Most medical laser applications are intended for the ablation, resection or vaporisation of tissue or the coagulation of body fluids. Examples include:

• stopping bleeding
• correcting short- or far-sightedness by targeted corneal ablation
• using laser radiation as a scalpel in surgery
• breaking down kidney stones or gallstones (lithotripsy)
• removing benign neoplasms of the skin, virus-related skin lesions, and skin lesions deemed to be preliminary stages of cancer.

The lasers used in these areas usually belong to the highest classes: 3R, 3B and 4. These lasers are dangerous in the event of direct exposure or, in the case of class 4 lasers, even with diffusely scattered radiation – especially for the eyes. Expertise as a laser safety officer is required for their operation. However, the course required for this is geared towards questions of occupational safety and by no means provides the necessary specialist knowledge for medical or cosmetic applications of optical radiation on humans.

Photodynamic therapy

In photodynamic therapy (PDT), tissue is removed or damaged using light in combination with special substances known as photosensitisers. These cause the target tissue to become particularly sensitive to light so that it can be selectively destroyed by the radiation while sparing the surrounding tissue. PDT is primarily used to treat skin conditions, including certain forms of skin cancer or their preliminary stages, as well as in ophthalmology, including to treat age-related macular degeneration. Lasers or incoherent light sources serve as the radiation source for PDT.

Lasers or optical radiation sources with comparable effects (such as IPL systems) are increasingly being used for cosmetic purposes, even outside of the clinical setting. In both cases, the aim is to achieve effects that generally involve exceeding the limit values for occupational safety or the recommendations of the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The same devices can be used in both medical and cosmetic applications. Their use in the cosmetics or wellness sector is not restricted to individuals with medical training, nor does the law currently stipulate that treatment must be performed under medical supervision.

Examples of the use of lasers for cosmetic purposes include:

• permanent hair removal (epilation)
• removal of scars or vascular lesions such as “spider veins”
• fat reduction (“body shaping”)
• tattoo removal.

The effects of laser radiation on biological tissue vary depending on the wavelength, intensity and duration of exposure, and on the properties of the tissue, which lead to differences in how the radiation is reflected, scattered and absorbed.

Water absorbs optical radiation above all in the UV and longer-wavelength infrared regions. As biological tissue generally contains a very high proportion of water, it absorbs short- and long-wavelength laser radiation very strongly. In the visible and near-infrared region, the degree of absorption is determined by the haemoglobin and melanin (the brown pigment in skin).

The type and degree of tissue response are essentially dependent on the irradiance and the duration of exposure.

Relatively long durations of exposure in the range of minutes and irradiances in the range of watts per square centimetre trigger what are known as photochemical effects. In the region of visible or infrared light, certain biological molecules can absorb the incident laser radiation. The molecules become excited and release their energy to oxygen molecules, for example, resulting in the formation of highly reactive radicals that can go on to damage other biological substances (such as DNA or proteins). Laser radiation in the UV region can also cause direct damage to DNA. If damage of this kind is not repaired, it can lead to lasting changes in the genetic material (mutations) and increase the long-term risk of developing cancer.

Depending on the duration of exposure, different thermal effects can be observed at medium to short exposure times ranging from a few seconds to milliseconds and irradiances ranging from a few watts per square centimetre to approximately 1 megawatt per square centimetre (1 megawatt per square centimetre = 1,000,000 watts per square centimetre). These effects range from slight warming of the tissue to the denaturing of proteins and boiling of water in tissue and cells and ultimately to the charring and blackening of tissue. Due to heat conduction, tissue outside of the radiation’s actual target volume is also heated at varying speeds and may be damaged as a result. This thermal effect has therapeutic applications in areas such as laser surgery.

At very short durations of exposure ranging from nanoseconds to microseconds and irradiances ranging from megawatts per square centimetre to 1 gigawatt per square centimetre (1 gigawatt per square centimetre = 1,000 megawatts per square centimetre), the tissue is “vaporised” and removed almost explosively, while the surrounding tissue is barely heated at all. This effect is used for the targeted treatment of eye defects in ophthalmology, for example.

Even higher irradiances ranging from gigawatts per square centimetre to terawatts per square centimetre (1 terawatt per square centimetre = 1,000 gigawatts per square centimetre) lead to the formation of plasma (free electrons and ions) in the tissue. This plasma expands and collapses again in an incredibly short time, producing a shock wave that propagates through tissue and destroys it mechanically. This effect of laser radiation serves as the basis for the fragmentation of kidney stones and gallstones (known as “lithotripsy”).

Because of its special optical properties, the eye is particularly sensitive to optical radiation and therefore also to laser radiation.

Eyesight

Radiation in the visible region is particularly important for eyesight, as this radiation passes through the cornea, lens and vitreous body to reach the retina. Short-wavelength infrared radiation (IR-A, “near infrared”) also reaches the retina. Radiation in the UV and far-infrared region, on the other hand, is already absorbed by the cornea or the lens.

Focusing of light

Particular attention must be paid to the image-forming properties of the eye, which have a strong focusing effect on visible and near-infrared light. As a result, parallel rays of light such as those present in laser radiation are focused onto a single point of the retina. This effect can be compared to the “burning glass effect” seen with a magnifying glass in sunlight. It increases the power density of the laser beam, which is generally already very high, by a factor of some 10,000 to 500,000 by the time the beam reaches the retina. As a result, a laser beam arriving at the cornea with an irradiance of 25 watts per square metre can reach an irradiance of up to 12.5 megawatts per square metre at the retina.

Retinal damage

This can result in varying degrees of retinal damage. People generally do not notice small spots on the retina where blood has coagulated and blood capillaries are damaged. However, if these spots are larger or they accumulate in one area, they will result in defects in the visual field. Other possible consequences include the detachment of parts of the retina or massive bleeding inside the eyeball. Laser damage to the area of sharpest vision, the macula, is particularly serious, as it can significantly reduce or even eliminate the visual acuity as well as colour perception. If the so-called blind spot, where the visual nerves emerge into the retina, is damaged by a laser beam, this can result in total blindness.

Damage to the cornea and conjunctiva

In contrast, laser radiation in the UV region primarily affects the cornea, the conjunctiva and the lens. At relatively low irradiances, this results in very painful inflammation of the cornea (photokeratitis) and the conjunctiva (photoconjunctivitis). Reversible corneal clouding can occur at higher radiation intensities, while irreversible corneal clouding and cataracts can occur from approximately 50 kilojoules per square metre.

Cataracts are also possible in the longer-wavelength infrared region, while only the cornea is damaged at wavelengths from approx. 2,500 nanometres.

In general, the skin can withstand significantly higher intensities of laser radiation than the eye. The penetration depth of radiation and therefore the effect on different layers of skin depend strongly on the wavelength.

As skin absorbs radiation very strongly in the UV and far-infrared regions, the effects are essentially limited to its uppermost layers. On the other hand, the penetration depth of radiation is relatively high in the visible and near-infrared regions, so damage can occur even in the hypodermis.

Health effects depend on the irradiance and duration of exposure

Depending on the irradiance and duration of exposure, a variety of health effects can occur. At a fairly low irradiance and duration of exposure, these include erythema (reddening of the skin or “sunburn”) in the UV region, while various photochemical and thermal reactions can be observed in the visible region and, as the wavelength increases, only thermal effects occur in the infrared region. At higher powers, these burns can lead to serious blistering and subsequent scarring.