Blindness is PERMANENT so please bare with me and read on...  LASERs emit highly excited and focused light (a coherent beam).  That is what makes them very visible and why they are used for pointing devices, laser levels, weapon sights, and for just having plain old fun.  The "fun" part is where problems can occur.  I will be the first to admit, being a gadget freak, that the idea of having a laser as small as a pen that can pop balloons, cut tape, beam a spot of light on the moon, is very appealing.  Think of the fun to be had...  The problem is that all lasers are not equal nor realatively safe.

Class I, II, & IIa lasers are low powered and considered pretty eye and tissue safe. A person's blink reflex prevents pain and retinal damage when the light is viewed directly (the e۠ye's retina is critical to vision).   Class IIIa lasers are h۠igher powered than Classes I-IIa and are also considered safe providing one doesn't stare into the beam (blink reflex usually prevents this).  Class III lasers include common laser pointers, laser levels, weapon sights, etc...  Class I-IIIa lasers are 5mW or less in power.  Problems really begin with Class IIIb - IV lasers (those of the "really cool" high powered lasers you might find on ebay).

Think before you buy!  Class IIIb and IV  visible light lasers are in the 5mW to 500mW power range.  They are capable of damaging or destroying retinal tissue in 1/1000th of a second or less, depending on wavelength (~ 380-750nm for visible light) and power, through direct viewing of the beam (shining it towards the eye) and specular reflection (a mirror-like reflection).  Class IV lasers will also damage retinal tissue with diffuse reflected light (light bouncing off non-specular surfaces such as a flat painted bedroom wall, etc...).  Retinal eye damage is PERMANENT as this tissue does not regenerate.  Damage will result in anything from blurred vision to permanent blindness.  This is why the FDA regulates the sales of Class IIIb and Class IV lasers.  They are very dangerous in untrained hands, even if the user is wearing proper protective equipment (proper eye protection, etc...).  A light beam that can reach the moon and bounce back to be seen on earth will certainly reflect off things in your neighborhood and eventually enter someones eyes, potentially destroying tissue.  Class IIIb and Class IV lasers also are capable of destroying other body tissue such as skin (remember they cut tape, light matches, and pop balloons).  They just don't burn tissue, they also can cause acoustical damage (like shock waves) and photochemical (cellular damage/mutation).  All educational, government, and commercial industries require a laser safety course completion before an individual is allowed to use these types of lasers.