Saline is a solution of salt (sodium chloride) dissolved in purified water. The standard formulation, known as “normal saline” or 0.9% saline, contains 9 grams of sodium chloride per liter of water. That specific concentration was chosen because it closely matches the salt level in human blood, making it safe to use in the body without damaging cells.
What’s in Normal Saline
Normal saline has just two ingredients: pharmaceutical-grade sodium chloride and sterile water. Each 100 milliliters contains 900 milligrams of sodium chloride, which works out to about half a teaspoon of salt per pint. The sodium chloride used in medical saline must meet strict purity standards set by the United States Pharmacopeia. It has to dissolve into a perfectly clear, colorless solution with a near-neutral pH.
This simplicity is the point. Normal saline delivers fluid and sodium to the body without introducing anything extra. It contains 154 milliequivalents per liter each of sodium and chloride, which is slightly higher than actual blood levels. Blood plasma has a sodium concentration closer to 140 milliequivalents per liter, so normal saline isn’t a perfect match for your blood’s chemistry, even though it’s close enough for most purposes.
Why 0.9% Is the “Normal” Concentration
The 0.9% concentration dates back to the work of a Dutch physiologist named Hamburger, who tested how red blood cells reacted to different salt solutions. He found that 0.9% sodium chloride had a freezing point similar to mammalian blood, and when red blood cells were placed in this solution, they didn’t burst or shrink. A solution with less salt causes red blood cells to swell and rupture (a process called hemolysis), while a saltier solution causes them to shrivel. Because 0.9% preserved cells intact, Hamburger called it “normal” or “physiological” saline.
In technical terms, 0.9% saline is isotonic, meaning its concentration of dissolved particles matches blood plasma’s range of 285 to 295 milliosmoles per liter. This balance matters because fluids flow across cell membranes toward higher concentrations. When an IV fluid matches the body’s concentration, it doesn’t force water into or out of your cells.
Other Concentrations and What They Do
Not all saline is 0.9%. Doctors use different concentrations for different problems.
Hypertonic saline has a salt concentration above 0.9%. Because it’s saltier than your blood, it pulls water out of swollen tissues. This makes it useful for reducing brain swelling after a head injury or for correcting dangerously low sodium levels in the blood.
Hypotonic saline has a salt concentration below 0.9%. It works the opposite way, pushing water into cells. This is used when sodium levels in the blood are too high and need to be diluted, though it has to be given slowly to avoid complications.
How Medical Saline Differs From Balanced IV Fluids
Normal saline contains only sodium and chloride. Your blood, however, also contains potassium, calcium, magnesium, and natural buffers that keep it from becoming too acidic. Balanced IV fluids like lactated Ringer’s solution are designed to more closely mimic this full chemistry. Lactated Ringer’s has a sodium concentration of 130 milliequivalents per liter (lower than saline’s 154), plus small amounts of potassium, calcium, and a buffering agent called lactate.
The chloride content in normal saline is notably higher than what’s in your blood. When large volumes are given, this excess chloride can shift the blood toward acidity. Balanced solutions avoid this by replacing some of the chloride with buffering compounds like lactate, acetate, or gluconate. For routine hydration and many medical situations, though, normal saline remains one of the most widely used IV fluids in the world.
What’s in Store-Bought Saline Products
The saline you buy at a pharmacy for nasal rinsing or contact lens care isn’t always just salt and water. Commercial nasal sprays commonly include buffers like sodium phosphate to keep the pH stable, along with preservatives such as benzalkonium chloride and benzyl alcohol to prevent bacterial growth in the bottle after opening. These additives are generally present in very small amounts, but they’re worth knowing about if you have sensitivities.
Preservative-free versions are available and come in single-use vials or cans that don’t need antimicrobial additives because they’re sealed until the moment you use them.
Making Saline at Home: What to Know
Many people mix their own saline for nasal rinsing using salt and tap water. This carries a real, if rare, risk. The CDC has documented fatal brain infections caused by amoebas, specifically Naegleria fowleri and Acanthamoeba, that can live in tap water, household pipes, and water heaters. These organisms are harmless if swallowed but can cause nearly always fatal infections if they enter through the nose and reach the brain.
If you make saline at home, the CDC recommends using water that has been boiled for at least one minute and then cooled, distilled water, or water that has passed through a filter with a pore size of 1 micron or smaller. The ratio for a basic isotonic solution is roughly a quarter teaspoon of non-iodized salt per 8 ounces of water, though pre-measured salt packets designed for nasal rinsing are the easiest way to get the concentration right.