To kill germs, it’s important to wash your hands in warm water, right? Wrong, according to Vanderbilt University experts. Although hot water can kill bacteria, human skin can’t safely tolerate a high enough temperature for a long enough time. E. coli, for example, can stay alive at temperatures up to 55°C (131°F) for over 10 minutes. But just 30 seconds of skin exposure to water this hot can cause deep second-degree burns.
The widespread belief that warm water is more hygienic than cold might seem harmless enough. Warm or cold, people are still washing their hands—a crucial step for preventing the spread of infection. Yet there are hidden environmental costs to this enduring myth, according to a new study in the International Journal of Consumer Studies.
“In the United States, hot water heaters are fueled primarily by fossil fuels, which have a number of negative environmental impacts,” says Amanda Carrico, PhD, a research assistant professor at the Vanderbilt Institute for Energy and Environment and faculty fellow with the Vanderbilt Climate Change Research Network.
Cleanliness Is Next to Wastefulness
“Most concerning is the emission of greenhouse gases, such as carbon dioxide, which contribute to global climate change,” says Carrico, lead author of the new research. “There is overwhelming scientific evidence that greenhouse gases from energy production and other sources are causing the climate to warm. This warming poses profound risks to human health and well-being in the United States and worldwide.”
What does that have to do with washing your hands? “Our analysis found that the use of warm or hot water for handwashing results in over 6 million metric tons of carbon dioxide emissions every year,” Carrico says. “To put that number in context, it is more than twice the total annual emissions of the Bahamas.”
“In the United States, we need about two coal-fired power plants just to provide enough energy for people to wash their hands with warm or hot water,” Carrico adds. “The fact that many people choose elevated water temperatures due to the incorrect belief that it is better at killing germs is the reason we decided to write this paper.”
Why We Believe What We Believe
The myth that warm water is better than cold for handwashing has a grain of truth to it. If your hands are dirty with oil or grease, warm water may do a better job of removing it. Plus, many people know that heat kills bacteria. “I believe that this knowledge was misapplied without recognizing that there are limits to what our skin can tolerate,” Carrico says.
Warm water is also more comfortable, which may encourage people to scrub up for longer. That’s important, because the Centers for Disease Control and Prevention (CDC) says you should rub your soapy hands together under clean running water for at least 20 seconds—about the time needed to hum the “Happy Birthday” song twice. Yet many people might be perfectly fine with turning on just the cold water if they knew it was better for the environment.
From a health perspective, washing hands in overly warm water may actually be counterproductive. “Hot water can cause scalding, damaging the skin and leading to painful burns,” says Carrico. “In addition, even water considered warm by most individuals can irritate the skin. When skin is irritated, it is less resistant to bacteria, potentially causing an increase in the spread of germs.”
The Psychology of Washing Hands
Still, people cling to the belief that warmer water is better for handwashing, and some of the reasons may be more psychological than practical. In our culture, we tend to associate handwashing with not only clean hands, but also a clean conscience—think of Lady Macbeth’s compulsive hand scrubbing as she tries to wash away her guilt.
We may also associate handwashing with washing our hands of past decisions, even if they don’t have a moral connotation. In one interesting experiment, college students who had been given a choice of music CDs seemed to feel less need to justify their choice later if they had washed their hands in the interim.
For some people, washing their hands in the hottest possible water might seem more purifying, at least on a subconscious level. This may be most obvious when taken to an unhealthy extreme by individuals with obsessive-compulsive disorder who develop washing rituals. But even those of us without OCD may feel that warmer water somehow leaves us feeling cleaner, even when there’s no rational basis for feeling that way.
Carrico’s research suggests that it’s time to reexamine such beliefs. “My advice is, first and foremost, to wash your hands,” Carrico says. According to the CDC, it’s one of the best ways to prevent the spread of infection and illness. But unless you have a medical condition such as Raynaud’s disease, which makes the fingers extra-sensitive to cold, try using cooler water and see if it’s tolerable for you. As Carrico says, “It’s better for your hands and for the planet.”