottled purified water simply takes that treatment several steps further, reducing dissolved solids to minimal levels.
The Mineral Question
Stripping water of all minerals raises a legitimate health consideration. Calcium, magnesium, sodium, and potassium in drinking water contribute to bone structure, electrolyte balance, metabolic function, and hormone regulation. While drinking water isn’t your primary source of these minerals (food is), the supplemental intake from water may still matter.
A report in the Medical Journal, Armed Forces India reviewed evidence on demineralized water and found that long-term consumption of water low in calcium may be associated with higher fracture risk in children, certain neurodegenerative diseases, and complications during pregnancy. Low magnesium intake from water has been linked to higher cardiovascular risk, with an expert consensus group concluding that the protective association between hard water and lower heart disease risk is probably valid, with magnesium being the likely contributor.
This doesn’t mean purified water is dangerous. It means that if purified water is your only water source, you may want to ensure your diet compensates for the minerals your water no longer provides.
Why Purified Water Tastes Different
Minerals give water its flavor. Remove them, and the taste goes flat. Many bottled water brands address this by adding minerals back after purification. Common additions include calcium (often as calcium sulfate or calcium lactate), sodium (as sodium bicarbonate or sodium sulfate), potassium (as potassium bicarbonate or potassium chloride), and magnesium (as magnesium carbonate or magnesium sulfate). The goal is a pleasant, slightly mineral taste without reintroducing contaminants.
Magnesium can create an astringent or slightly bitter taste when present at higher concentrations, which is why manufacturers keep levels carefully controlled. Sodium at around 125 mg/L tends to hit a flavor sweet spot in otherwise mineral-free water. If you’ve noticed that different purified water brands taste noticeably different from each other, the mineral blend added back is why.
Purified vs. Distilled Water
Distilled water is a type of purified water, but not all purified water is distilled. The difference is the method. Distilled water specifically uses evaporation and condensation. Purified water can be produced by any method that brings dissolved solids below the required threshold, including reverse osmosis or deionization.
In practice, distilled water tends to be the purest option since the phase change from liquid to gas to liquid is extremely effective at separation. That’s why medical facilities and laboratories rely on it. For everyday drinking, purified water produced by reverse osmosis works just as well and costs less to produce.
Common Uses Beyond Drinking
Purified or distilled water is recommended for several household and medical devices. CPAP machines used for sleep apnea are a common example. These machines use heated humidifiers, and tap water introduces minerals that build up as scale inside the components, making them harder to clean and potentially shortening the machine’s life. More importantly, tap water can contain biological contaminants that, when aerosolized and inhaled overnight, could lead to respiratory infections.
The same logic applies to steam irons, humidifiers, and car batteries. Minerals in tap water create deposits anywhere water is heated or evaporated. Using purified or distilled water in these devices keeps them running longer and reduces maintenance. For aquariums, home brewing, and automotive cooling systems, purified water provides a clean baseline that you can then customize with the specific additives each application needs.
Is Purified Water Worth Buying?
If your tap water meets EPA standards and tastes fine to you, purified bottled water offers minimal health advantage for everyday drinking. Its real value shows up in specific situations: when your local water supply has known contamination issues, when you need mineral-free water for a medical device, or when you want a consistent, neutral-tasting water regardless of where you are. Home reverse osmosis systems offer a middle ground, giving you purified water on tap without the ongoing cost of buying bottles. Just keep in mind that any purification system needs regular filter or membrane replacement to stay effective.