http://staff.uow.edu.au/ohs/workings...Guidelines.pdf
Class 1 lasers are those which are incapable of damaging the eyes or skin because of either engineered design or inherently low power output. The lasers used in CD players are the most common example of this category.
Class 2 lasers have sufficient power output to cause damage to the eyes if viewed continuously. However, their outputs are low enough to allow the natural aversion responses, such as blinking, to prevent damage. Additional hazard control measures take the form of cautionary signs or labels. The laser pointers often used by conference presenters are common examples.
Class 3A lasers have the potential to cause damage to the eyes from intra-beam viewing and precautions are required to prevent both direct viewing or viewing with optical instruments.
Class 3B (restricted) lasers operate at the same power levels as class 3A but have higher irradiance, i.e. higher power density, and consequently have some additional restrictions on use.
Class 3B lasers are more hazardous because of either higher output or operation outside visible wavelengths. In addition, specular reflections may also be hazardous. In general, more stringent controls are needed to prevent exposure.
Class 4 lasers are high power devices capable of producing eye damage even from diffuse reflection. Skin damage is also possible from even brief exposures. Class 4 lasers may also constitute a fire hazard. Examples of class 4 lasers include entertainment lasers, surgical lasers and those used in the plastic, wood and metal fabrication industries.
I'm an astronomer. I treat my class 3a laser with utmost respect; its a great tool, but I value my sight. If you have a 3b device (a modded pen device above 5mW for eg) then you need to be either licenced or locked up.
In the UK, where health and safety is taken seriously, they are illegal above 1mW (class 2) for general use.
I don't like to preach to grown ups, but this is a beginners forum ...