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“It is true that some viruses are readily inactivated by alcohol; however, some are not. Viruses consist of nucleic acid (either RNA or DNA) surrounded by a capsid (protein shell). Some viruses have an additional external layer or wrapping known as an envelope. The envelope is created from a piece of phospholipid membrane that comes from the infected host cell during the “budding” process when viral particles leave the infected cell. Enveloped viruses are referred to as lipophilic viruses, because of their lipid envelope, while non-enveloped viruses are referred to as non-lipophilic viruses.

“Generally, enveloped (lipophilic) viruses are susceptible to alcohol: Herpes simplex virus (HSV), human immunodeficiency virus (HIV), influenza virus (Flu), respiratory syncytial virus (RSV), vaccinia virus, Hepatitis B and C viruses are considered susceptible to alcohols.

However, certain nonenveloped (nonlipophilic) viruses such as hepatitis A and enteroviruses, which are both responsible for viral gastrointestinal infections are less susceptible. Depending on the alcohol concentration of the hand-cleanser and time of exposure to the alcohol, hepatitis A and other nonlipophilic viruses may not be eliminated.

“Alcohol kills microorganisms by denaturing proteins and the most effective alcohol preparations have contain 60-95% alcohol. Higher concentrations are less effective because proteins are not denatured easily in the absence of water. Most commercial hand-cleansers contain between 65-70% alcohol. Nonenveloped viruses require slightly higher alcohol concentrations for reliable inactivation than are found in many commercial hand-cleansers (70-80%).

“The effectiveness of alcohol in ridding hands of potentially infectious microorganisms is dependent not only on the alcohol concentration, but the time of exposure, and the volume of alcohol as well. It is important to ensure adequate contact between the cleanser and the then hands. Although the optimal volume of alcohol-based cleanser has not been determined, it is suggested that if hands feel dry after rubbing hands together for 10-15 seconds, an insufficient volume was likely applied.”

However, this is only half the equation.

Very, very few pathogens can successfully create a runaway infection with minimal contamination, such as Tularemia (rabbit fever), that only needs 10 to 50 bacteria to establish itself. Most bacteria and viruses require tens or hundreds of thousands or even millions, all at once, especially if the immune system has been exposed to it, or a similar pathogen, before.

For example, if you have overcome an infection of an ordinary H1N1 flu strain, a “second cousin” to that strain may require millions of viruses to establish an infection, and even then it will not be severe. But a “fifth to tenth cousin” of that strain might need far less to establish itself, and the disease itself with be harsher.

And this is the value of washing the hands, or at least using hand sanitizer. Such a volume of pathogen has to be carried in a glob of contamination. If you can wash the glob off, it will take enough of the pathogen with it so the *remainder* won’t be able to infect you.

This is why there is a rule that if you can physically see contamination, you should use soap and water. Only if the hands are (relatively) clean should you use hand sanitizer.

And dirty hands are, in many circumstances, absolutely essential to the spread of cold and flu viruses.

Experiments have shown that the ideal environment to spread viruses by coughing and sneezing is at 40F and low humidity. As the temperature and humidity increase, colds and flu become much more dependent on hand contamination.

Ironically, air conditioning does much to increase the spread of colds and flu by both stagnating and recirculating air. If instead windows are opened, air circulation inside and out radically reduces the amount of pathogen floating about on drops of moisture.

One final note, viruses that cause intestinal problems resulting in diarrhea and/or vomiting, can be readily wafted into the air with toilet flushing. So it is not a bad idea when those diseases are about, to put a cup of chlorine bleach in the toilet after use, a minute or so before flushing.

And then to wash hands before exiting the bathroom.