well you should be concerned about fires and take some precautions in preventing them, the lights like hids such as a high pressure sodium, or metal halide are safe and are commonly used in both indoor and outdoor lighting already and are not really a fire hazard providing they are mounted secure to prevent them from falling,and all the ballast components is in a ballast box or housing and the lights located a safe distance from flamable materials so that even if the cooling fan failed to come on it wouldnt start a fire. you should also make sure that your house wiring and circuit are adequete size to supply the lights used, and would even suggest you spend a little more for a good timer and not use the cheaper 15 amp timers that you plug into the wall.
you might due a search on leds, some have experimented with them, but dont think they will ever be used with good results for growing plants.