Does Sunlight Kill Mold?
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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.
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.
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Hello,
Thank you for all the info. I am reading 1-2 articles everyday since last week. And getting really annoyed bu realising mold everywhere afterwards 🙁
I was wondering; does heat kills mold? When i realized i have mold on the end of my wood cutting board first i ve used sandpaper to clean it and then heated/burnt it with a lighter.
And now im wondering if its ok to use or not..
Hi there! The issue with wooden cutting boards is that typically, if you can see mold, the mold has already rooted deeper into the cutting board where you can’t see. It makes it very difficult to remove all of the mold. If you have a moldy cutting board, my advice is to always dispose of it.
thank you! when it comes to wood- if its already covered with oil (either synthetic for floors or natural like tung oil for cutting boards) then it gets moldy- does it still mean it penetrated inside despite the oil coverage?
It still can penetrate, especially if the oil finish is older.
here’s an excerpt on UVC from the US FDA:
“UVC radiation from the sun does not reach the earth’s surface because it is blocked by the ozone layer in the atmosphere. Thus, the only way that humans can be exposed to UVC radiation is from an artificial source like a lamp or laser.”
in regards to UVB radiation, about 95% of it is absorbed by the atmosphere. so what we’re left with is the least harmful radiation which is UVA. so if you’re looking to nuke something specifically with UVC radiation (which has the shortest wavelength and is the most harmful type of UV), you can’t do it by leaving it out in sunlight since that form of UV doesn’t reach the earth’s surface.
Hi,
Just wondering if you could give us some advice on how to successfully remove mildew from household items. We had a bunch of stuff stored in the basement of our rental unit. Dog and cat beds, blankets, towels, furniture, tent, garage items (as we did not have enough room to store them in the garage), mirrors, weights, a table, a mattress. Is there anyway to salvage all of these items? I just placed most of these in the sun. Still need to clean some off. Not sure how to correctly do this without wrecking them. I have read a mixture of vinegar and water. Not sure what to do, though. Any advice you could provide would be appreciated.
Also, the issue with the basement is that the house is old and water comes in cracks. We do clean it up to the best of our ability but cannot get at all of it.
Hi Dustin. Porous items are tricky. A mattress cannot be safely cleaned unfortunately. I don’t really have a suggestion for you on that.
Anything that can go in a washing machine can be laundered with this: https://www.citrisafe.com/product/remedy-laundry-liquid-detergent-32-oz/?wpam_id=65
The other items you mentioned can be cleaned with this: https://www.amazon.com/Superstratum-Mildew-Stain-Remover-32-ounce/dp/B095J4QSNG?tag=moldhelp-20&ref_=as_li_ss_tl Spray it on, leave it for ten minutes, spray again and wipe clean.
Do you have a dehumidifier in the basement to help deal with the increased moisture levels?
I live in Hawaii and have a very stubborn mold issue in my car. It grows on both the leather seats and plastic pieces. I use a product that is supposed to kill and control the growth of mold, but it always comes back after a few weeks. Any recommendations on products and strategies to exterminate this mold? I greatly appreciate your time and any info you are willing to share! Mahalo!
Hi Rob! Yes, you will need to do a chlorine dioxide bomb! I will send you an email with more information.
Can you please advise how to get mold and mildew out my car. It has been cleaned by pro, but still smells moldy. I have had lung cancer twice and I’m concerned about riding in the car. Is there any way to de-mold the car to the point that I am not afraid to ride in it. It stinks like mold and mildew to me. My husband thinks it is strong chemical cleaning agents I might be smelling.
Hi Shannon! Here is a link to my article on car mold: https://moldhelpforyou.com/car-mold/ This should answer all your questions!
Hey Jennifer! I appreciate the info. Any advice for a home that has mold in the crawl space that is leaking into the home through the floor vents. We are moving out to get out of the atmosphere but all our belongings….because it’s circulating through the air we assume all is contaminated. We have Lyme and had quite the blow of the combo of mold and lyme. Will wiping down and sun be a wise move on non poris belongings? There is no visible mold on anything. If it’s not visible would even just wiping down wood items like tables and book shelves be enough with sitting them in the sun? We got ride of all beds and furniture that may have absorbed. Any advice would be great as we are trying to heal and start over Thanks heather.
Hi Heather! I would read through this article and follow the cleaning protocol here: https://moldhelpforyou.com/small-particle-cleaning/
I am so sorry you are going through this!
why is it when I’m exposed to black mold it attacks my joints like gout does it’s not as bad but I gotta eat tons of nsaids to function right….
This is because you are also getting attacked by mycotoxins that the mold releases. Mycotoxins are responsible for the bulk of the health issues that correspond to mold exposure.
