The biological impacts of laser radiation on human body are closely related to the wavelength, irradiance, and exposure duration. Laser radiation spans wavelengths from about 10,000 nanometers (nm) to approximately 200 nm, encompassing the infrared, visible light, and short-wavelength ultraviolet (UV) regions.

The specific health hazards associated with laser radiation primarily arise from the exceptionally high power density and intense collimation (focusing) of the laser beam. Given the limited penetration depth of optical radiation in biological tissue, its primary impact is on the skin and eyes. The eyes, in particular, faces greater vulnerability due to their unique optical properties.

Effect on Biological Tissue

The impact of laser radiation on biological tissue varies based on factors such as wavelength, intensity, duration of exposure. It also depends on the tissue properties affecting how the radiation is absorbed, scattered, and reflected.

Water, prevalent in biological tissue, strongly absorbs short- and long-wavelength laser radiation, particularly in the UV and longer-wavelength infrared regions. In the visible and near-infrared spectrum, absorption depends on haemoglobin and melanin content. The tissue response type and magnitude relies on irradiance and exposure duration.

Extended exposures in the minutes range and irradiances in watts per square centimeter induce photochemical effects. Visible or infrared light absorption by biological molecules can generate reactive radicals that damages substances like DNA or proteins. UV laser radiation can directly harm DNA, potentially leading to genetic mutations and an increased cancer risk.

For medium to short exposure times (seconds to milliseconds) and irradiances (from a few to around 1 megawatt per square centimeter), thermal effects manifest. These effects range from tissue warming to protein denaturation, water boiling, tissue charring, and blackening. Therapeutically, this thermal effect finds application in laser surgery.

Very brief exposures (nanoseconds to microseconds) and high irradiances (megawatts to 1 gigawatt per square centimeter) cause almost explosive vaporization of tissue, with minimal heating of surrounding areas. This effect is utilized in targeted treatments, such as correcting eye defects in ophthalmology.

Extremely high irradiances (gigawatts to terawatts per square centimeter) result in plasma formation within tissue, generating a shock wave that mechanically destroys the tissue. This process is employed in lithotripsy, breaking down kidney stones and gallstones.

Effect on the Eye

Due to its distinctive optical characteristics, the eye exhibits heightened sensitivity to optical radiation, including laser radiation.

• Eyesight: Visible and short-wavelength infrared radiation are crucial for eyesight, as they penetrate the cornea, lens, and vitreous body to reach the retina. Conversely, UV and far-infrared radiation are absorbed by the cornea or lens.
• Focusing of Light: The eye's image-forming properties focus on visible and near-infrared light, similar to a magnifying glass concentrating sunlight. This effect intensifies the laser beam's power density by a factor of 10,000 to 500,000, reaching up to 12.5 megawatts per square meter at the retina, exacerbating the inherent high power density.
• Retinal Damage: Laser focusing can lead to retinal damage, often unnoticed initially. Coagulated blood spots and damaged capillaries may result in visual field defects. Severe consequences include retinal detachment or significant intraocular bleeding. Damage to the macula, responsible for sharp vision, can severely impair visual acuity and color perception, while laser impact on the blind spot may cause total blindness.
• Damage to the Cornea and Conjunctiva: UV laser radiation predominantly affects the cornea, conjunctiva, and lens. At lower irradiances, this can induce painful inflammation, such as photokeratitis and photoconjunctivitis. Higher intensities may cause reversible corneal clouding, while irreversible clouding and cataracts may occur at approximately 50 kilojoules per square meter.
• Cataracts are also a risk in the longer-wavelength infrared region, whereas wavelengths around 2,500 nanometers primarily damage the cornea.

Effect on the Skin

The skin generally exhibits greater resistance to laser radiation than the eyes and tolerates significantly higher intensities. The impact on distinct skin layers depends on the radiation's penetration depth, which varies with wavelength.

• Skin Resistance and Wavelength Effects: The UV and far-infrared regions prompt strong absorption in the skin's upper layers and confine effects to the surface. Conversely, the visible and near-infrared regions penetrate deeper, potentially affecting the hypodermis.
• Health Effects Based on Irradiance and Duration: The health effects depend on irradiance and exposure duration. Lower irradiance and brief exposure may induce erythema (skin reddening or "sunburn") in the UV region. In the visible region, various photochemical and thermal reactions may occur that progress to predominant thermal effects in the infrared region. Elevated powers can result in severe burns and lead to blistering and subsequent scarring.

Legal Regulations for Medical Lasers

• Medical devices are subject to the Medical Devices Act (MPG). In connection with the Medical Device Operator Regulation (MPBetreibV), the MPG regulates aspects including the requirements for medical devices and for their construction, operation, and application.
• Devices that the manufacturer has registered not as medical device but rather as a consumer product are subject to the Product Safety Act (ProdSG).
• The occupational safety provisions are regulated in the Ordinance on Artificial Optical Radiation (OStrV).
• The Ordinance on Protection against the Harmful Effects of Non-Ionizing Radiation in Human Applications (NiSV), which is in force since 31.12.2020, regulates the requirements for the operation of laser devices and other powerful optical radiation sources when used for cosmetic and other non-medical purposes. Some applications, such as the removal of tattoos with lasers, may only be performed by licensed physicians with appropriate training or continuing education. For applications that are not subject to physician's reservation, from 31.12. 2022 onwards, requirements for specialist knowledge must be fulfilled and proven.

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