Why Is 70% Isopropyl Alcohol (IPA) a Better Disinfectant than 99% Isopropanol, and What Is IPA Used For ?

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. This post will help you identify key uses, best practices, and proper disinfection with isopropyl alcohol. Likewise you can see our other posts on IPA as a universal cleaner and the importance of high-quality USP IPA.

Isopropyl alcohol, particularly in solutions between 60% and 90% alcohol with 10 – 40% purified water, is rapidly antimicrobial against bacteria, fungi, and viruses. Once alcohol concentrations drop below 50%, usefulness for disinfection drops sharply. Notably, 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% IPA 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.

Solutions > 91% IPA do kill bacteria, but sometimes require longer contact times for disinfection, and enable spores to lie in a dormant state without being killed. In this analysis, a 50% isopropyl alcohol solution kills Staphylococcus Aureus in less than 10 seconds (pg. 238), yet a 90% solution with a contact time of over two hours is ineffective. Some disinfectants will kill spores, which are classified as chemical sterilants. So why do higher alcohol solutions yield fewer results for bactericidal and antimicrobial outcomes?

Some bacteria transform into spore cells when external conditions are unfavorable; the result is reduced metabolic activity, higher ‘cidal’ resistance, and immunity from alcohol-based disinfectants. Spores lie dormant, and once conditions become favorable again, the microbe converts back to a vegetative state and grows actively. When examining the effectiveness of IPA, accurately translating its benefits and shortcomings require distinctions of identity, purity, sterility, and intended use. Disinfection, unlike sterilization, does not provide sporicidal attributes.

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.

70% isopropyl alcohol upholds key requirements for use as a bactericidal in cleanrooms or medical facilities, but also for general purposes. 70% IPA/30% water solutions produce less vapor and odor, therefore reducing risks of toxic fumes or combustion. When isopropyl alcohol reacts with air, light, and oxygen, it forms unstable peroxides which increase the likeliness of explosion, especially when heated with aluminum. IPA volatility increases with storage time and alcohol concentration, especially when exposed to light over multiple years after opening.

70% IPA is less flammable but also offers a more economical price point for general wipe down and large-surface disinfection. Likewise, high-moisture alcohols evaporate slower and increase contact time without becoming immediately dry. If 70% IPA is so effective as both a general-purpose cleaner and disinfectant, why use 99% concentrations?

99% isopropyl alcohol is ideal as a solvent or cleaning agent for industries that produce water sensitive items, therefore rapid evaporation and low water content is favorable. 99% USP IPA provides the lowest presence of water and in some forms is free from denaturants. Computer technicians, medical device manufacturers, printed circuit board manufacturers, and soldering and rework technicians prefer immediate evaporation for work with sensitive devices such as integrated circuit adapters, computer chips, and circuit boards. 99% IPA evaporates cleanly and minimizes residual substances. Rapid evaporation reduces shelf life but is more effective against sticky residues, grease, and grime than 70% concentrations. Because isopropanol is hygroscopic, acetone may yield better grime fighting results for inks or oils.