Does UV Light Kill Mold? (3 Reasons It Works)

Licensed Inspector Arie Van Tuijl
Licensed Inspector Arie Van Tuijl

🕖Last Updated On May 10, 2022

It may seem weird that a spectrum of light can kill mold, but most people know the power of sunlight to cause rapid sunburns.

And the wavelength of sunlight that actually does the damage is known as ultraviolet light.

And it won't just kill mold, but it will also kill other microorganisms such as bacteria, viruses, and other types of fungi such as mildew.

I invite you to read my top 4 reasons as to why UV light can kill mold!

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In this HomeInspectorSecrets.com guide, I will go over:

  • The top 4 reasons that UV light will kill mold
  • Why shortwave frequency of UV kills mold spores
  • The 3 factors that maximizes ultraviolet light in killing mold
  • Home UV light products that are effective to stop mold and mildew

Below is my detailed list of how UV light kill mold. Keep reading to learn about different UV home products, and how to get the best results when using ultraviolet light for mold and mildew.

Does UV Light Kill Mold? (3 Reasons It Works)

  1. Sunlight Is A Disinfectant And Contains UV
  2. UV Light Destroys Mold From A Safe Distance
  3. UV Light Kills Mold Spores In The Air
  4. UV Light Uses A Short Wavelength of Light

Reason 1 - Sunlight Is A Disinfectant And Contains UV

Mold can't grow in direct sunlight because it naturally contains UV light. 

Have you noticed that you only see fungus growing in shaded areas?

And even though fungus dies in sunlight, it is actually the invisible portion of the light spectrum that destroys the mold. This is known as ultraviolet light which has a shorter wavelength than the light humans can actually see. In fact, mold destroying UV light from the sun will also destroy bacteria, viruses, and other types of fungi.

I remember watching a television show called Man Vs. Wild where the host disinfects stagnant water by placing a clear water bottle in direct sunlight. Over a matter of hours, the water became sterilized and safe to drink. Sunlight is a great natural disinfectant that has been used for hundreds of years.

Reason 2 - UV Light Destroys Mold From A Safe Distance

Since UV light is a specific wavelength of light, it can kill mold or mildew from a distance.

And when it comes to black mold, no one wants to go near the stuff.

If mold is in a basement, bathroom, or crawlspace, you can leave an ultraviolet lamp in the area, use a remote control to turn it on from a safe distance, set a countdown timer on the unit (turns off automatically), and just leave the home.

When you come back, the mold will be disinfected and ready for cleaning or removal. In general, it will take 1-3 hours to disinfect fungus using ultraviolet light.

The great advantage of ultraviolet light is that you don't have to use harsh chemicals or go anywhere near the mold when it is still active. If you disturb mold while it is still growing, it can release harmful spores and mycotoxins into the air which may circulate throughout the entire home — and into your lungs.

Reason 3 - UV Light Kills Mold Spores In The Air

And ultraviolet light won't just kill mold on surfaces, but it can kill the mold spores floating in the air.

Mold spores are the reproductive units (similar to seeds) of the fungus, and these spores can be allergenic and toxic to humans if inhaled.

One common characteristic of mold exposed to the human body is itchiness but also coughing, sneezing, wheezing, and other allergic symptoms. As mentioned earlier, you don't want to be exposed to powdery mold when cleaning it without wearing PPE (personal protective equipment).

A popular UV-C light product for homes are HVAC UV light sanitizers. These UV lights are installed in the air conditioning ductwork and kills mold on the surfaces of the duct but can also kill mold in the air. 

Reason 4 - UV Light Uses A Short Wavelength of Light

Ultraviolet light uses electromagnetic waves to penetrate through microscopic organisms such as mold.

The wavelengths are of various sizes — between 100 and 280nm — and known as the Germicidal UV-C that kills microorganisms. For mold destroying UV light sanitizers, the most widely used wavelength is 253.7 nanometers when used as an surface/air disinfectant.

The ultraviolet light irradiates the immediate environment (air and surfaces) in which it is placed such as in an indoor HVAC system. And when the germicidal rays come across molds, they penetrate into the nuclei of the molds, damaging the nucleic acids — until the organism becomes disabled. 

Ultraviolet light is used in different industries for microbial decontamination such as in hospitals, water treatment, air purification, and food sanitization.

There have been numerous scientific studies showing that UV-C is effective in killing microorganisms. The short wavelength known as UV-C is responsible for disrupting the nuclei of microorganisms which either kills it or prevents it from reproduction.

Read Also >> See Our Pictures Of Mold Inside HVAC Systems

What Are The Risks of Using UV Light To Kill Mold?

As with any home product, there will be pros and cons when using it. UV lights are great products, but they can be used incorrectly and lead to health problems. And if a UV light is used to replace mold remediation, it can be harmful because you aren't truly solving the problem. Here are a few risks of using UV light to kill mold:

Damages Skin & Eyes

When using UV-C light as a disinfectant, the biggest risk is damage to your eyes and skin. Similar to sunlight exposure, it can quickly cause damage to your skin (similar to a sunburn) and eyes. UV light exposure can even result in blindness due to damage of the corneas. 

With UV lights for HVAC systems, it shouldn't be a problem because it is installed inside of the ductwork so humans are never exposed to the radiation. Most HVAC UV-C lights even have 'sight glasses' so that you can know the ultraviolet light is on and working without having to open up the HVAC system.

