Sterile water and saline are often mistakenly considered interchangeable solutions, especially given their frequent appearance in medical and first-aid settings. These two liquids are distinctly different, and understanding this difference is a matter of safety in health applications. The presence or absence of salt fundamentally changes how each solution interacts with the human body’s biological systems, dictating their appropriate uses, from diluting medicine to irrigating tissues.
Defining the Chemical Composition
Sterile water is ultrapurified water (H₂O) that has undergone rigorous processes like distillation or filtration to eliminate all microorganisms and impurities. This process ensures the water is nonpyrogenic, meaning it does not contain substances that could cause a fever when introduced into the body.
Saline, frequently referred to as normal saline, is a solution containing a precise amount of sodium chloride (NaCl)—common table salt—dissolved in sterile water. The standard medical concentration is 0.9%, meaning there are 0.9 grams of salt per 100 milliliters of solution. This specific percentage is formulated to mimic the salt concentration found naturally in human blood.
Tonicity and Cellular Interaction
The most significant difference between the two solutions is their tonicity, which measures the osmotic pressure gradient they create across a cell membrane. Human cells are surrounded by a semipermeable membrane that regulates water movement, a process known as osmosis, where water moves from lower to higher solute concentration.
Saline, at 0.9% concentration, is an isotonic solution because its solute concentration closely matches that of human plasma and intracellular fluid. When cells are placed in an isotonic solution, there is no net movement of water across the cell membrane, meaning the cells neither swell nor shrink, maintaining their optimal size and function. This compatibility makes saline the preferred solution for intravenous (IV) administration.
Conversely, sterile water contains virtually no solutes, making it a hypotonic solution relative to human cells. When hypotonic water contacts tissue, the high concentration of solutes inside the cell creates an osmotic pull, causing water to rush in. This rapid influx causes cells to swell and can lead to lysis, where the cell membrane bursts.
Practical Applications and Safety
The differing tonicity of each solution dictates its specific medical applications. Normal saline is the default solution for any application requiring direct contact with tissues and cells, especially in large volumes. This includes flushing open wounds, rinsing contact lenses, and serving as the base for many IV fluids.
Sterile water, due to its hypotonic properties, is restricted to applications where it will be mixed with a solute before administration or where cellular integrity is not a concern. It is frequently used to dilute powdered medications before injection, where the medication provides the necessary solutes to make the final solution isotonic. It is also used in equipment rinsing or in nebulizers, where the water is vaporized and inhaled. Confirming the label is important, as only an isotonic solution like 0.9% saline is considered safe for direct tissue contact and systemic introduction.