Understanding infrared light.

What is red light phototherapy?

Discovered in 1967 by Hungarian physician Endre MESTER during his research on wound healing in rats, red light therapy has gradually gained recognition. Today, it is widely used for skin and hair care, both at home, thanks to LED masks, and in specialised institutes. Red light corresponds to wavelengths between 620 and 750 nanometers, a range perceptible to the human eye. Near-infrared light, often associated with it, emits at wavelengths between 750 and 1,200 nanometers, and is not visible. Although different in their perception, these two lights are complementary: red light primarily acts on the skin surface and in the superficial dermis, while infrared can penetrate up to 4 to 5 centimetres deep into tissues, reaching muscles and joints.

At the cellular level, the action of red light is based on a photobiomodulation mechanism. When photons reach the tissues, they are absorbed by intracellular receptors, primarily cytochrome c oxidase in the mitochondria, which is essential for cellular respiration. This absorption triggers several cascading effects, leading notably to an increase in tissue repair, a reduction in oxidative stress, modulation of inflammation, and stimulation of blood circulation.

Le spectre du soleil. — The sun spectrum.
Source : HAMBLIN M. & al. Infrared and skin: Friend or foe. Journal of Photochemistry and Photobiology B: Biology (2016).

What are the effects of red light on the skin?

The efficacy of red light in skincare is currently supported by numerous studies. When it penetrates the epidermis, it interacts with various cells and skin components, thereby triggering the mechanisms responsible for its multiple benefits.

Red light interacts with keratinocytes.
In keratinocytes, the predominant cells of the epidermis, red light significantly stimulates cell proliferation. Concurrently, it enhances mitochondrial activity, resulting in an increased production of ATP, the energy required for optimal cell function. This dual action promotes re-epithelialisation, that is, the skin's ability to rapidly regenerate after an injury or assault. Strengthening this superficial layer improves the skin's protection against external aggressions and significantly reduces transepidermal water loss.
The red light interacts with the melanocytes.
Melanocytes, responsible for the production of melanin during the process of melanogenesis, also respond to red light irradiation. However, the responses observed in melanocytes are not as well understood as those of fibroblasts and keratinocytes. Depending on the experimental context, different authors have observed either inhibition or activation following exposure to red light. On one hand, red light irradiation has been used for repigmentation therapy in vitiligo, based on the stimulation of melanocytes. On the other hand, some authors establish a correlation between exposure to red light and the reduction of melanin content and the activity of tyrosinase, an enzyme essential to melanogenesis. Therefore, further studies are necessary to provide a more comprehensive understanding of melanocyte responses to red light.
Red light interacts with fibroblasts.
At the dermis level, fibroblasts also benefit from the effects of red light. This irradiation stimulates their activity and promotes the synthesis of extracellular matrix components, such as the collagen, elastin and hyaluronic acid. These molecules respectively ensure the firmness, elasticity and hydration of the skin. Moreover, red light activates DNA repair processes, thus helping to protect fibroblasts against premature ageing induced by oxidative stress. This dermal stimulation allows for the strengthening of skin density, improving skin texture and reducing the appearance of wrinkles.
Red light interacts with the sebaceous glands.
Sebaceous glands, responsible for sebum synthesis, are organs located within the dermis. Studies conducted on human sebocytes, the primary cells of the sebaceous glands, have shown that exposure to 650 nm red light can suppress lipid synthesis, thereby reducing sebum production. This property is particularly beneficial for individuals with oily skin.
The red light interacts with Langerhans cells.
Finally, Langerhans cells, which are responsible for skin immunity, have their activity restored under the effect of red irradiation. This light acts particularly by rebalancing the production of cytokines, chemical messengers involved in the immune response. Infrared light therapy also promotes an anti-inflammatory response, allowing the skin to be less reactive to daily external aggressions.

Targeted Compounds	Effects	Effects on the Skin
Keratinocytes	Stimulation of cellular proliferation, mitochondrial respiration, and re-epithelialisation	Smoother skin and strengthened skin barrier
Melanocytes	Influences melanogenesis, an action still poorly understood	Reduction in vitiligo hypopigmentation, more even skin tone
Fibroblasts	Stimulation of collagen, elastin, and hyaluronic acid synthesis, and DNA repair	Firmer skin, increased hydration, and reduction of wrinkles
Sebaceous Glands	Inhibition of lipid synthesis by sebocytes	Reduction of shiny areas
Langerhans Cells	Restoration of immune activity and anti-inflammatory action	Reduction in redness, improved defence against external aggressions

The effects of red light on the skin.

Adapted from: BAPTISTA M. S. & al. Red-light photons on skin cells and the mechanism of photobiomodulation. Frontiers in Photonics (2024).

Infrared light therapy and hair: benefits to be aware of?

Primarily recognised for its positive effects on the skin, the use of red light is also beneficial for the scalp, particularly for stimulating hair growth. This is linked to the activation of mitochondria, the cells that produce ATP, following their irradiation by red light. The increase in cellular energy allows for better activity of the hair follicles and thus enhanced hair growth. A study conducted with human hair follicles showed that their irradiation with a 650 nm red light extended the anagen phase, which corresponds to the period when hair is growing. Other biological processes are involved, including the reduction of scalp inflammation through the blocking of the migration of leukocytes, immune system cells, as well as the increase in the diameter of blood vessels which ensures better delivery of oxygen and nutrients to the hair follicles.

A clinical study involving 47 women suffering from varying degrees of androgenetic alopecia examined the effects of red light on hair growth. The participants were divided into two groups: an active group and a placebo group. The active group used a device with 21 diode lasers and 30 LEDs, all emitting red light at 655 nm. The placebo group used an identical device but with incandescent red lights, without therapeutic effect. The participants used the device at home every other day for 16 weeks, for a total of 60 sessions. At the end of the experiment, the researchers measured a 37% increase in the number of hairs in the active group compared to the placebo group.

Red light appears to hold some interest for individuals looking to thicken their hair.

The contraindications of red light therapy.

Although red and infrared light therapy offers numerous benefits for skin and hair, it's important to note that this technique is not suitable for everyone. Indeed, light therapy is not recommended for individuals with blood clotting disorders, such as haemophilia, as it can cause bleeding or bruising. The same applies to individuals with open wounds. This contraindication also applies to people suffering from active or suspected cancers. Indeed, the wavelengths of red and infrared light can stimulate the growth of cancer cells, potentially exacerbating the disease.

Furthermore, red light therapy is incompatible with photosensitising medications, such as oral retinoids prescribed for severe or persistent acne, as their combination can cause significant skin irritation. It's also worth noting that direct exposure of the eyes to red light can be dangerous and cause retinal damage. Therefore, it is strongly advised to wear protective glasses during sessions. Lastly, the effects of light therapy on pregnant or breastfeeding women have not been sufficiently studied, so its use in this population is discouraged as a precautionary principle, unless it has been prescribed by a healthcare professional.

Note : To safely enjoy the benefits of red light therapy devices at home, it is essential to closely follow the manufacturers' recommendations, particularly regarding the duration and frequency of exposure. Excessive use of these devices could lead to undesirable effects, such as redness, tingling, or burning sensations.