Does Hot Water Clean Better Than Cold Water?

published Jun 3, 2019
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Credit: Joe Lingeman/Apartment Therapy

Welcome to Clean IQ, where we uncover the chemistry of common life and explore the science behind your favorite cleaning solutions.

Some friendly reminders that the planet is dying, and it’s all our fault: More than 60% of Americans still wash their laundry in warm water, and of the total emissions produced and energy used in a single load of laundry, 75% is a result of heating the water itself. Given these numbers, and the fact that switching to cold water also saves you money, you might be asking: Why are we still stuck on the heat cycle?

Does hot water clean better than cold water?

“The perception that hot water cleans better than cold stems from the way we did laundry years and years ago,” says Kay Gebhardt, trained chemist and senior scientist for sustainability and authenticity at Seventh Generation. “Back then, heat was useful because it sped up the cleaning process when detergents and machines were less efficient.”

Credit: TanyaRozhnovskaya

To understand this better, we need to look at how water and detergents interact. Hot water has more kinetic energy than cold water, meaning the molecules in the water move around more quickly. When these molecules interact with the surfactants in classic detergents, the greater energy they possess means they are able to more quickly agitate the surfactants (water-fearing molecules that grip dirt and, like kids darting away from a wave on the beach, move the dirt off your clothes and away from the oncoming tide), thereby sloughing away stains with greater speed.  

When washing was done by hand, the addition of boiling water made what was a deeply onerous and days-long task somewhat less so; and, when the first electric washing machines were introduced in the early 1900s, hot water was necessary in order to “activate” the rudimentary detergents. (However, the necessity for hot water also meant the machines were prohibitively expensive to use.)

Credit: Joe Lingeman/Apartment Therapy

Washing Clothes in Cold Water vs. Hot Water

Despite the best practices of the past, modern laundry detergents are formulated to work just as well in cold water. “The new detergents use enzymes that are cold water stable,” says Gebhardt. “They literally cut up the soils and that allows the surfactants to move the stains off the clothing” without the use of hot water. These cold water detergents are every bit as effective as traditional detergents, but use a fraction of the energy that hot water requires.

And the benefits of cold water washing go beyond cost and energy savings. Cold water is much less damaging to clothing fibers; there are also many stains—especially protein-based stains, like blood—that will actually be locked into the fabric by hot water.

Why, then, are we still turning up the heat?

“A lot of consumers think that hot water sanitizes clothing,” Gebhardt theorizes. “The truth is, unless you have a sanitize cycle, the water just isn’t hot enough. Only the dryer can sanitize, although sun drying is just as effective for that purpose.” Sanitation, too, is only really necessary when the soiled clothes are harboring nasty bacteria, such as fecal matter on cloth diapers, or vomit resulting from an illness. In those instances, Gebhardt says, hot water is the way to go.

She also makes an exception for geography. “I live in Vermont,” she says, “and in the winter, our water is really, really cold. That does become an issue when you’re washing. If the water is at the level of freezing, the surfactants will get really sluggish and the detergents won’t work as well.” In winter months, she’ll turn the dial for stubborn loads.

What about washing dishes in cold water?

It’s wise to stick with warm-to-hot water when you’re hand washing, but not for the reason you think. When it comes to food-borne bacteria, water temperature (at least at temperatures your body can stand) doesn’t seem to make a difference: A 2017 study in the Journal of Food Protection found that cold and lukewarm water were just as efficient as 100-degree hot water at removing bacteria during a wash. It’s the detergent that’s key when you’re hand-washing, and it probably needs warmer water to work: Since most dishwashing detergents are surfactant-based, they aren’t formulated for cold water, and require the thermal energy of hot water to create that speedy grease and debris-removing effect.

“For hand washing, honestly, much of it is comfort,” adds Gebhardt. “Washing dishes in cold water is just painful.”

Credit: Joe Lingeman

But turn to automatic dishwashers, and the same cold-washing rules apply as in your laundry machine—provided you’re using the correct detergent.

“The reality is that most cleaning in a dishwasher happens because of the agitation, not because of the detergent,” says Gebhardt. “That said, powder detergents will struggle to dissolve in cold water. Think about dissolving sugar into tea—it’s much harder to get sugar to dissolve in iced tea than it is in hot tea.” If your goal is to save energy, she recommends using a liquid detergent, running on a cooler cycle, and skipping the heat dry.

Worried about germs? Unless you’re running a special sanitizing cycle every time (which would be quite an energy drain), your dishwasher likely isn’t hitting the temperature required to sanitize your dishes—150 degrees, according to the National Sanitation Foundation standard (your regular wash cycle likely peaks at around 120 degrees).

The bottom line: While hot water can, in some instances, speed up the cleaning process, for most modern tasks, cold works just as well—and has immense benefits to the environment. As Gebhardt pointed out, 92% of the environmental impact in Seventh Generation energy audits—and that includes transportation and manufacturing—comes from consumers employing heat when using their products.

That’s a huge number—and one we’re capable of making smaller, at no cost to cleanliness.

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