The part of the human body that is most sensitive to light and can most easily be damaged by lasers is the human eye. Common sense precautions such as not shining a laser into your own, or anyone else's eyes, will help prevent vision loss from laser exposure.
The major danger of laser light shows is hazards from beams entering the eye since this is the organ most sensitive to light. The simplest way to explain this is to say, "just as a magnifying glass can be used to focus the sun and burn wood, the lens in the human eye focuses the laser beam into a tiny spot than can burn the retina".
Lasers in the visible and near infrared range of the spectrum have the greatest potential for retinal injury, as the cornea and lens are transparent to these wavelengths and the lens can thus focus the laser energy onto the retina. The maximum transmission by the cornea and lens, and the maximum absorption of laser energy in the retina occurs in the range from 550 nM to 400 nM. Argon and YAG lasers operate in this range clearly making them the most hazardous lasers. Wavelengths of less than 550 nM can cause a photochemical injury similar to sunburn. Photochemical effects are cumulative and result from long exposures (over 10 seconds) to diffuse or scattered light.
Laser damage and retinal burns can occur when a laser beam enters the human eye. Lasers are a 'point source' of light much like the sun thus the eye focuses on infinity when viewing lasers especially in a darkened space where no other light sources are present. Laser beams are almost parallel thus the lens of the human eye will focus them down to a small spot. A laser beam with low divergence entering the eye can be focused down to a spot 10 to 20 microns in diameter.
The laws of thermodynamics do not limit the power of lasers. The second law states that the temperature of a surface heated by a beam from a thermal source of radiation cannot exceed the temperature of the source beam. The laser is a non-thermal source and is able to generate temperatures far greater than it's own. A 30 mW laser operating at room temperature is thus capable of producing enough energy (when focused) to instantly burn through paper!
Due to the law of the conservation of energy, the energy density (measure of energy per unit of area) of the laser beam increases as the spot size decreases. This means that the energy of a laser beam can be intensified up to 100,000 times by the focusing action of the eye. A one watt laser beam when focused down to a small spot can produce temperatures higher than the surface temperature of the sun! Thus even a low power laser in the milliwatt range can cause a burn if focused directly onto the retina.
NEVER point a laser, even a laser pointer, at someone's eyes no matter how low the power of the laser.
Structure of the eye
The part of the eye that provides the most acute vision is the Fovea Centralis (also called the Macula Lutea). This is a relatively small area of the retina (3 to 4%) that provides the most detailed and acute vision as well as your colour perception. This is why you move your eyes when you read or when you look as something; the image has to be focused on the fovea for detailed perception. The balance of the retina can perceive light, and movement but not detailed images (peripheral vision).
If a laser burn occurs on the Fovea, you can loose most of you fine (reading and working) vision in an instant. If a laser burn occurs in the peripheral vision it may produce little or no effect on fine vision. Repeated retinal burns can lead to blindness.
Fortunately the eye has a self defense mechanism, the blink or aversion response. When a bright light hits the eye it tends to blink or turn away from the light source (aversion). This MAY defend the eye from damage where very lower power lasers are involved but cannot help where higher power lasers are concerned. By the time the eye reacts, the damage is already done. Due to the focusing effect discussed above, a one watt laser beam entering the eye can be focused to 100,000 watts per square centimeter of power at the retina.
Eye damage can also occur when laser beams are scanned across the eye even for very brief periods. The amount of exposure is difficult to estimate as 'dwell' or 'transit' time must be taken into account in your calculations. For example a 1 mW laser illuminating a 7 mm aperture (the average size of the dark adapted iris) for one second is the equivalent of a 10 mW laser illuminating the same 7 mm aperture for 1/10 of a second.
The international laser safety standard, IEC-825, defines a short exposure as 2.5 mW per square centimeter. Each jurisdiction has it's own maximum exposure levels for laser radiation. You should consult with your local regulatory authorities and get their official methodology and formula(s) for calculating the MPE (Maximum Permissible Exposure).
Symptoms of a laser burn in the eye include a headache shortly after exposure, excessive watering of the eyes, and sudden appearance of many 'floaters' in your vision. Floaters are those swirling distortions that occur randomly in normal vision most often after a blink or when you have had your eyes closed for a couple of seconds. Floaters are caused by dead cell tissues that detach from the retina and choroid and float in the Vitreous Humour. Ophthalmologists often dismiss minor laser injuries as floaters due to the very difficult task of detecting minor retinal injuries.
Exposure of the skin to high power laser beams (1 or more watts) can cause burns. At the under five watt level, the heat from the laser beam will cause a flinch reaction before any serious damage occurs. The sensation is similar to touching any hot object, you tend to pull your hand away or drop it before any major damage occurs.
With higher power lasers, a burn can occur even though the flinch reaction may rapidly pull the affected skin out of the beam. These burns can be quite painful as the affected skin can be cooked and forms a hard lesion which takes ages to heal.