Does Sunlight Kill Mold?

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Written ByJennifer Updated

One of the most frequently asked questions I get when I am on a consultation call with someone who is determining what mold contaminated items can be saved is “does sunlight kill mold?” The reasoning behind the question typically stems from the logic that you don’t see outdoor mold growing out of control everywhere so something must be preventing it from growing or slowing the rate of growth.

We know that mold typically needs moisture or another food source to grow. We know it needs oxygen. We know it is happiest at warmer temperatures. And we know that mold prefers dark places but won’t shy away from light and bright if other conditions are met. So it does stand to reason that sunlight isn’t mold’s most favorite thing. While outdoor mold can be a big issue due to the never-ending supply of moisture and food, it can be argued that it is kept in check by the sun.

Let’s take a deeper dive into sunlight and mold. I think you might be surprised at the real relationship between these two.

Mattresses stacked on a wooden bedframe with a green wooden ladder from bottom to top. Handmade wooden planters, stained. They are getting sunshine in a grassy yard.

How Does Sunlight Kill Mold?

You probably have heard of Ultraviolet (UV) rays which are produced by the sun. These are those pesky rays that you get warned about. But they do have some very good qualities as well.

UV-C is a very short length of UV light. One of it’s superpowers is producing radiation that destroys fungal cells. It kills mold and other bacteria by penetrating the organism’s cells and damaging the DNA within. More specifically, UV-C rays work by disrupting the nuclei of microorganisms.

Once a mold spore’s DNA is damaged it can no longer reproduce. So instead of spreading or perpetuating itself, the mold cell eventually dies.

Although there is outdoor mold, the UV-C rays control mold concentrations which is why it doesn’t grow literally everywhere outside.

How Quickly Does Sunlight Kill Mold?

With direct exposure to the sun’s ultraviolet light, it takes 1-3 hours to damage a mold spore’s DNA and stop it from reproducing.

Does Indirect Sunlight Kill Mold?

No. It’s as simple as that. Window glass and cloud cover filter out UV light and therefore the sun coming through will have little to no impact on mold.

Are There Disadvantages To Using Sunlight To Kill Mold?

  • First, sunlight is not a replacement for remediation and mold removal. If you have a mold issue in your home, there is no way of fixing it with the sun.
  • While the sun may indeed render mold spores unable to replicate, it won’t remove the mold or mold spores from whatever item you have basking in the sun. You still need to remove as much mold from the item first and then allow it to get some sun.
  • The sun can only impact visible mold.  The light has to actually reach the mold for it to have any type of effect. If mold is growing from the inside out, the sunlight will not be able to reach all of the mold spores, only those on the surface of the object.
  • If the moldy object is coated in dust or debris, it can also greatly lower the sun’s UV rays from reaching the mold spores.
  • Trichothecenes resist sunlight. Trichothecenes are just one variety of mycotoxins. The concern with this is if you have a highly contaminated environment and are relying on just the sun to clean your items, you may be leaving behind an untold number of health damaging mycotoxins.

Is It Worth It To Use Sunlight To Kill Mold On Items?

If you have an item with surface mold, there is a pretty good chance that after you properly clean off the mold that the sunlight will prevent any mold from returning. So direct sunlight for 3 hours will likely tackle any mold spores that were inadvertently left behind on an object after a mold removal protocol was followed.

The types of items that tend to have the most success in the sun are:

  • Freshly washed clothes, towels, sheets, and other thin fabric items
  • Plastic items
  • Leather items (be careful as sunlight can bleach leather)
  • Houseplants
  • Some wood items although this can be tricky since mold really loves to root deep into wood

Remember – anything you decide to set in the sun should have all visible mold removed first.

As a mold professional I personally would not rely solely on the sunlight to kill mold. But it is a great compliment to other mold removal techniques.

A glass door partly open with sunlight streaming in

Sources:

Jennifer

Jennifer is a Certified Mold Inspector from the International Association of Certified Mold Inspectors. She has also completed the esteemed Mold Masterclass. Jennifer is currently enrolled in the Professional Home Inspection Institute's Certified Residential Mold Inspector and Mold Remediation programs as part of her Continuing Education Units from the Institute of Inspection Cleaning and Restoration Certification (IICRC). She is a member of the International Society for Environmentally Acquired Illness. Jennifer’s earlier years afforded me the opportunity to graduate with honors with a Bachelor's Degree in Ethnic Studies.

Having gone through her own mold nightmare spurred Jennifer on to become a certified mold inspector with the goal of helping others avoid extreme situations.

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