Does UV Light Kill Mold? (How Effective?)

Do you want to know if UV light is actually effective in killing black mold? How does it work?

Mold in your a/c system is a terrible thing to experience --- but there are solutions. Here is my review on the best UV lights to kill mold in your air ducts.

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

does uv light kill mold
  • How ultraviolet light kills mold
  • Why hospitals use UV light to disinfect operating rooms
  • A quick review of how to install uv lights for hvac systems
  • Other ways to kill mold in air conditioners
  • The common causes of mold in the central ac

Since I know how scary it is to find mold hanging around your air vents and interior air handler --- I have put together this in-depth guide on the effectiveness of germicidal ultraviolet light to kill mold. I frequently recommend installing UV light bulbs to my home inspection clients.

Let's get started with this guide!

Read Also: Top 5 Best UV Lights For HVAC

Will UV Light Actually Kill Black Mold?

It has been proven in numerous studies and actual application that UV light (at a specific frequency) will kill mold, mildew, and other pathogens. UV light can be so effective because it blankets the environment in light --- disinfecting the whole area --- which would be very difficult to do using manual cleaning methods.

Ultraviolet light is used in numerous industries for microbial decontamination such as formedical purposes,  water treatment, and air purification. The benefits of UV light has been known  since the early twentieth century. The Berkeley Lab proved that ultraviolet lights is germicidal when it produces short wavelength frequencies. The short wavelength known as UV-C is responsible for destroying the nuclei of microorganisms. These short wavelengths either kill the mold cells totally or make sure they are unable to reproduce until they soon die out.

Don't underestimate the power of ultraviolet light. When installed on HVAC systems, UV lights have sight glasses so you can verify that it's on without endangering your eyes. If you plug in a UV light outside of your furnace, it can damage your skin and your eyes --- be careful!

The effectiveness of UV light for mold control is actually dependent on the intensity of the light and the amount of time that the molds are exposed to the light. So if you use a UV light with the wrong wavelength or if the mold  is not fully exposed to the light...then the UV light may not kill all of the mold in your air conditioner or furnace.

For UV light to actually kill mold, there has to be a balance between it's intensity and exposure period to the mold. The other factors that determine if UV light will actually kill mold are the humidity of the environment, the UV system design, and where the UV light bulb is installed.

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

How Does UV Light Compare To Other Ways of Killing Mold In My A/C?

Besides using ultraviolet radiation to kill mold, there are a few other basic techniques that can be used to control mold and mildew in an air conditioning system. Each of these techniques have their advantages, disadvantages, and particular uses. 

Manual Cleaning of Mold From The HVAC

Physically cleaning mold is a very common way of killing mold and according to the United States Environmental Protection Agency --- there are certain regulations which must be followed before contractors use this method.

Manual cleaning of mold in the AC unit requires that you find a good mold removing agent. One commonly used agent is a baking soda detergent solution because it contains some bleaching properties and can remove discolored patches or growth such as mold from surfaces.

The air conditioner must first be turned off. Using a rag or brush, you scrub off the surface where the molds have grown. If you have ever done this before, you will know that this manual mode of killing molds consumes a lot of time and requires a lot of energy. You have to be careful that you are thorough and don't miss any areas.

It's important to wear proper protection when cleaning mold because it is unhealthy to be so close to molds. The cleaning agents could also react with your skin during application --- so you need to be extremely meticulous (Cornell Cooperative Extension).

If you have visible mold in your ac unit, it's recommended to physically clean the mold before installing a UV light. With a high quality UV light and proper installation, cleaning mold from your air conditioner should be unnecessary afterwards. And remember to change your UV light bulb every year or at least clean the bulb annually --- dust reduces the bulb's effectiveness.

Fogging Machines

The main type of fogging machine used for killing mold is called a cold thermal fogger, and it used to spray a breathable disinfectant throughout the air duct system. Please use a botanically based disinfectant --- do not spray bleach throughout your air ducts! A plant based disinfectant such as the solutions made from Benefect are safe to use in your air ducts.

Since a UV light is only installed onto an air conditioner, it will only inhibit and kill mold growth at that particular location rather than the rest of the air ducts and vent covers. However, in my experience, 95% of the mold in an HVAC is at the indoor furnace or air handler. Sometimes though, mold is found on the vents and elsewhere in the duct system. In this case, a fogging machine is the solution.

A fogging machine sprays a breathable disinfectant throughout the entire duct system, coating the metal ducts and air vents with a botanical disinfectant. Besides killing mold, the botanical spray will also odorize the air ducts and has a pleasant smell.  Of course, to keep getting benefits from a fogger, it is recommended to fog the air ducts a few times each year. 

The process for fogging air ducts is simple. Turn the a/c on at the thermostat so that the blower fan is running. Then you simply spray the botanical solution into either a return vent (remove the filter) or you spray it directly into the air handler with the cover off. Make sure to avoid saturating the furnace filter, and you should also probably avoid any electronic components of the a/c. Even though it is a fine mist, if you directly spray this onto an HVAC circuit board, it can cause damage.

does uv light kill mold in hospitals

Why Does Mold Grow In My AC System?

