Red Light Therapy (RLT), also known as photobiomodulation, is a non-invasive treatment that uses low-level light in the red and near-infrared wavelengths, typically between 600 and 900 nanometers. This light is absorbed by cells to support various functions, including skin health, inflammation reduction, and pain management. With the growing use of convenient at-home RLT devices, users often ask if it is possible to have too much of this therapy. The answer is yes, and understanding the science behind light dosage is essential to achieving the intended benefits.
The Biphasic Dose Response
The effectiveness of red light therapy is governed by the biphasic dose response. This concept, often described using the Arndt-Schulz Law, dictates that a biological response to a stimulus is not linear, but rather follows an inverted U-shaped curve. A dose that is too low may yield no noticeable effect, while an optimal dose provides the greatest therapeutic benefit.
Exceeding the optimal light dose can decrease or negate the positive cellular effects. Light energy is absorbed by the mitochondria, stimulating the production of Adenosine Triphosphate (ATP). However, too much light energy can overstimulate the mitochondria, leading to an excessive production of reactive oxygen species (ROS).
While a moderate increase in ROS promotes healing, a surplus can inhibit the cellular repair processes the therapy is meant to enhance. Delivering too high a dose risks triggering photoinhibition, which is counterproductive. The goal is to stimulate the cells just enough to trigger a beneficial response without causing this inhibitory effect.
Acute Signs of Overexposure
When a user moves past their optimal dose, the body signals overexposure. The most common acute sign is skin irritation, which manifests as temporary redness (erythema) that lasts longer than a few minutes after the session ends. Individuals may also experience a sensation of warmth or a mild burning feeling, similar to a very mild sunburn.
Overexposure can also cause localized dryness or increased sensitivity of the skin in the treated area. Beyond the skin, some people report temporary side effects such as mild headaches or eye strain. These effects serve as a clear indication that the duration or intensity of the session should be reduced.
These acute signs are reversible and temporary, unlike the damage caused by UV radiation. If these symptoms occur, decrease the session time or increase the distance from the device to prevent further discomfort. Using the body’s immediate feedback allows the user to quickly recalibrate usage to a more effective dose.
Key Variables Determining Safe Usage
Avoiding overexposure requires managing the variables that determine the total light energy delivered. The first factor is the device’s power output, or irradiance, measured in mW/cm\(^2\). High irradiance devices, such as clinical units, require shorter session times compared to lower-powered home devices to deliver the same therapeutic dose.
The distance between the user and the light source is important because light intensity decreases as distance increases, following the inverse square law. Moving a home device from a few inches away to a foot away significantly reduces the light energy (fluence) reaching the skin. For general skin benefits, a distance of 15 to 30 centimeters is recommended, though treatments for deeper tissues may require the device to be closer.
Session duration and frequency must be managed to prevent photoinhibition. Typical recommendations are 10 to 20 minutes per treated area, performed three to five times per week. Using the device daily or for long sessions is counterproductive, as cells require time to process the light energy and complete the repair cycle between treatments.
Individual skin sensitivity and medication use are additional considerations. Certain photosensitizing drugs, including some antibiotics (tetracyclines), NSAIDs, and topical retinoids, can increase the skin’s reaction to light exposure. Individuals taking these medications should consult a healthcare provider and begin with a shorter session time to gauge their response.