Sun Light Therapy Through Clothing! How Much Light Penetrates Clothes?
Should you wear clothing during Red Light Therapy? No, Red Light Therapy should be applied to bare skin for the most consistent dosing.
However, it is often recommended to get light therapy from sunlight. Unfortunately in many places we are unable to enjoy sunlight fully exposed in bare skin.
The culture demands modesty when outdoors or in public places. This causes many people prefer to do light therapies indoors where they can be comfortable without clothing.
To motivate people to go outside for sunlight, it is often claimed that Near-Infrared light or Sunlight can penetrate clothing.
But how much sunlight can penetrate clothing? Is it significant enough to gain the benefits equivalent to light therapies?
Results:
We tested sunlight through various common clothing materials.
Tops and T-Shirt materials offer the best penetration of Near-Infrared light through typically ranging from 40 to 60%.
This can lead to a meaningful dose of NIR from sunlight in just 30 minutes of being outdoors, even while wearing clothes.
Pants materials (jeans, sweat pants, yoga pants, khakis) block nearly all Visible light and only allow a very small percentage of Near-Infrared through. This appears to be due to the tight weave and heavy dyes used.
White clothing lets the most Visible light through, while Dark Colored clothing naturally blocks most of the Visible spectrum. The Red and Gray clothes allow more Red wavelengths through.
Even a double-layer of T-Shirt material can allow for a significant portion of Near-Infrared Light penetration. This also works with Polos and Light Sweatshirts.
Recommendations:
This may be an important consideration for dosing, as the combination of using Red Light Therapy and time outdoors may lead to excessive dosing.
Many people intuitively reduce Red Light Therapy exposure during the summer as they are already getting it for free from sunlight, even when clothed.
So, we should be choosing clothing based on their optical characteristics to modulate our sunlight therapy exposure.
Light colored, minimal thickness, loosely woven clothing will allow the most light through.
Dark colored, thick, multiple layered, tightly woven clothes will block or absorb most light.
Bare Skin for Best Light Therapy:
Most PBM and LLLT studies will apply Red/NIR Light Therapy directly to bare skin.
For targeted treatments this works well, as you only need to expose the area of the skin being treated. The rest can remain clothed.
For full body treatments it is recommended to undress as much as possible. At home or given a private room in a clinic, this is no problem.
Style vs Optics:
Clothing may allow some light through, but it could cause extremely inconsistent dosing. Perhaps if all clothing was uniform and standardized, then we could make a correction factor for dosing.
Humans erroneously chose their clothing based on style and fit. Instead, we should be choosing clothing based on their optical properties to optimize the potential sunlight therapy benefits.
There are already many variables in Red Light Therapy dosing, clothing would just add an unnecessary layer. Similar to how many researchers prefer skin contact as that eliminates the variability of skin reflection losses and trying to maintain a consistent distance.
As in many sciences, it is often helpful to eliminate or control extraneous variables.
UV and Vitamin D Through Clothes:
Historically, a main reason for getting into sunlight was for the UltraViolet (UV) light exposure and Vitamin D production. The more skin we can expose to sunlight, the more IU (International Units) of Vitamin D is produced.
"In summer and spring, with 22% of uncovered skin, 1000 IU vitamin D doses are synthesized in 10-15 min of sun exposure for adults." [1]
Too much UV exposure can cause sunburns and is often correlated with photoaging, skin disorders, and skin cancers.
For Sun Protection and UV protection, it is often recommended to wear physical barriers like long-sleeved clothing and hats to prevent sunburn.
As such, many investigations on the penetration of UV light through clothing have been conducted. Unfortunately they typically only focus on the UVA and UVB ranges - and not so much into the Visible or Infrared ranges.
However, there are some factors in UV blocking clothing that will also apply to Red and NIR light penetration.
UltraViolet Protection Factor (UPF):
Some UV light can penetrate though clothing. So it has been important to quantify the percentage (%) the transmits through.
Depending on the type of material, the color, and weave of the fabric will affect the UV transmittance. Some special treatments or coatings are being developed to enhance UV blocking clothing.
The UltraViolet Protection Factor (UPF) is the rating given for the protection given from clothing. This is similar to the Sun Protection Factor (SPF) of sunscreens, but the UPF is more common for clothing.
It is important to note that you don't necessarily need fancy clothing with UPF-rating labels.
"Clothing not specified as “sun-protective” is capable of blocking UVR with varying levels of success, depending upon specific dye and weave patterns. " [2]
They can typically be clothing with strong colors/dyes and tight weaves that will be good at blocking UV light.
