Does sunlight kill mold?

Does sunlight kill mold?
The role of sunlight in combating mold.

Does sunlight kill mold? Yes, it does! But keep in mind that there are limitations to using the sun. Here we will cover these limitations and answer some commonly asked questions.

Topics Covered

  • How does sunlight kill mold?
  • How long does sunlight take to kill mold?
  • Does indirect sunlight kill mold?
  • What are the disadvantages?
  • Conclusion

How does sunlight kill mold?

Sunlight contains UV rays, including UV-A, UV-B, and UV-C. UV-C rays have the shortest wavelength and are the most effective at killing mold and other microorganisms.

When mold is exposed to these UV-C rays, the high-energy radiation penetrates the mold cells and damages their DNA. With damaged DNA, mold spores cannot replicate or reproduce. So instead of spreading like they normally would, the mold cells eventually die off.

How long does sunlight take to kill mold?

The time it takes for sunlight to kill mold can vary depending on several factors, such as the intensity of sunlight, the type of mold, and the conditions in which the mold is present.

It generally takes about 1-3 hours of direct sun exposure to damage a mold spore’s DNA and stop it from reproducing.

However, it’s important to note that the effectiveness of sunlight is influenced by the duration of exposure. Longer exposure times can enhance the mold-killing effect. It’s also crucial to ensure that the sunlight engulfs the entire affected area and has a direct line of sight on the mold. On the other hand, if the mold is deeply rooted or growing from the inside out, sunlight may not be able to reach all of the spores effectively.

Does indirect sunlight kill mold?

Does indirect sunlight kill mold?
Does indirect sunlight kill mold?

No, indirect sunlight does not effectively kill mold. As mentioned, sunlight’s ability to kill mold is primarily attributed to the ultraviolet (UV) rays it emits, particularly UV-C rays. However, indirect sunlight, such as light filtered through window glass or affected by cloud cover, significantly reduces the intensity of UV rays reaching the affected area.

That is because window glass and cloud cover act as barriers which filter out a significant portion of the UV light. As a result, the limited UV rays that pass through indirect sunlight have diminished effectiveness in getting the job done. Mold requires direct exposure to UV rays for a prolonged period to experience the DNA damage and cell disruption necessary to inhibit its growth and reproduction.

It is important to ensure direct exposure to the sun’s UV rays in order for it to be effective. This typically requires moving affected items to an outdoor area where they can receive the direct sunlight they need.

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What are the disadvantages of using sunlight to kill mold?

While sunlight can be great at inhibiting mold growth, there are some disadvantages to relying solely on sunlight. These include:

Inability to reach hidden mold

Sunlight won’t reach mold that is hidden within porous materials or in areas with limited light penetration. If it is growing from the inside out or in hard-to-reach places, it won’t be adequately exposed to sunlight. And this greatly limits its effectiveness in killing the mold.

Lack of comprehensive mold removal

Sunlight does not physically remove the mold or spores from the affected items or surfaces. Proper cleaning and remediation techniques, including physical removal and addressing the underlying moisture issues are still necessary.

Dust and debris interference

If mold-affected surfaces or objects are coated with dust or debris, it can significantly reduce the effectiveness of sunlight in reaching the spores. Dust and debris can block or diminish the UV rays, thus limiting their ability to damage the mold cells.

The sun damages certain materials

Prolonged exposure to sunlight can cause fading or discoloration of various materials, such as fabrics, upholstery, artwork, photographs, and painted surfaces. UV rays can break down the pigments or dyes present in these materials, which eventually leads to noticeable color changes over time.

Avoided this by limiting the exposure of direct sunlight on sensitive or valuable items. You can also periodically rotate the items being treated to minimize the risk of damage.

The sun has limited effect on trichothecenes

Trichothecenes are a type of mycotoxin produced by certain molds, such as the Fusarium and Stachybotrys genera. These spores are particularly problematic as they are known for their toxic properties and potential health risks.

Trichothecenes are relatively stable compounds and can withstand tough environmental conditions, including exposure to sunlight. They are highly resistant to degradation by UV light and other external factors.


The sun is effective at killing mold on surfaces exposed directly to sunlight. The duration of exposure and the intensity of sun play significant roles in its effectiveness. However, light may have limitations in reaching deeply rooted or hidden mold within porous materials or areas with limited light penetration.

And although sunlight is effective when it comes to mold control, it should be complemented by other proper remediation techniques, including actual physical cleaning and addressing the underlying moisture issues that caused the problem in the first place.

Further reading

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