This might seem counter intuitive, but 70% alcohol is a actually a better disinfectant than 99% alcohol!

Isopropyl alcohol (2-propanol), also known as isopropanol or IPA, is the most common and widely used disinfectant within pharmaceutics, hospitals, cleanrooms, and electronics or medical device manufacturing. Different solutions, purity grades, concentrations, and alcohol types yield beneficial cleaning and disinfection properties when applied correctly; or dangerous consequences when used improperly.

Why Is 70% the Most Effective Concentration of Isopropyl Alcohol for Disinfection?

70% Isopropyl alcohol with 30% purified water, is rapidly antimicrobial against bacteria, fungi, and viruses.

Once alcohol concentrations drop below 50%, usefulness for disinfection drops sharply and higher concentrations of alcohol don’t generate more desirable bactericidal, virucidal, or fungicidal properties.

The presence of water is a crucial factor in destroying or inhibiting the growth of pathogenic microorganisms with isopropyl alcohol. Water acts as a catalyst and plays a key role in denaturing the proteins of vegetative cell membranes. 70% alcohol solutions penetrate the cell wall more completely which permeates the entire cell, coagulates all proteins, and therefore the microorganism dies. Extra water content slows evaporation, therefore increasing surface contact time and enhancing effectiveness. Isopropyl alcohol concentrations over 91% coagulate proteins instantly. Consequently, a protective layer is created which protects other proteins from further coagulation. Higher than 70% solutions do kill bacteria but may require longer contact times for disinfection, and enable spores to lie in a dormant state without being killed.

As an example, in lab tests, 50% isopropyl alcohol kills Staphylococcus Aureus in less than 10 seconds however a 90% solution with a contact time of over two hours was ineffective. Some disinfectants will kill spores with exposures times that exceed 3-12 hours, which are classified as chemical sterilants.

Proper Uses of Isopropyl Alcohol Require Distinction Between Sterilization and Disinfection.

Terms like disinfection and sterilization are often misunderstood and used interchangeably. The Center for Disease Control (CDC) defines terminology clearly:

Unlike sterilization, disinfection is not sporicidal. A few disinfectants will kill spores with prolonged exposure times (3–12 hours); these are called chemical sterilants. At similar concentrations but with shorter exposure periods (e.g., 20 minutes for 2% glutaraldehyde), these same disinfectants will kill all microorganisms except large numbers of bacterial spores; they are called high-level disinfectants. Antiseptics are germicides applied to living tissue and skin; disinfectants are antimicrobials applied only to inanimate objects. In general, antiseptics are used only on the skin and not for surface disinfection, and disinfectants are not used for skin antisepsis because they can injure skin and other tissues. Virucide, fungicide, bactericide, sporicide, and tuberculocide can kill the type of microorganism identified by the prefix. For example, a bactericide is an agent that kills bacteria.

Isopropyl alcohol is excluded from classification as a high-level disinfectant because of its inability to eradicate bacterial spores and hydrophilic viruses such as polio. Its low-level categorization outlines effectiveness for noncritical patient care devices such as blood pressure cuffs. IPA is also commonly applied during cleanroom wipedown for disinfecting tools and packaging that must pass into ultra-clean spaces.

This information from Production Automation Corporation