The Hot Water Reflex
You've been doing it your whole life. The moment you turn on the faucet to wash your hands, you reach for the hot side. Posters in restaurant bathrooms instruct employees to use warm water. The FDA's Food Code recommends that plumbing in food establishments deliver water at 100°F for handwashing. Your parents told you hot water kills germs. Everyone told you hot water kills germs.
Nobody checked. Not rigorously, anyway. Not until a team at Rutgers decided to run the numbers.
Twenty Sessions, Three Temperatures, One Answer
In 2017, food scientist Donald Schaffner and colleagues designed one of the most thorough tests of this assumption ever conducted. They contaminated the hands of 21 volunteers with a harmless strain of E. coli (ATCC 11229) and had each person wash their hands 20 times over a six-month period at three different water temperatures: 60°F, 79°F, and 100°F. They also varied the amount of soap and compared antimicrobial versus plain soap (Jensen et al., 2017).
Cold water removed the same amount of bacteria as hot water. Mean reductions ranged from 1.9 to 2.3 log CFU across all conditions, and water temperature contributed no significant variance. A 60°F wash killed just as many germs as water forty degrees warmer.
This wasn't an anomaly. In 2002, Michaels and colleagues tested an even wider range, from 4.4°C to 48.9°C, and found the same thing: water temperature had no measurable effect on bacterial removal (Michaels et al., 2002).
Why Temperature Is Irrelevant
The intuition behind hot water feels solid. Heat kills bacteria. Pasteurization works at 161°F. Boiling water sterilizes surgical instruments. But the flaw is scale: the water coming out of your kitchen faucet tops out at about 120°F, and your hands can't tolerate much above 105°F without discomfort. Those temperatures are nowhere near what's needed to thermally destroy bacteria during a 20-second wash.
What actually removes bacteria from your skin is mechanical action. Friction from rubbing. The surfactant properties of soap disrupting lipid membranes. The rinsing itself carrying loosened microbes away. Soap molecules have one end that binds to water and another that binds to oils and dirt. When you lather, these molecules surround bacteria and pull them free. None of that chemistry requires heat.
"People need to feel comfortable when they are washing their hands," Schaffner said. "But as far as effectiveness, this study shows us that the temperature of the water used didn't matter."
Six Billion Kilowatt-Hours of Wasted Energy
Here is where the finding bites. Carrico and colleagues at Vanderbilt University estimated that Americans could save approximately 6 billion kWh of energy annually by switching to cooler water for handwashing (Carrico et al., 2013). That is roughly equivalent to the entire annual electricity generation of Hoover Dam, or enough to power 600,000 average American homes for a year.
The FDA's own guidelines drive the waste. The Food Code recommends that food establishment plumbing deliver water at 100°F for handwashing. Many states interpret this as a hard requirement. Restaurants, cafeterias, hospitals, and schools across the country heat water to meet this standard, burning natural gas or electricity to achieve a temperature that produces zero measurable difference in bacterial removal.
Schaffner has argued publicly that the guideline should change. "Instead of having a temperature requirement, the policy should only say that comfortable water needs to be delivered."
The Strongest Case Against
The most reasonable objection is that the Jensen study used a laboratory surrogate and a standardized inoculation protocol rather than testing against the full diversity of pathogens encountered in real kitchens and bathrooms. Real-world hand contamination involves varying bacterial loads, viral particles, and organic matter like food residue that may interact differently with temperature.
A second, subtler concern involves compliance. Warm water may encourage people to wash longer simply because it's more pleasant, and longer washing does remove more bacteria. If cold water made people rush through a five-second rinse rather than a full 20-second lather, the theoretical equivalence would collapse in practice.
These are fair points. But they concern human behavior, not microbiology. Nothing in the chemistry of surfactants or the biology of bacterial adhesion depends on the temperature range between 60°F and 100°F. No study in the literature has found otherwise.
What We Didn't Prove
This study tested bacterial removal, not viral removal. While the mechanical principles are the same, specific viral pathogens like norovirus have different adhesion characteristics and may warrant separate investigation. The sample of 21 volunteers, though each was tested 20 times, limits demographic generalizability. All testing used soap; the results should not be extrapolated to water-only rinsing, where temperature might play a different role by affecting the skin oils that trap pathogens.
The study was partially funded by GOJO Industries, makers of Purell hand sanitizer. This creates a potential conflict of interest. The findings align with independent work by Michaels et al. and with basic surfactant chemistry, but the funding relationship should be noted.
The Bottom Line
Soap removes bacteria through chemistry and friction. The temperature of the water in your tap is nowhere near hot enough to kill germs thermally, and every study that has tested the question has found no difference between cold and hot water for handwashing. What matters is soap, time, and technique. Not temperature.
The hot water myth persists because it feels right. Heat connotes cleanliness. Cold feels insufficient. But feelings are not data, and the data here are consistent: 60°F water, soap, and 20 seconds of lathering will protect you exactly as well as scalding your hands ever did.
What You Can Do
Skip the hot tap. When you wash your hands, use whatever temperature is comfortable. Cold water works identically to hot for removing bacteria.
Focus on duration. Twenty seconds of lathering with any soap matters far more than water temperature. Scrub between fingers, under nails, and the backs of hands.
Save energy at home. If your water heater serves your bathroom and kitchen sinks, you can reduce the temperature setting without compromising hygiene. (Dishwashers and laundry need separate consideration.)
Ask about the rule. If you operate or manage a food service establishment, the Jensen study provides peer-reviewed evidence that the FDA's 100°F handwashing requirement lacks scientific support. Several food safety experts have called for the guideline to be updated.