The UPF Number describes the 1/X penetration of UV light through. So we can consider the 1 minus 1/X to know how much UV light is blocked.
5 UPF = 1 - 1/5 = 80% blocked = 20% UV penetrates
15 UPF = 1 - 1/15 = 93.3% blocked = 6.7% UV penetrates
30 UPF = 1 - 1/30 = 96.7% blocked = 3.3% UV penetrates
50 UPF = 1 - 1/50 = 98% blocked = 2% UV penetrates
Generally a UPF of 30 or higher is recommended for good quality UV protection.
The typical range of UPF of common clothes have been tested, they vary greatly from 8 to 80 UPF. [3][4]
Some studies have investigated the loss of UPF after washing the clothing. As clothing is repeatedly washed, it loses fabric and dyes will fade. This usually lowers the UPF. [5]
This would generally hold true for Red/NIR light penetration. Older clothes that have been washed many times will allow more light through than brand new versions of those clothes.
Variables in the Optics of Clothing:
Key Term: Transmittance is the proportion of light that passes through a material, not being reflected or absorbed.
If fish-net clothing was still in fashion (was it ever?), then we could get plenty of sunlight through our clothing. But also no UV protection either.
The optical transmittance of light through a fish-net shirt will be quite different than winter outerwear.
While this image exaggerates the extremes of clothing, in-between these extremes there is a gradient of infinite combinations of clothing and materials that will affect the optical transmittance reaching the skin through clothing.
Some of the factors that may affect light through clothes:
- Thickness
- Number of Layers
- Color
- Material Type (cotton, polyester, etc.)
- Thread Count / Mesh size / Porosity / Weave Style
- Dyes (i.e. even for the same color, a different type of chemical could be used for the dye)
- Other treatments like waterproofing or UPF (SPF but for clothes)
One study notes that the porosity, thickness, and number of layers seemed to have the strongest effect on blocking UV light. [6]
This will hold true for Red and NIR light as we will see later.
100% of the Intensity Through Tiny Holes:
The mesh size and porosity refers to the tiny holes between the stitching. Imagine the fish-net stitching but on a microscopic scale.
If the fabric is loosely oven, there will be larger openings for light to come through. If it is tightly woven, then the gaps will be much smaller restricting light through. This is often called the porosity of the clothing.
"According to the “hole effect”, clothing is naturally porous, containing holes through which UVA and UVB are absorbed into the skin at 100% intensity [9]." [2]
However, when we take intensity measurements, the sensor will essentially be taking an average intensity between the pores and the blocked areas.
Microscope images of weaves can be found here.
Similarly, for Sunlight and Red/NIR penetration through clothing, there will be small holes where 100% of the light goes through at full intensity.
Fabrics with looser weaves and higher porosity will naturally allow more light through.
Check For Yourself:
When shopping, you may just hold the clothing up to the light and look for how much light comes though.
The T-Shirt Materials generally show a lot of light coming through.
In the above picture, we can even see the sunlight through the black T-shirt.
When I hold a Jeans material up to the sunlight, there is barely any light that I can see coming through.
In other words, there are some quick ways you can assess if an piece of clothing will let significant light though. Just hold it up to the light while shopping!
Unfortunately we cannot possibly test every brand or style or material. The goal is to understand some of the parameters to get a general understanding of the optics of clothing.
This way rather than memorizing specific brands, we can apply what we learned to a wide set of clothing.
Research on Visible and Infrared Light Penetration through Clothing:
Only a few studies have investigated the transmittance of light through clothing in the Visible and Near-Infrared ranges.
Author and Researcher Scott Zimmerman provided us with the spectral diagram below.
Of note is Dr. Zimmerman's research on the crucial role of Near-Infrared stimulating melatonin production in the mitochondria. [10] Thus confirming the importance of getting outside in the sunlight, even wearing clothing.
Although it is a logarithmic scale, it shows both White and Black T-Shirts having substantial transmittance of Near-Infrared up to about 2000nm. From 2000nm to 3000nm there is detectable penetration, which means it covers most of the Mid-Infrared region as well.
One article measured light transmittance through various textile samples and showed about 30% through the Visible and Near-Infrared range (400nm to 1400nm).[7]
Another article showed that untreated cotton textile had 35-40% transmittance in the 400nm to 1400nm range. [8]
The same article showed that untreated polyester textile had 50-60% transmittance in the 400nm to 1400nm range. [8]
Another article tested an 850nm laser through various layers of clothing and found significant transmittance through. [9]
Experiment Description:
In this experiment, we took various common clothing and tested them with the HopooColor Spectoradiometer. We will compare the intensity through the clothing to the intensity of unfiltered sunlight at the same time.
