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.
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.
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