UV Light for Mold Removal: Does It Really Work?

Most people know that one of the best ways to treat mold on a household item like a blanket or piece of upholstered furniture is to bring it outside and put it in the sun. If you didn’t know this, try it. The combination of air circulation and sunlight usually does the trick, although it may take a few days of dry weather.

The sun is a full-spectrum light source, which means all light frequencies are present, including those humans can’t see. Higher frequencies, which are those above the visible spectrum, make up ultraviolet (beyond violet) light. Having the most energetic frequencies, UV light can wrinkle your skin, give you sunburn, damage your eyes and even give you skin cancer. But does UV light kill mold, and if so, is it the most effective way to do it?

Let’s find out with the help of Forrest Webber, an electrician who founded a cleaning company, and HVAC expert Kevin Goude.

Does UV Light Kill Mold?

“Yes,” says Webber. “It messes with the DNA of the mold spores, rendering them unable to reproduce and party all over your basement.” To understand why, you need a little help from the science you may remember from high school.

Light isn’t exactly a wave in the conventional sense, but it behaves like one, and it has a wavelength. Ultraviolet light has a short wavelength, which means it has a high frequency, and according to an equation made famous by physicist Max Planck (Energy = h x frequency, where “h” is a minuscule number called Planck’s constant), high-frequency light is high-energy light. That’s why UV light can damage and kill mold spores, whereas visible and infrared light can’t.

How long does it take to kill mold with UV light?

If you use an artificial UV light source, whether the light can kill mold depends on the intensity of the source, its frequency of the light and the distance between the light source and the mold. According to Dara Greany, CEO of LEDLightExpert.com, it takes an average of 90 minutes to kill mold with UV light, but it can take three to five hours if you have a weak light source and a room full of mold to kill.

What Type of UV Light Kills Mold?

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“UV-C light can be effective at killing mold spores,” says Webber. So what’s UV-C light? Consider that there’s an entire spectrum of UV radiation, from barely visible violet light to highly energetic X-rays, gamma rays and beyond. Scientists divide the UV spectrum into three classes: UV-A, UV-B and UV-C. UV-A radiation is closest to the visible spectrum and has the lowest energy while UV-C is farthest from visible light and has the highest energy.

The sun showers the earth with all three classes of ultraviolet radiation, but UV-C is so energetic that it interacts with the atmospheric ozone layer, and most of it gets absorbed (which is a good thing, or we’d all be toast). Because it has so much energy, UV-C radiation has proven effective for killing viruses, bacteria and mold, but because it doesn’t occur naturally at ground level, it has to be artificially produced. Accordingly, there are several types of lamps on the market today that generate UV-C radiation.

What Type of Mold Does UV Light Kill?

Does UV light kill mold? Yes, but the problem is that it has to be able to contact the organisms directly. Webber puts it like this: “Mold is a sneaky little thing. It likes to hide in places where the light doesn’t shine (insert joke here). So unless your mold problem is lounging somewhere in the direct line of fire, UV light might not reach it, and those spores will just keep plotting their grand comeback. Think of UV light as a laser pointer in a game of whack-a-mole—great for the ones it hits, but it won’t reach the sneaky spores deep in the walls or under the carpet.”

Are There More Effective Alternatives to Kill Mold?

“I don’t recommend relying on UV lights as your only solution for mold control,” warns Goude. Instead, he suggests using it as a supplemental mold control method, not the main one. So what’s the main one? “You need to lower indoor humidity to 50 percent or below by fixing any plumbing leaks, improving ventilation with open windows and exhaust fans, and ensuring good air circulation with ceiling fans.”

Depriving new mold colonies of moisture will prevent them from growing, but what to do about existing colonies? Webber recommends using what he calls the Dynamic Duo: “Vinegar is your best friend. It’s cheap, it’s non-toxic, and mold hates it. Mix white vinegar with water and go to town on moldy surfaces.”

You’ve probably heard that bleach effectively kills mold — and there’s plenty of evidence that it does — but its high surface tension prevents it from penetrating porous surfaces and getting to the roots. Consequently, when you use it to clean mold from wood and stone, there’s a good chance the mold grows back. Vinegar has no such issue, but if you don’t have any, you can do what the EPA recommends: Clean moldy surfaces with detergent and water and dry them thoroughly.

About the Experts

• Forrest Webber became an electrician after graduating from Tulane University. He subsequently became an electrician and later founded Alabama-based Bear Brothers Cleaning.
• Kevin Goude is the owner of First Choice Heating & Air, a South Carolina company specializing in HVAC maintenance and repair.

Sources

• Greany, Dara: UV Lights — How Do They Kill, and How Long Does It Take a UV-C Light to Kill?; 2023.
• U.S. Environmental Protection Agency: What Are the Basic Mold Cleanup Steps?; 2024.
• Eco Flood & Mold Remediation: Never Use Bleach to Treat, Clean or Kill Mold? Read Why!; 2021.