Then we can use the difference to calculate the percentage of the spectrum passing through the materials. Often we take at least 2 measurements of sunlight one at the beginning of the test and one at the end to make sure the sun hasn't substantially shifted over the time period of testing.
Sunlight Through Pants:
The pants we tested appeared to block the most light. We tested some jeans, sweatpants, yoga pants, and khakis.
All of the pants materials appeared to have a tighter mesh, not having much porosity for light to come through. They are quite dense compared to the shirt materials we will test in the next section.
Practically all the Visible light light is blocked by all of these pants. Only one of the Khaki pants showed some visible light transmittance.
However, we start to see one interesting trend emerge. There is significantly more light passing through above 700nm in the Near-Infrared range.
Key Term: Attenuation is the reduction of magnitude, effect, or value. Past tense: Attenuated.
The online doctors were correct, Near Infrared does pass through clothing. However the intensity is greatly attenuated.
We get about 30mW/cm^2 of sunlight in the NIR range of 700nm to 1000nm in this measurement.
The light colored jeans, sweatpants, and yoga pants allowed about 7mW/cm^2 of NIR though.
The dark colored jeans and khakis only transmitted 1 to 4 mW/cm^2 of NIR.
At best, you could get 4-7 mW/cm^2 of NIR though pants. In 30 minutes of direct sunlight exposure, that would be a dose of 7.2 - 12.6 J/cm^2. That is something!
Effect of Clothing Color:
We can often feel the effects of clothing color in light. Dark clothes often feel warmer since they absorb more light and transfer the heat to the body. Lighter clothes feel cooler likely because they are reflecting more light away.
Hence, in the spring and summer often it is normal to wear lighter colors. Since those will feel cooler in the sun and heat. In fall and winter then darker colors are worn to absorb more warmth. So perhaps humans are already subconsciously wearing clothing for their optical properties.
Here we test 5 identical T-Shirts from the same brand and style. The only difference was the color. The fabric is 65% polyester and 35% cotton. They are George brand from the Men's section of Walmart.
Again we see all the colors transmitting Near-Infrared above 750nm through at about 45 to 65%. Which is quite substantial. The Black and Blue shirts absorbed most of the Visible Light and only allowed Near-Infrared through.
The Red colored shirt blocked most visible light except Red, allowing about 40% of Red light through in addition to the NIR. The Red having about 9mW/cm^2 in the 600nm-700nm range and also the Gray shirt had 6mW/cm^2 in the same range.
And the White shirt had a high penetration of all wavelengths >500nm of about 63%.
This means the Near-Infrared intensity in the range of 700nm-1000nm was about 13mW/cm^2 to 16mW/cm^2 for black, blue, and gray (in that order).
With the White and Red shirt being the highest NIR intensity about 18mW/cm^2. In just 30 minutes of direct outdoor exposure would be 32 J/cm^2 of dosage through these shirts!
Two Layers and Color Transmittance:
Normally I wear a white Hanes 100% cotton undershirt with my t-shirt outershirts. So then I tested the transmittance again with 1 layer of the undershirt and the outer layer of each of the same colors tested above.
The white undershirt consistently reduced the light transmittance by 55-60% for all wavelengths and combinations of shirts.
But still leaving a substantial amount of NIR transmittance for all the combinations of about 30-40%! So even with 2 layers of t-shirts, it is possible to get significant penetration of NIR light.
Effect of Clothing Materials:
Here we found as many white clothing as possible of various thicknesses, styles, types and brands.
The only thing we kept constant is that they were all White colored! Except a few were cream colored.
These were various brands or random unbranded graphic t shirts we could find. We found some polos and thin sweatshirts as well.
Without doing a full analysis, we see all the white clothing having a substantial penetration between 40 to 70% in the Red/NIR range.
The thinnest T-shirt materials naturally had the best penetration above 60%. The slightly thicker T-shirts and even Polos were 50-60% penetration. And only the thickest clothing like sweatshirts had 40-50% penetration.
But overall it is clear that most white clothing can allow through substantial amounts of Red and Near-Infrared light, and even a lot of Visible light too. Truly, wearing white clothing outdoors can be a full-spectrum light therapy.
Conclusions:
While clothing has been traditionally thought of as providing warmth and protection from sunlight - a new perspective will be the optics of common clothing and how that affects light therapy dosing.
We show that most tops like T-Shirts allow substantial Near-Infrared light through, even through 2 layers and with various colors.
