Does UV Light Kill Mold? (2020 Ultimate Guide)

Do you want to know if ultraviolet light will actually kill mold?

UV radiation has been in use for years by hospitals to disinfect operating rooms and medical equipment.

In this guide, I will be going over...

  • How UV light kills and prevents mold growth
  • The different types of germicidal UV light products to kill mold
  • Why I always recommend installing a UV light in your HVAC system

Let's get started with this guide!

does uv light kill mold

Will UV Light Actually Kill Mold?

Under the right conditions and with a properly designed product, germicidal UV light can be effective in killing mold --- and preventing it from taking root.  UV light has been in use for many years as a disinfecting technology to not only kill mold, but also bacteria, viruses, and other pathogens.  

In fact, as early as 1878, Arthur Downes wrote a scientific paper describing the sterilization of bacteria to short-wavelength light.

Ultraviolet light acts very similarly to humans getting a sunburn. Sunburns are caused by longer wavelength light, but the principle is very much the same.

Mold and other microbes have far less protection against the germicidal effects of light than humans.

Read Also: What Are The Signs Of Black Mold In Air Vents?

The UV-C Short Wavelength

Scientists have discovered that there is a certain wavelength range where microorganisms are vulnerable to damage --- and this is the UV-C short wavelength range --- specifically in the 100 to 280 nanometer range.

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 microogranisms. 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: What Are The Best UV Lights For HVAC Systems?

How Does UV Light Kill Mold?

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

The wavelengths are of various sizes --- between 255 and 280nm --- known as the Germicidal UV-C that kills microorganisms. The ultraviolet light irradiates the immediate environment in which it is placed such as in an indoor air conditioner.

And when the germicidal rays come across molds, they penetrate into the nuclei of the molds, damaging the nucleic acids present --- until their the organism becomes disabled. 

According to the research work found in the US National Library of Medicine on Light-Based Anti-Infectives, it was reported that the disinfectant mechanisms of UV light cause actual damage to DNA by changing the arrangement of the molecules.  

When the DNA "arrangement" is changed, it causes defects in the cell reproduction of the mold and it will eventually lead to cell death --- when the cells die, the mold dies.

Read Also: 7 Pictures Of Mold In Air Ducts

does uv light kill mold in hospitals

What Are The Best Conditions For 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 UV light is on without directly looking at it. At this level of intensity, exposure can damage your eyes and skin.

Read Also: What Are The Best Honeywell UV Lights For HVAC?

#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 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 growing in direct sunlight. Natural sunlight has a weak germicidal effect on microbes.

#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.

Read Also: How To Install An HVAC UV Light?

What Are The Different Types of Mold-Killing UV Lights?

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

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. Usually composed of one or two UV bulbs, 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: 7 Pictures Of Mold In Air Ducts

does uv light kill mold (hvac uv light)

UV Light Water Sterilizer

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.

does uv light kill mold (uv water sterilizer)

UV Light Whole House Water Sterilizer

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 lamps will turn off automatically if it is held upright in order to prevent damage to your eyes.

does uv light kill mold (portable uv disinfection lamp)

Portable UV Lamp Sanitizer

What's The Bottom Line On UV Light Killing Mold?

UV light can be highly effective in killing and controlling mold in either an HVAC system, water supply, or even for food.

Hospitals, food processing centers, and water treatment facilities have been using germicidal UV light for years, and I'm sure UV will continue to be used to sterilize a variety of objects.

Other Home Guides

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

13 thoughts on “Does UV Light Kill Mold? (2020 Ultimate Guide)”

  1. Hey Michelle,

    There are portable UV light sanitizers on the market, you can use these in your bathroom as short term measure, and they are generally in the 25 to 50-watt power range.

    With these portable UV lights, you still don’t want the light exposed to your skin or eyes, and you want to buy one with a timer that will automatically turn on & off.

    The longer term solution though is to have an effective bathroom exhaust fan to remove moisture. Bathroom exhaust fans should be on for at least 20 minutes after each shower.

    If there is mold growing on the caulking, I would try scrubbing the caulking/grout first with a cleaner to see if it can be removed. If not, you may have to re-caulk or re-grout the bathroom.

    Silicone caulking won’t allow mold to grow on it, but if caulking is never cleaned, there can be a thin dirt film on the caulking that allows mold to take root.

    Good luck!

    Reply
  2. We have/had mold in the air handler and coil area of our furnace. A local HVAC company installed an apco uv light kit that supposedly kills mold, viruses, bacteria, etc., along with purifying air and stopping household odors. I would like to understand what actually happens to the mold that exists already within the furnace – does it just die and remain there as a stain? Does it die and get released into the rest of the duct system, hopefully being caught by the filter? Does it get burned away by the uv light? How long does this process take? Do I need to call in an abatement company to clean out my unit? Hoping for some understanding and direction – Thanks so much!

    Reply
    • Hi Sherri,

      If there is existing mold in the air handler, the UV light will kill the mold, and it will turn into a type of “mold dust”. Unfortunately, this mold dust can still by highly allergic. Whenever I do a home inspection, I always recommend cleaning the air handler first of any visible mold, and then installing the UV light to prevent the mold from growing back.

