Distilled water is water that has been purified by boiling it into steam and then condensing that steam back into liquid. This process removes up to 99.5% of impurities, including bacteria, heavy metals, dissolved minerals, and most chemical contaminants. What you’re left with is nearly pure H₂O, with virtually zero dissolved solids.
How Distillation Works
The process is straightforward. Contaminated or mineral-rich water is heated until it evaporates into steam. Because inorganic compounds, heavy metals like lead, and large organic molecules have higher boiling points than water, they stay behind in the boiling chamber. The steam rises, moves into a cooling system, and condenses back into liquid water in a separate, clean container.
This collected water typically measures around 0.5 parts per million (ppm) or less of total dissolved solids. For comparison, average unfiltered tap water sits around 100 ppm. That difference reflects just how thoroughly distillation strips everything from the water.
The heat involved also kills bacteria, viruses, and parasitic cysts, making distillation one of the most effective single-step purification methods available. There is one limitation worth knowing: volatile organic compounds like benzene and toluene have boiling points close to water’s, so they can evaporate along with the steam and end up in the final product. Most home distillers add a carbon filter to catch those compounds before condensation.
How It Differs From Purified and Spring Water
Distilled water is technically a type of purified water, but not all purified water is distilled. Purified water is a broad category that includes any water treated to meet a specific purity standard, whether through reverse osmosis, carbon filtration, deionization, or distillation. The method varies, but the goal is the same: remove contaminants to very low levels.
Spring water comes from an underground source and naturally rises to the surface. It retains its original mineral content, which is part of its appeal for drinking. Standard drinking water (tap water) comes from a municipal supply and is treated to be safe for consumption, often with added fluoride. It still contains dissolved minerals and trace amounts of treatment chemicals like chlorine.
The key distinction with distilled water is that it removes not just contaminants but also beneficial minerals like calcium and magnesium. That’s what makes it ideal for certain equipment and lab work, but it also raises questions about whether it’s the best choice for everyday drinking.
Is Distilled Water Safe to Drink?
Yes, distilled water is safe to drink. It won’t harm you in the short term. The real question is whether drinking it exclusively over months or years could affect your mineral intake.
Drinking water isn’t your primary source of calcium or magnesium. Most of your mineral intake comes from food. However, research published in the Medical Journal of the Armed Forces India found that even in countries with adequate diets, food alone may not fully compensate for the absence of calcium and especially magnesium in drinking water. The supplemental contribution from water, while small in percentage terms, appears to have health significance beyond what the numbers suggest.
An expert consensus group convened by the World Health Organization examined the long-term health effects of consuming demineralized water. Their conclusion: the hypothesis that harder water (water with more dissolved minerals) is associated with a lower risk of cardiovascular disease is probably valid, with magnesium being the more likely contributor to that benefit. This doesn’t mean distilled water is dangerous. It means that if you drink it exclusively, you may want to ensure your diet is rich in magnesium and calcium, or consider a mineral supplement.
There’s also a minor chemical quirk. Distilled water absorbs carbon dioxide from the air, forming trace amounts of carbonic acid. This gives it a slightly acidic pH, typically around 5.5 to 6.5 after sitting exposed to air. That’s roughly comparable to coffee or tea and poses no health risk, though some people notice a flat or slightly sharp taste compared to mineral-rich water.
Common Uses for Distilled Water
Distilled water is essential for applications where mineral buildup or bacterial contamination would cause problems. CPAP machines, used for sleep apnea, are one of the most common household examples. Manufacturers recommend distilled water because minerals in tap or spring water cause scaling inside the humidifier chamber, reducing performance and creating an environment where bacteria can grow. Using distilled water reduces infection risk and keeps the machine working properly.
Car cooling systems benefit from distilled water for similar reasons. Minerals and chlorine found in tap water accelerate corrosion of engine parts and create calcium carbonate deposits that clog cooling passages. Mixing coolant with distilled water instead of tap water minimizes these risks.
Hospitals, laboratories, and manufacturing facilities use distilled water to prevent limescale buildup on equipment and to ensure consistent, uncontaminated results in testing and production. Steam irons, aquariums, and home brewing are other common applications where mineral-free water matters.
How to Store Distilled Water
Unopened, commercially bottled distilled water lasts indefinitely in a cool, dark place. The water itself doesn’t expire, but the container can eventually leach trace chemicals into the water, especially if it’s plastic and stored in heat or direct sunlight.
Once you open a bottle, air introduces microorganisms and particles that compromise purity. Opened distilled water generally stays good for one to four weeks if you reseal the container tightly and store it in a clean environment. For long-term storage or applications requiring high purity, glass containers are the better choice. Glass doesn’t release chemicals the way some plastics can, keeping the water cleaner for longer.
If you’re using distilled water for a CPAP machine or other medical device, it’s worth replacing opened bottles regularly rather than stretching a single container over many weeks. The whole point is purity, and that degrades with each exposure to open air.