Most pants materials seem to block all visible light and only allow through a small amount of infrared.
It is quite possible that we are getting therapeutic doses of Near-Infrared outdoors while clothed in just under 30 minutes of time outside.
Then the next complication is the light reflection from human skin after the light passes through the clothing. If about 40-60% of the Red/NIR from Sunlight is reflected from the skin, then the dosing would be much lower, but still possibly therapeutic in 30-60 minutes outdoors.
Again, why precision light therapy dosing should always be on bare skin, no clothing or other barriers in the way.
Overall, this is great news for enjoying some of the benefits of light therapy from sun in public areas through proper clothing selection. Clothing can protect us from UV and sunburns, while still enjoying the exposure to longer wavelengths.
References:
[1]
Religi A, Backes C, Chatelan A, Bulliard JL, Vuilleumier L, Moccozet L, Bochud M, Vernez D. Estimation of exposure durations for vitamin D production and sunburn risk in Switzerland. J Expo Sci Environ Epidemiol. 2019 Oct;29(6):742-752. doi: 10.1038/s41370-019-0137-2. Epub 2019 Apr 16. Erratum in: J Expo Sci Environ Epidemiol. 2019 Oct;29(6):862. doi: 10.1038/s41370-019-0143-4. PMID: 30992519.
https://pubmed.ncbi.nlm.nih.gov/30992519/
[2]
Lu JT, Ilyas E. An Overview of Ultraviolet-Protective Clothing. Cureus. 2022 Jul 27;14(7):e27333. doi: 10.7759/cureus.27333. PMID: 36043025; PMCID: PMC9414157.
https://pmc.ncbi.nlm.nih.gov/articles/PMC9414157/
[3]
Cole Y, Ilyas AM, Ilyas EN. Assessment of UV Protection for Children's Summer Clothing. Cureus. 2023 Aug 25;15(8):e44137. doi: 10.7759/cureus.44137. PMID: 37753032; PMCID: PMC10518426.
[4]
Aguilera J, Navarrete-de Gálvez E, Sánchez-Roldán C, Herrera-Ceballos E, de Gálvez MV. Sun-protective Properties of Technical Sportswear Fabrics 100% Polyester: The Influence of Moisture and Sweat on Protection against Different Biological Effects of Ultraviolet (UV) Radiation. Photochem Photobiol. 2023 Jan;99(1):184-192. doi: 10.1111/php.13679. Epub 2022 Aug 4. PMID: 35877464; PMCID: PMC10086949.
[5]
Fernau E, Ilyas SM, Ilyas EN. The Impact of Routine Laundering on Ultraviolet Protection Factor (UPF) Values for Commercially Available Sun-Protective Clothing. Cureus. 2023 Jul 21;15(7):e42256. doi: 10.7759/cureus.42256. PMID: 37605709; PMCID: PMC10440054.
[6]
Aguilera J, Navarrete-de Gálvez E, Sánchez-Roldán C, Herrera-Ceballos E, de Gálvez MV. Sun-protective Properties of Technical Sportswear Fabrics 100% Polyester: The Influence of Moisture and Sweat on Protection against Different Biological Effects of Ultraviolet (UV) Radiation. Photochem Photobiol. 2023 Jan;99(1):184-192. doi: 10.1111/php.13679. Epub 2022 Aug 4. PMID: 35877464; PMCID: PMC10086949.
[7]
Mahltig, Boris & Leuchtges, Giulia & Holstein, Pauline. (2022). T-shirts: An overview and comments on price range, functional materials and European production. Tekstilna Industrija. 70. 4-13. 10.5937/TEKSTIND2204004M.
[8]
Hybrid sol-gel materials for realization of radiation protective coatings—a review with emphasis on UV protective materials
- August 2021
- Journal of Sol-Gel Science and Technology
DOI:10.1007/s10971-021-05558-2
- License
- CC BY 4.0
[9]
Near Infrared Transmission through Various Clothing Fabrics Aamer Saleem, Céline Canal, Lee AJ Davis, Roger J Green and David A Hutchins* School of Engineering, University of Warwick, Coventry CV4 7AL, UK
Saleem et al., J Textile Sci Eng 2013, 3:2 DOI: 10.4172/2165-8064.1000129
[10]
Tan DX, Reiter RJ, Zimmerman S, Hardeland R. Melatonin: Both a Messenger of Darkness and a Participant in the Cellular Actions of Non-Visible Solar Radiation of Near Infrared Light. Biology (Basel). 2023 Jan 6;12(1):89. doi: 10.3390/biology12010089. PMID: 36671781; PMCID: PMC9855654.