Does UV Light Kill Mold? (3 Reasons UV Light Is A Fungus Killer)

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.

UV light with a short wavelength—such as 253.7 nanometers—will kill existing mold and prevent it from growing again.
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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.

The story of using UV light to kill microbes goes all the way back to 1878 when Arthur Downes wrote a scientific paper describing the sterilization of bacteria to short wavelength light.

UV light has been used in a variety of industries for things like:

  • Water disinfection
  • Mold remediation
  • Food sterilization
  • Hospital operating room sterilization
  • HVAC air purification

But how exactly does UV light kill mold?

Ultraviolet light can kill mold on surfaces or in the air within a few hours of contact. UV light is a mold killer because the UV-C light penetrates deep into the nucleus until the organism becomes disabled and dies. The damaged DNA will prevent further reproduction of the mold spores.

According to the research by the US National Library of Medicine on Light-Based Anti-Infectives, it was reported that the disinfectant mechanisms of UV light cause direct damage to DNA by changing the arrangement of the genetic molecules — leading to reproductive defects and cell death.

In this guide, I will go over a few of the top reasons that UV light is used to kill mold, some of the risks involved with ultraviolet rays, and the optimal conditions for using UV light.

3 Reasons To Use Mold Killing Ultraviolet Light

There will always be an advantage to killing mold if you can do it from a distance. Mold can release harmful mycotoxins and allergic spores into the air. Anytime mold is removed from a home, wearing protective gear is advisable. Here are the top reasons UV light is used to kill mold:

#1. Sunlight Is Already A Natural Disinfectant (And Contains UV Rays)

Mold loves darkness, and it simply can't grow in direct sunlight. 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.

#2. UV Light Destroys Mold Without Touching It

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.

#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. If you want to see what black mold growth looks like inside HVAC systems, I invite you to check out my guide of pictures of mold inside ductwork.

Risks Of Using UV-C Light

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 three of the top 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. 

I recently wrote a guide you may find interesting on the signs of black mold on air vents which shows you how to identify mold inside HVAC systems.

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.

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.

The Science of UV Light Mold Disinfection

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UV light has been studied in laboratories for over one hundred years and it is known to kill viruses, bacteria, and fungi (mold). 

The UV-C Short Wavelength

Scientists discovered a long time ago that there is a certain wavelength range where microorganisms are vulnerable to damage — and this is the UV-C short wavelength— specifically the 100 to 280 nanometer wavelength zone. Besides UV-C, there is also UV-A and UV-B which has different uses but generally not used to kill mold and mildew.

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.

How Does Ultraviolet Light Destroy Mold?

Ultraviolet irradiation works as a disinfectant and uses electromagnetic waves to penetrate through microscopic organisms.

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. 

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Three Factors of Germicidal Light Effectiveness

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. If you want to learn how to install a UV light to kill mold in your HVAC system, check out my installation guide for HVAC UV lights.

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.

Ultraviolet Light Home Products

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.

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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). If you want to learn more about UV lights for HVAC systems, I invite you to read my guide on the best HVAC UV lights to kill mold.

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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38 thoughts on “Does UV Light Kill Mold? (3 Reasons UV Light Is A Fungus Killer)”

  1. I I have a crawl space that I jest remediated from mold do to a pipe leak I was thinking to run a 100 feet Eletric wire whit socket whit UVc light to have more disinfect I well light at night for 3 H but I really do not know were to by the bulb can you help me thanks Sabrina

    Reply
    • To be honest, I wouldn’t go that route. UV light needs line of sight for it to kill mold, and in a crawlspace, I think it is unlikely to reach most areas due to joists and possibly insulation. I think it may be better to put in a wifi connected humidistat so you can monitor the moisture levels. If the humidity is kept below 40-50%, it is almost impossible for mold to grow. That means having adequate ventilation in the crawlspace.

      There are mechanical crawlspace vents that you can buy to improve the ventilation such as this one with a 220-cfm rating.

  2. Hi Arie. I have a 10×12 outdoor screened in porch that gets no direct sunlight. Have patio furniture and the cushions keep getting mold on them every few weeks. Could I use a UV-C lamp that has 60 Watt Bulb. Can I just set it in the middle of the porch at night for 2 hours and it would kill the mold. Would I need to do this every night or just maybe once a week for 2 Hours to maintain. Was looking at the nVc Cleanaire UV-C Lamp at Home Depot. Or would you suggest a different one and wattage??? Thanks.

    Reply
    • Hey RB,

      I think the Cleanair lamp could work, I don’t have experience with it on furniture TBH. And yes, I imagine you will have to do it regularly every few days or every week to maintain it.

      I was looking at the manual of this lamp and it says you have to keep it within 3.5-feet for surfaces under the ‘additional notes’ section, so that may be an issue. You can see the manual here.

      You may also want to try a low tech solution first such as this patio furniture protector by Mold Armor. It claims to prevent mold/mildew for up to 3-months, then you respray.

      Arie

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Arie Van Tuijl

Arie Van Tuijl

I am a licensed home inspector in two U.S. states and the founder of Home Inspector Secrets. After performing hundreds of inspections, and seeing thousands of house defects, I realized people would love to have an online resource dedicated to home maintenance. I write detailed home guides and product reviews sprinkled with inspection tips. You can read my bio here.

About Home Inspector Secrets

Home Inspector Secrets is an online resource for owners, buyers, and sellers to understand all aspects of home maintenance. We have detailed home guides, product reviews, inspection advice, and much more.

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Looking to learn more about home maintenance? Check out our other informative home product reviews and guides!