      In addition, a UV light will only kill mold if it has a direct line-of-site to the mold. As you know, a furnace or heat pump has many different areas that make it difficult to reach with light (like the blower fan, evaporator coil, etc.).

      There are UV lights that are designed for air treatment rather than surface treatment. These UV lights are installed at the return side of the air handler, and these tend to be higher quality and more effective than the “surface treatment” UV lights which are installed on the supply side. However, they are also more expensive.

      Anyways, please clean the air handler of all visible mold, this is an important step.

      Good luck!

      Reply
    • Hi Pam,

      Ordinary grow lights do not have any type of anti-microbial effect on plants (at least nothing substantial). There are actually products designed to emit UVC light to sterilize plants. Perhaps the largest company is based in the Netherlands and called CleanLight. One of there products is called the Hobby Unit Crop Protector, check it out here.

      Good luck!

      Reply
  3. Hello there, my parents live on a fix income. They live in a mobile home that I am sure has black mold. The roof leaks and dripping rain is caught by pans in a few parts of the house. My father almost lost his life do to a fungus infection. Possibly valley fever. We live in the valley of California. He’s in the last stage of lung disease. I need molds spores fungus and viruses and bacteria to be killed. I am about to go buy a uvc 40w light off ebay. Should I buy a stronger one? I think I found a 60w uvc light. They create ozone. Dont think I need ozone. I just need to kill these living things before they kill my dad. Thank you

    Reply
    • Hello Teresa,

      Sorry to hear about what’s happening with your parents home and your Dads health.

      If the UVC light creates ozone, that isn’t a problem, as long as you air out the home after use — you don’t want to breathe in ozone, it can be hazardous.

      In fact, there are ozone generators made to kill mold such as the OdorStop OS3500UV Ozone Generator.

      Generally, the higher the wattage, the more powerful the UVC, but 40-watt may be fine. I am not really familiar with mobile homes, I have never inspected them. I would definitely look into the ventilation system and see if there is any fungal growth there. I would also buy a cheap humidistat that will tell you the relative humidity of the mobile home.

      I would also focus on the basics, such as fixing the leaks, cleaning any visible mold, and making sure the indoor humidity stays within 30% to 50% all day long as recommended by the CDC. Buy a dehumidifier if necessary. Mold requires moisture for it to grow.

      What I did recently for my bedroom is buy a 20-inch box fan for $19, and then I taped a 20-inch x 20-inch HVAC filter to it (arrow pointing in direction of air flow). That also can really help with the indoor air quality. Just change the filter every few weeks.

      Good luck!

      Reply
  4. Clothes closet is near the bathroom with an exhaust fan. Mold gets on shoes etc. What wattage of a UV bulb should I use for inside closet.

    Reply
    • Hi Betty,

      Just for a small closet, anything in the 20 to 40 watt range should be fine. I would recommend buying one with a timer. You don’t want UV-C light on your skin or eyes — please be careful.

      Also, the real problem is excess moisture in that area from the bathroom. According to HVI, bathroom fans should be left on at least 20-min after showers. Replacing your wall switch for one that includes a timer is highly recommended, here is one on Amazon made by GE.

      There are also bathroom fans with humidity sensors that will turn on/off automatically. Here is my article on that; Best Bathroom Fans With Humidity Sensors.

      Good luck!

      Reply
  5. I had black mold in my utility closet which is next to my bathroom, which has a window but no exhaust fan. I totally changed drywall, and repaired, however, I am petrified it will come back. So I bought the following light: BOAITE UV Germicidal Lamp UVC with Ozone Light Bulb E26 25w 110v Cleans Air
    However, I read that Ozone could hurt your lungs, was this the right purchase, and if so, how long should I keep it on, because it really smells weird after one minute. PLEASE HELP, I am really afraid my lungs will be damaged with this Ozone light and is killing me instead of the mold!.

    Reply
    • Hi Vivien,

      Ozone can be effective in removing odor and killing mold but it is detrimental to your lungs. Most manufacturers recommend letting the room air out for 30-60min before reusing that area — or at least wait until all of the ozone smell is gone.

      I can’t really say whether it was the right purchase, but the true cause of the mold is the excess moisture from the bathroom. You really need to install a bathroom exhaust fan which all modern bathrooms have. In fact, you are supposed to use the fan during any shower and 20-30min after each shower to remove moisture. Mold requires moisture to grow.

      Windows just don’t really cut it. Depending on where you live, the outdoor air can also contain excessive humidity if you just open a window.

      I would also buy a cheap humidistat (~$10) and monitor the humidity of your closet. Ideally, the humidity shouldn’t go above 60% but between 30% and 50% is what you should shoot for.

      Good luck!

      Reply

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Arie Van Tuijl

Arie Van Tuijl

Arie is the founder of Home Inspector Secrets, an online resource dedicated to helping people understand how homes work. He is a licensed home inspector in two U.S states and owns a residential and commercial inspection company. To ask Arie a question, please use the comment box at the bottom of the relevant article.

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