What concentration of alcohol is commonly used as a disinfectant?

• A. 50%
• B. 70%
• C. 90%
• D. 30%

Answer: (B)

Alcohol kills microbes mainly by denaturing proteins and dissolving membranes, and water plays a crucial role in that process. At around seventy percent, there’s enough water to enable thorough protein denaturation and to help the alcohol penetrate the cell, while still maintaining strong antimicrobial activity. If the concentration is too high, like ninety percent, proteins on the outer surface coagulate too quickly and form a barrier that limits deeper penetration, reducing disinfection effectiveness. If the concentration is too low, such as thirty or fifty percent, the alcohol isn’t able to denature proteins efficiently or disrupt membranes reliably. So, seventy percent hits the right balance for effective surface disinfection.