Handwashing science in the time of the coronavirus

9Health Expert Dr. Payal Kohli explains the science behind hand washing and sanitizing.

With the spread of the new coronavirus, COVID-19, everyone has been talking about hand hygiene but here is the science behind why hand hygiene works.

Studies show that washing hands with soap and water for 15-20 seconds can reduce bacterial counts by 90% and when an additional 15 seconds will reduce counts by close to 99.9%.  Soap and water work by mechanically removing the bacterial and virus particles from your skin and so does scrubbing your hands. 

Scrubbing your hands creates friction which lifts off dirt, grease, and organisms.  It is recommended to wash hands for ~20 seconds (with an additional 20-30 seconds for drying).  Some soaps have an antibacterial called triclosan which can reduce bacterial counts on your skin but the FDA has said that this is no more effective than non-antibacterial soap and water. Drying hands is important because wet hands are like a sponge for more germs.

Alcohol kills bacteria and viruses by changing the shape of (“denaturing”) the proteins that help the bacteria and viruses to survive.  So, in contrast to soaps, sanitizers work by “killing the germ”.  Most sanitizers have a 62% alcohol concentration.  Alcohol alone dries the skin so often sanitizers are mixed with hand conditioners.  There may be some viruses that are not effectively killed with hand sanitizers.  And, it is important to use enough sanitizer so that your hands are well covered. 

There are advantages and disadvantages to each approach.  Because of the convenience, compliance or use may be higher among those who use hand sanitizer compared with those who wash their hands.  But, certain types of organisms (like cryptosporidium, norovirus and C. difficile) can only be removed with handwashing.  In cases when the hands are visibly soiled (such as after gardening, playing sports, camping or going fishing), handwashing may be more effective than hand sanitizers because protein and fats on your hands reduce the effects of alcohol-based sanitizers.   

Below are instructions from the World Health Organization on the correct way to wash your hands with soap and water as well as with alcohol-based sanitizer.