Air conditioner systems are vulnerable to the growth of microorganisms because mold spores are naturally present in the air. Just think of how mold suddenly starts growing on old fruit and food -- it is a natural process because mold spores are everywhere. Unfortunately, the interior air handler or furnaces ticks off the three requirements for mold growth. These three things are...

  1. Darkness

  2. Food source (dust)

  3. Moisture

Your air conditioner clearly has those three things, and even more so if your A/C is very dusty. Dust, which is main dead human skin cells, can easily accumulate inside your air handler, especially on the housing, the blower fan, and coils. 

Besides dust, the air conditioner generates a lot of water as part of the air conditioning process --- in the industry we call this water "condensate". This is why all air conditioners (the inside unit) will have white pvc pipes to carry away the water to a drain (usually a floor drain).

In addition to dust and moisture, the interior air handler is very dark. Light in general is a natural disinfectant. Even non UV-C wavelengths still have a weak disinfectant property. It is even a recommended survival strategy to sterilize water in a clear plastic using only sunlight.

How Is  A UV Light Bulb Installed On My HVAC System?

In short, the ultraviolet light bulb is screwed onto the cover of the furnace or air handler --- and it is placed so that the light is spread with as much surface area as possible. You want the ultraviolet light to cover the evaporator coil, the blower fan, and all of the furnace housing.

Most UV lights are simply plugged into a nearby 120 volt outlet, it's really that simple. The UV light will have a sight glass so that you know that it's on. You do not want to have UV light damage your skin or eyes --- only turn plug it in when installed!

Finding A Skilled Contractor

If you want the UV light professionally installed, I always recommend Thumbtack because you can quickly get free estimates, read trusted reviews, and instantly chat with contractors about jobs.

Read Also: How To Install a UV Light in Your HVAC (Is It Easy?)

Why Do Hospitals Use UV Light To Kill Pathogens?

It is becoming more common for hospitals to use germicidal UV light to disinfect contaminated operating rooms as well as for air purification. The presence of microorganisms in a hospital cannot be permitted at all, so hospitals want to ensure that they kill all pathogens --- and UV light is one of the best ways to do that.

Mold contamination of HVAC systems is a relatively common issue, but in a hospital setting it can be dangerous. Here are the reasons why hospitals use UV light to kill pathogens:

Air Purification

If there is any place where the quality of air should be given maximum attention, then it should be in a hospital. The presence of mold in a hospital fills the air with hazardous mold spores which could contribute to the sickness of patients. Hospitals install UV lights to remove all sort of microorganisms as well as mold which can result in infectious diseases --- making the air quality much healthier.


One of the places where mold quickly grows is on organic matter. Organic matter provides nutrient for molds, so whenever foods are stored, especially in a damp environment, the growth of mold can quickly contaminate the food making it unhealthy for consumption. Therefore, hospitals use UV lights to ensure that the places where foods are kept are not vulnerable to the effects of mold and the foods are not contaminated.

Furniture & Hospital Equipment

Hospitals want to ensure that the furniture that are used by the patients do not allow the growth of mold or pathogens which could contribute to a patient’s illness. It would be very difficult to thoroughly sanitize hospital equipment by physical cleaning without UV light. Ultraviolet light is an excellent guarantee that hospital beds and equipment are thoroughly disinfected.

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

I don't think it can be disputed that UV light is highly effective in killing and controlling mold in an HVAC system. Hospitals have been using germicidal UV light for many years now, and I'm sure UV will continue to be used to sterilize a variety of objects.

The great thing about UV light is it's on all day long, constantly guarding your HVAC against mold growth --- actually stopping mold from the very beginning rather than cleaning it after it has taken root. 

Other Home Guides

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

2 thoughts on “Does UV Light Kill Mold? (How Effective?)”

  1. Hey Michelle,

    The UV lights I recommend are for HVAC installations. I think there are a few UV lights on the market for non-HVAC applications. BUT, if they are strong enough to kill mold, then they can also damage your skin and eyes — this is why HVAC UV lights have sight glasses.

    If there is mold on the tile grout or caulking, it would be easier just to clean it with a bleach solution, and then to re-grout or re-caulk. Most modern caulk already has anti-fungal chemicals included as well. Bleach effectively kills mold, maybe better than UV light — it’s just that we can’t spray air ducts with bleach for obvious reasons.

    If the mold is on the drywall, I would first try to clean it off with a bleach solution, but if it is embedded deep within the drywall, it will have to be removed and replaced.

    Even if you used a UV light to kill mold in the bathroom, it would regrow back shortly, and you would have to keep doing it.

    I hope I answered your question.


Leave a Comment

Arie Van Tuijl

Arie Van Tuijl

Arie is the founder of Home Inspector Secrets, an online resource dedicated to helping owners and buyers understand homes. He is a licensed home inspector in two U.S states and also owns a residential and commercial inspection company.

About Home Inspector Secrets

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