Hi I could use your help. I recently moved my most prized belongings to a climate controlled storage unit in Nebraskas that was built brand new. I have a storage unit on the ground floor and recently found out that the owner did not seal the floors in the unit, just the hallways. That made no sense to me. About a month ago a sprinkler broke in the loading bay area and released gallons of water, the units on the ground level which I also believe are just cement slab on grade or dirt got wet. When I checked out my unit later that day the floor was moist and I had a lot of cardboard boxes with items packed into my unit. The boxes were moist and sagging, even the ones on the shelving unit I have in there. I believe the humidity was very high in there probably for a few days as he was trying to dry it out with fans and dehumidifiers. I believe he is running way too high on his humidity levels just in general because we placed a dehumidifier in the hallway recently just weeks after the water accident and the average reading is at 62%. I know mold grows at 60 or above. I’ve been running a commercial dehumidifier in my unit that I borrowed from him for weeks now trying to keep it dry as I can in my unit and took most items out of their cardboard boxes which were damp inside an discarded the cardboard. I’ve set thing out on tables. I got the levels from the low 60’s in my unit to come down to mid 50’s however there is a section of the floor that a hydrometer keeps detecting as high risk and measures about 17% and when I stick the probes into the expansion joints of the cement which feels like porous dirt and gravel I get a beeping alert and a wet reading on the hydrometer. In that one spot in the unit the humidity won’t go below 57-58%. He has taken no responsibility for having high humidity levels in his facility and since my items already were exposed to moisture high humidity after the sprinkler broke I fear I mold could be on my items. I have seen traces on paper and cardboard and wood. My precious oil paintings on canvas have been sitting out drying near the dehumidifier for 3 weeks now on a table in there. I brought a very special and expensive framed print home and rested it up against a box sitting upright and it transferred a white fine powdery substance to the box. I’ve have seen this white film on everything in my unit. What could it be? I don’t know what to do. I hope it is something like salts or something else that is dissolving in the air and not a white powdery mildew, is there such a thing? What do you suggest I do and should I move my paintings out of there asap? Can I wipe things down with a diluted vinegar to get rid of the film or layer of dust? Any suggestions you have I am desperate and waiting to move upstairs into a dry unit once I have wiped things down and put items in sealed totes with silica bags.
I’m sorry you’re dealing with this. Based on what you’ve described, I think there are several separate issues occurring, and it’s important not to assume that every white or discolored material is mold although it is possible.
First, a relative humidity averaging around 62% is certainly higher than I would like to see in a climate-controlled storage facility. While mold does not automatically begin growing at 60% RH, maintaining humidity below about 50% provides a much greater margin of safety, especially after a water event. A sprinkler discharge that wets concrete floors, cardboard, and stored contents can temporarily raise the humidity enough to support mold growth if materials remain damp for more than 24–48 hours.
The damp cardboard boxes are the greatest concern. Cardboard readily absorbs moisture and can support mold growth when it remains wet. Discarding the damp boxes was a good decision. Your efforts to lower the humidity within your individual storage unit are also appropriate.
Regarding the moisture readings in the concrete, it’s important to understand that pin-type moisture meters are not designed to determine whether concrete is “dry” in the same way they are used on wood. Concrete naturally contains moisture, and expansion joints often retain moisture much longer than the surrounding slab because they can wick water from below or collect moisture within the joint. Elevated readings in those areas do not necessarily mean there is active mold growth, but they may indicate that moisture is still migrating through the slab or that the slab has not fully dried from the sprinkler incident.
The white powder you’re seeing may or may not be mold. Several possibilities exist:
Efflorescence, which is a white crystalline mineral deposit left behind as moisture moves through concrete or masonry and evaporates.
Paper or cardboard fibers.
Mold or mildew growth, although mold cannot be identified reliably by color alone.
Unfortunately, no one can determine whether a white film is mold simply by looking at it. The only way to know with confidence is through laboratory analysis of a tape lift or bulk sample.
My biggest concern would be your artwork. Original oil paintings, canvas, paper, photographs, books, and other irreplaceable items are much more sensitive to prolonged humidity than most household belongings. If you have access to a storage location that is consistently maintained below about 50–55% relative humidity and has not experienced a recent water event, I would move those valuable pieces as soon as practical. The sooner they are placed into a stable, dry environment, the lower the risk of deterioration or microbial growth.
I would also avoid wiping valuable artwork with vinegar. Vinegar is acidic and can damage many finishes, paper products, textiles, and artwork. For valuable paintings, prints, photographs, and antiques, surface cleaning should be approached very cautiously. If visible contamination is present on artwork, consultation with a professional art conservator is the safest approach.
For most non-porous household items, a microfiber cloth slightly dampened with water and dawn dish soap or an appropriate cleaner is generally sufficient to remove settled dust. Porous items such as books, paper, unfinished wood, and fabrics require a different approach depending on whether they are simply dusty or have actual microbial growth.
If you continue using the storage facility, I would recommend:
Keeping the humidity inside your unit below approximately 50–55% if possible.
Storing items in clean plastic totes with tight-fitting lids rather than cardboard boxes.
Allowing belongings to dry completely before sealing them in containers.
Continuing to monitor humidity with a calibrated hygrometer.
Inspecting sensitive belongings periodically for any signs of moisture or microbial growth.
Based on everything you’ve described, I think your concern is reasonable, particularly because of the sprinkler incident and the elevated humidity afterward. However, I would avoid assuming that every white film or powder is mold until it has been properly identified. If you’re seeing persistent visible growth or continue noticing musty odors despite maintaining lower humidity, having a few representative samples evaluated by a qualified laboratory can provide much more certainty and help determine whether remediation of the contents is necessary.