[lazide]

Do you have a link to the studies? I definitely notice a huge difference in washing with soap in cold water (camping), which is hard to not leave a film, and warm water which does a pretty effective job quite quickly with the same exact soap.

[e12e]

I didn't encounter any good primary sources, but a few references that seemed to indicate no difference in bacteria reduction between warm/cold water for washing hands. It may be that cold water is simply good enough.

[lazide]

The tricky part here is how the study is constructed. If you take a dry swab with a known bacterial culture, wash in water for x minutes at temp y, and graph it, that might make sense. It just doesn’t reflect real world situations, which almost always involve some kind of biofilm, oil/grease, dirt or similar mass in a complex interaction.

Especially once lipids get involved (almost all hydrophobic), especially in combination with other types of substances like oil soaked dirt or dead skin (like what you get on a doorknob at the micro level), it just doesn’t work the same way on the same timescales with colder temps - it fundamentally can’t, chemistry doesn’t work that way. [https://www.chemguide.co.uk/physical/basicrates/temperature....]

If you want the same speed of reaction - aka same amount of oil converted to a not oil - with lower temps (and hence lower molecular velocities and less Brownian motion) you need more reactive chemicals and more aggressive physical action to get compound A in contact with compound B enough to have the reaction you need. It’s as true for soap as it is for anything else.

And this all of course has a giant * on it, since above a certain point the energy in the system causes parts of it to disassociate or form unwanted side reactions, so there is a limit where it ‘blows apart’, ‘crashes’, or ‘can’t stick’ to extend the analogy. I can’t think of any soap that would have that problem at temps within human ‘not going to immediately get 3rd degree burns’ ranges.