Red light therapy (RLT) involves applying red and near-infrared light therapeutic wavelengths to the body. Have you ever wondered what happens when body or scalp hair sits between the RLT device and skin?
Hair can block light from hitting the skin, but this effect is not the same for everyone. For most people, a small number of fine body hairs do not interfere with red light therapy. However, dense or darker hair can create a significant physical and optical barrier — especially while doing scalp treatment.
Here is what you need to know before shaving, extending treatment times or reconfiguring your device settings.
How Hair Absorbs, Reflects, and Scatters Light
Not all of the photons emitted from a red light therapy device reach your target tissue when that light is directed towards your skin. Since hair is positioned in the light’s path, it can interact with light therapy before it reaches the skin.
Some of the light is absorbed, and other rays get reflected or scattered at a different angle. This is why the skin under the hair may receive a lower dose of light than it would on bare skin. [1]
This effect depends largely on how much scalp hair is present. Just a couple of fine hairs leave plenty of exposed skin between single hair strands. A denser layer of units makes a bigger obstacle to cross.
Hair does not necessarily always block all light. Few light photons can still pass through small gaps between hair or strands to reach the skin.
Does Dark or Dense Hair Block More Light?
Hair color and density also matter because hair contains melanin, a pigment responsible for maintaining hair color.
Dense and darker hair usually has a higher amount of melanin than very light or gray hairs. If they are situated between a red light device and the skin, it can interfere with light flow because melanin is capable of absorbing light at numerous wavelengths.
Hair Density is also equally important.
Imagine two people are treating their legs with different types of hair density. One has thin, widely spaced hairs and the other has thick, densely packed dark hair. The light coming directly to the skin may differ, even when two people use the same device at a similar distance.
| Hair Characteristic | Potential Effects on Light Delivery |
| Sparse, fine hair | Typically leaves more skin exposed directly. |
| Dense hair | Can physically prevent more incoming light. |
| Thick/coarse hair | May cause more obstruction and shadowing. |
| Dark hair | Contain more melanin, so may absorb more light. |
| Light hair | Can partially scatter or obstruct light. |
| Long hair | Covers more area and create additional layers of barriers. |
How Scalp Hair Changes Light Delivery to Follicles
The scalp is one of the places where hair interference becomes especially important.
The hairs can overlap each other, reflect light, scatter it and even create shadows on some parts of the scalp. A red light therapy device has a fairly clear passageway to the skin when someone is using it on an almost hairless scalp. The person with thick, long hair produces a significantly more complex optical environment.
This can lead to strands of hair overlapping, and absorbing or scattering light onto the scalp below, creating shadows over areas of the head. A red light panel that is aimed toward the top of thick hair does not necessarily mean that an equivalent amount of light reaches beneath the scalp. [2]
For this reason, devices that are specially designed for scalp applications may utilize configurations such as caps, helmets and closely positioned light sources. It's not about lighting the hair itself. The aim is to deliver the light exposure that is intended to heal the scalp.
Panels vs. Wraps and Mats: Why Device Contact Matters
Depending on the type of red light therapy device, body hair can still be a significant factor in reducing the light exposure.
A panel is typically designed to operate at a specific distance from the body. As the light makes its way through the air before hitting the area that will be treated, hairs hovering over the skin may absorb or deflect some of it.
Since wraps, belts and flexible mats all have different operating principles. [3] Most of them are made to be worn directly on or very close to the body. Dense hair can still prevent perfect contact between the light-emitting surface and skin.
| Device Type | Operating Position | Impact of Body/Scalp Hair |
| RLT Panel | Distant from skin | Hair my obstruct and spread light. |
| Wrap / Belt | On or near skin | Can physically prevent incoming light. |
| Flexible Mat | Direct contact | Gaps from the thicker hair. |
| Scalp Cap / Helmet | Around scalp | Dense hair block light flow to scalp. |
| Handheld Device | Light Hair | Easy to reposition around scalp |
Do You Need to Shave Before Red Light Therapy?
Shaving is not automatically required for most RLT applications.
In instances where there is relatively little hair in the area of treatment and much of the skin can be seen between individual hairs, a red light device can be applied directly on the skin.
When hair is thicker, denser or longer, it covers a large percentage of the treatment area; shaving or trimming may be more beneficial. However, trimming alone can be enough to boost the light exposure.
In most cases, shaving the head is not a reasonable or even convenient option, especially for scalp treatment. There are also practical reasons not to shave right before a session. Shaving may cause temporary skin irritation and small cuts or razor burn. Therefore, if your skin is not in the best condition 48 hours after shaving, you need to wait until sensitivity has settled before using a device.
Practical Ways to Improve Skin Exposure Without Increasing the Dose Blindly
If hair seems to obstruct your treatment, the first response should not be to just double the session duration. The dose of red light therapy is determined by many variables such as irradiance, wavelength, treatment distance, exposure time and the device itself. Without knowing the others, increasing one will create a dose that is very different from what was intended. [4]
If you suspect body or scalp hair is interfering with red light exposure, there are simple things that can be done before prolonging treatment time：
- Expose the skin where possible: Reposition, part or pin longer hair strands so that the light can more easily reach the treatment area.
- Reduce hair Density: If the hair body is too dense – one may need to trim it. Trimming simply allows direct exposure to the treatment site without shaving.
- Keep the recommended treatment distance: With panels, always follow the recommended distance guidelines given by the manufacturer.
- Improve contact with wearable devices. If using a wrap or flexible mat that touches the skin, be specific with where you place it to avoid unnecessary hair bunches underneath.
Final Thoughts
Body hair may absorb, scatter or physically block some of the light rays before it reaches the skin, thus reducing direct red and near-infrared light therapy exposure.
You don't need to have a 100% barrier-free scalp; instead, create a setup that allows for comfortable and consistent exposure of the target area. Minor tweaks like keeping hair off the skin, ensuring good device position and sticking to your schedule could be more beneficial than introducing big changes to your therapy routine.
References
[1] Lanzafame, Raymond J et al. “The growth of human scalp hair in females using visible red light laser and LED sources.” Lasers in surgery and medicine vol. 46,8 (2014): 601-7. doi:10.1002/lsm.22277
[2] Cleveland Clinic. “Red Light Therapy: Benefits, Side Effects & Uses.” Cleveland Clinic, 1 Dec. 2021.
[3] Kim, Jin Hee et al. “Assessment of Effects of Low-Level Light Therapy on Scalp Condition and Hair Growth.” Indian journal of dermatology vol. 68,4 (2023): 487. doi:10.4103/ijd.ijd_59_23.
[4] Lanzafame, Raymond J et al. “The growth of human scalp hair mediated by visible red light laser and LED sources in males.” Lasers in surgery and medicine vol. 45,8 (2013): 487-95. doi:10.1002/lsm.22173.
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