Red Light Therapy (RLT) and Near-Infrared (NIR) light treatments use specific wavelengths to stimulate cellular function, a process known as photobiomodulation. For the treatment to be effective, light energy must reach the target tissue beneath the skin. While the answer is complex, clothing substantially diminishes the therapeutic effect of RLT. The degree of reduction depends heavily on the type of light used and the specific fabric barrier.
The Biological Mechanism of Light Absorption
For red and near-infrared light to initiate a therapeutic effect, it must first be absorbed by specific components within the cells. The primary target for this light energy is the mitochondria, which generate adenosine triphosphate (ATP), the main energy currency of the body.
The light’s photons are absorbed by an enzyme called cytochrome c oxidase (CCO) located in the inner mitochondrial membrane. This absorption temporarily disrupts the binding of nitric oxide, a molecule that can inhibit cellular respiration. By freeing the CCO, the light allows oxygen to be used more efficiently in the electron transport chain. This process increases ATP production, fueling cellular repair and regeneration.
Penetration Depth: Red Versus Near-Infrared Light
Red light therapy devices use two distinct wavelength ranges that penetrate tissue to different depths. Visible red light (630 to 670 nanometers) is absorbed mostly in the superficial layers of the body. This range is effective for skin treatments, such as promoting collagen production and reducing surface-level inflammation.
Near-infrared (NIR) light (810 to 850 nanometers) is invisible and penetrates much deeper. These longer wavelengths are absorbed less by water and hemoglobin in the blood, allowing them to reach underlying muscle tissue, joints, and even bone. NIR light can penetrate up to five centimeters into the body, making it the preferred wavelength for deep muscle recovery, joint pain, and nerve repair. This physical difference in penetration depth means NIR light has a slightly better chance of passing through thin obstructions than visible red light.
How Fabric Affects Light Transmission
Clothing acts as a significant obstacle because fabric is designed to absorb and scatter light, preventing it from reaching the skin’s surface. Studies indicate that even thin fabrics can cut the light intensity reaching the skin by 50 to 80 percent. This dramatic reduction in therapeutic dose would require a substantial and often impractical increase in treatment time to achieve the same cellular benefits.
The color of the fabric is a major variable, as dark colors absorb a much greater amount of light energy across the entire spectrum. Clothing that is black, dark blue, or dark red will absorb nearly all incoming red and near-infrared light, rendering the treatment ineffective. Conversely, light-colored or white fabrics reflect less of the therapeutic light spectrum and allow a higher percentage of photons to pass through.
The material and thickness of the garment also determine how much light reaches the body. Thicker, denser materials such as denim, leather, or heavy cotton block almost all light transmission. Thin, loose-fitting, and sheer fabrics, like a lightweight, white t-shirt or thin mesh, offer the least resistance, especially to the deeper-penetrating NIR wavelengths.
For maximum efficacy, direct contact with bare skin is always the most effective approach. If clothing must be worn, choosing a loose-fitting, light-colored, and thin garment is necessary to allow any measurable energy to reach the target tissue.