Does Sunlight Kill Mold? Is It Really Effective?

You may have noticed that mold and mildew tend to grow in the shadier parts of your property (attics, basements, bathroom ceilings, etc), so why is this the case? Is the sunlight actually killing the fungi or does it simply prefer being in the dark? Does the light need to be natural in order to kill it or can it be synthetic? In this article, we answer all these questions and more, so, for all you need to know about whether or not sunlight kills mold and mildew, keep reading.

Can sunlight kill mold

Does sunlight kill mold?

We’ve established that fungi don’t grow in sunny spots in the home, but it’s not quite as simple as sunlight killing it outright, there’s a little more to the story, so let’s dive a little deeper into the details.

UV light

Both mold and mildew are killed when they are exposed to UV radiation for extended periods of time. The sun emits UV rays in three classifications, UVA, UVB, and UVC.

Whilst UVC radiation will kill mold and bacteria by damaging their DNA, nearly all of the UVC rays emitted by the sun are absorbed by the earth’s ozone layer, the same layer that prevents us humans from being burned to a crisp.

UVB and UVA rays can penetrate the earth’s ozone layer and reach the ground. These rays are not as powerful as UVC radiation, however, they have germicidal effects that can help to prevent mold and may be able to kill younger fungi whilst more mature patches will be able to withstand these rays.

So it is possible for natural sunlight to kill some fungi, but it takes time to be effective. Using a UV lamp that can emit UVC rays directly onto the mold is a faster and more efficient method.


Mold requires a humidity level greater than 55% in order to be able to live and breed. When sunlight hits pools of water, it heats the molecules and causes them to evaporate into the air, and if there is enough airflow, the moisture-laden air will be removed from the room, leaving no moisture for fungi to use for hydration.

In this instance, the mold wouldn’t “die” as such, but it would dry out and become dormant.

So, given enough time, sunlight can kill mold, but it also acts as a fantastic deterrent, as it can create an inhospitable environment for fungi by denying it the main components it requires to live and breed successfully.

What if the light is indirect?

We’ve seen that direct sunlight may be able to kill mold in its early stages but will struggle to do so when the growth is more mature, so if the sunlight is indirect and so much less powerful, the light will almost certainly not be able to kill the fungi.

This is not to say it is not useful, however. The very fact that a room has sunlight passing through it makes it a far less hospitable environment for mold to grow in, and so whilst it may not be useful in killing it, it certainly can help to prevent its growth initially.

The additional factor to consider here is that when sunlight passes through a pane of glass, the UV rays are filtered out, which protects us from harmful radiation. This means that any light that enters a room through a window is void of UV rays, and so is ineffective at killing mold.

Does sunlight prevent mold?

Yes, sunlight prevents mold growth by reducing overall moisture levels and creating a dry and inhospitable environment that will provide very little for any mold spores that settle on surfaces.

How long does it take for it to kill mold?

Direct sunlight will kill young patches of mold much more quickly than established growth as they are more susceptible to the UVA and UVB rays that can reach the earth’s surface. In this instance, the mold would be killed within 1-2 hours of continuous, direct sunlight.

In order to kill more established mold and mildew, UVC rays are much more efficient. These rays can only come from artificial sources such as UV lamps. In this instance, even established mold would be killed within 2 hours of direct light exposure.

What about black mold?

True black mold (Stachybotrys chartarum), is very similar to other strains of mold. It has the same requirements in terms of hydration, nutrition, and oxygen, its main difference is that it is able to produce mycotoxins, which can be harmful to humans.

With this being said, direct sunlight would have just as much effect on black mold as it would any other strain. As long as you take away its source of moisture and nutrients, the mold will dry up and not be able to spread, making it easier to remove. The germicidal UVA and UVB rays are more effective on some strains of mold than others, and they may take more time to be effective, or may not be effective at all in actually killing the mold.

Are there any drawbacks to using sunlight to kill mold?

Some of the biggest drawbacks to using sunlight to kill mold, are that it can take a considerable amount of time to dry the mold enough to stop it from replicating, and also it may not kill the mold entirely.

Windows will also need to be open in order for any UV rays to affect the mold, and the light will also need to be direct for as long as possible, which can be very difficult to achieve. The amount moisture within the air and other environmental factors also play a large part in the efficacy of sunlight when it comes to killing mold.

Certain materials are also very difficult to treat with sunlight, anything with a porous surface for example would either take considerable time or may not be affected by sunlight at all if it could not penetrate deeper into the material. In this example, fungicidal solutions and chemicals would be more effective, as they would be able to seep into the material and kill the embedded mold or mildew.

What are some of the advantages?

Some clear advantages of using sunlight to remove mold are that it is environmentally friendly as it involves no chemicals, it is effective at drying the mold enough for it to be removed easily, and is of course, completely free.

Is it worth using sunlight to kill mold?

Unfortunately, the cons heavily outweigh the pros when using sunlight to kill mold. Using either specific chemicals, home remedies, and professional mold remediation is vastly superior to using sunlight. These methods are more effective and take considerably less time. If the main reason for using sunlight is that it is environmentally friendly, then many methods that are highly effective can be used instead, such as using UV light (which makes use of the UVC radiation we spoke of earlier), vinegar, baking soda, and plenty more.

Does light in general kill mold?

No, there are some sources that state that having lamps on may help to prevent mold growth, but this certainly won’t kill it. It is more likely that the heat that a lamp emits will reduce moisture levels and dry the mold out, preventing it from spreading rather than killing it. In order for the light to actually kill fungi, it must emit some kind of UV radiation, which standard lamps do not.

When should you call a professional?

Using home remedies and remediation products will be adequate to control the majority of mold growth. However, if after several attempts at removing the fungi yourself, you find it quickly returning, there may be a deeper issue within the property that needs to be investigated by a professional.

A remediation expert will be able to come into your home, assess it, and discover if there are any burst pipes, dampness, or any other source of moisture that mold can use, that may not be immediately obvious.

They will then be able to either deal with the issue themselves or will recommend what services need to be performed to rid you of mold for good.


Sunlight is not the most efficient way to kill mold, as the particular type of UV radiation required to kill fungi effectively does not reach the Earth’s surface and the germicidal properties of the UVA and UVB rays that can reach the Earth take considerable time to be effective at killing fungi. However, it can prevent its growth, as areas with direct sunlight are often warmer and less likely to have the kind of humidity levels required for its growth. This is why you most often see mold growing in dark, damp areas of the home shielded from sunlight. Using sunlight as a method of killing mold is ineffective, and often only dries it out, leaving it to become active once again when sufficient humidity levels are met.

Chris Walker

Chris Walker has struggled for several years with mold after buying his own property. After finding the solutions to several issues around his home, he decided to create this site in order to answer as many questions about mold and mildew as possible to help others dealing with the same problems.

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