If you plan on using a household or room UV sanitizer, people are more at risk since it isn't an enclosed system. However, virtually all ultraviolet disinfecting lamps have remote controls and countdown timers so that you never have to be exposed to the light.

And with portable or travel 'wand' UV sanitizers, you just to be careful that you never point the UV-C light onto your skin or eyes.

Doesn't Replace Mold Remediation

Unfortunately UV lights don't replace the need for mold remediation. In HVAC systems, if there is mold growth inside the furnace or ductwork, it needs to be cleaned first. If the powdery mold isn't cleaned first, the UV light will still kill the mold, but the mold will turn into a toxic dust and may get blown throughout the entire home. 

For HVAC systems, UV lights should be installed only after you clean the ducts and remove any visible mold. And if you use an ultraviolet light to kill black mold in a basement, crawlspace, or other area — you will still need to physically clean or remove the mold after it is killed. If you just leave mold in place, it will eventually re-grow if it can get enough moisture.

Read Also >> What Are The Signs of Black Mold On Air Vents?

Won't Solve Moisture Problems

And if you had some kind of roof leak, plumbing leak, or just high moisture in the basement — you will still need to solve that problem otherwise the mold or mildew will almost certainly happen again. Using UV light to kill mold is not a substitution for actually solving the water issue that led to the fungus problem.

You may want to hire a qualified home inspector or contractor to pinpoint the source of the moisture which is what led to the mold issue.

Determining the source of water or moisture can be tricky. I once inspected a garage that was covered in mold, and it happened because there was an uninsulated air duct that went through the garage. The moisture condensed inside the air duct, releasing moisture into the garage, and leading to mold growth.

Read Also >> How To Prevent Mold in Air Conditioners?

does uv light kill mold in hospitals

How To Get The Best Results When Using UV Light To Kill Mold?

The effectiveness of UV light in killing mold depends on multiple factors, and some of the most important things are:

1. Light Intensity

If the UV light is very weak, then it won't be nearly as effective. Most UV lights installed in HVAC systems have light bulbs in the 8-watt to 16-watt power range. In fact, these lights are so powerful that they even have sight glasses (peep holes) so you can verify the ultraviolet light is on without directly looking at it. At this level of intensity, exposure can damage your eyes and skin. 

2. Line of Sight

In order for UV light to work in killing mold, there has to be line-of-site. If the mold is growing behind an object, and UV-C light isn't directly hitting the mold, then the germicidal effects of the light will be largely non-existent.

The light has to actually reach the mold for it to have any type of effect. This is one reason why you never see mold or mildew growing in direct sunlight. Natural sunlight has a weak germicidal effect on microbes due to the ultraviolet spectrum.

How long does it take?

With direct exposure to ultraviolet light, it only takes 1-3 hours to kill mold.

3. UV Lamp Debris

If the UV light bulb is coated in dust or debris, it can also greatly lower its reach. This is why periodically cleaning the UV bulb or replacement is so crucial. A dirty UV bulb can halve the sanitizing power of a new & clean bulb.

What UV Light Products Can Kill Mold?

Since ultraviolet light can be so effective for disinfection, different types of products are available to homeowners in order to kill and prevent mold. Here are a few different types:

HVAC UV Lights

Perhaps the most common germicidal UV product is the HVAC UV Sanitizer. These custom UV lights are installed inside of your indoor air handler, furnace, or heat pump.

does uv light kill mold (hvac uv light)

HVAC UV lights will have usually have one or two UV bulbs, and these UV lights are normally on 24-hours a day inside of your air handler — killing microbes and mold.

There are even more premium models that will turn on automatically based on whether the A/C or heat has actually turned on.  This feature can significantly lengthen the life of the ultraviolet light bulb (one of the biggest ongoing expenses is changing the bulbs yearly).

Read Also >> What Are The Best UV Lights For HVAC Systems?

UV Light Water Sterilizers

There are also UV products designed to purify the entire homes water supply. These systems will kill microbes, germs, and pathogens that are in your water supply that either the municipal system misses or because you are on well water and need disinfection. These whole water purification systems are installed at the main water supply, usually installed immediately after the main water shutoff.

Portable UV Disinfection Lamps

Another increasingly popular product is the portable disinfection lamp. These products are designed to be very small and portable and can be used to quickly disinfect pillows, faucets, door knobs, and other small objects. Some of these portable UV wands will turn off automatically if it is held upright in order to prevent damage to your eyes.

Whole Room UV Germicidal Lamp With Timer

These are ultraviolet lamps left in the room with a countdown timer and remote control. You can place this household UV lamp in a basement or room, leave the area and turn it on with a remote control.

The remote is important so that the UV light doesn't damage your skin or eyes. The countdown timer will be adjustable so you can have the UV light turn off automatically in 15-min, 30-min, 1-hr, or 4-hrs.

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4 thoughts on “Does UV Light Kill Mold? (3 Reasons It Works)”

  1. I have a large vinyl collection which has been stored in the crawl space for 20 years.
    There is a musty smell about them and I was wondering if uv would be effective as a way to remedy this. Is so, can you offer a product please?

    Reply
    • It’s hard to say since I don’t know the size of the area. You should also consider improving the ventilation of the crawl space. You can install a crawlspace ventilator fan to increase the airflow.

      Reply

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