Today I was looking around in the local hardware store in the electrical section for some lighting. I have a small building (20 x 30) that I want to light. In the cold months of the year it is not likely it will be heated very warm and florescent lighting is out of the question because of this. Ahead of time I had decided to look into some mercury vapor lights similar to what is used as the typical 'security light'. Alot of these nowadays are sodium or metal halide. The dork that was working there said that you have to mount mercury vapor lights 25 feet high to allow the air to filter out the UV radiation coming off of the light. He mentioned metal halide and sodium would be acceptable. I don't believe I even told him where I wanted to mount the lights. I ignored him and went to look at the lights anyway. I find pictures on the box of many of these lights mounted on the outside of buildings at best 10 feet off of the ground on patios and such. I left them there because this is not something I need right away and I thought I'd do a bit more research. Mercury vapor, sodium, and metal halide are ALL arc discharge lamps and will ALL emit UV radiation when malfunctioning due to a broken outer bulb or some such failure. I asked another employee about it and they also mentioned that they needed to be mounted a certain height off of the ground. What I suspect the reason is for mounting these at a certain height or higher is to reduce the chance of breakage from step ladders and etc. So what's the story on ANY of these arc discharge lamps? I find it hard to believe that mercury is any worse than any of the others. Is there a good reason an outdoor light of suitable intensity cannot be mounted indoors with ample clearance around the light for cooling? In my case the lights will be greater than 10 feet off of the floor and there will likely be 2 or more feet of clearance to the roof which is sloping at about a 4/12 pitch.