Fluorescent lamps (straight, shattershields, U-tubes, and circular); mercury vapor lamps; metal halide lamps; and high-pressure sodium lamps contain mercury, which is harmful to people and the environment.
As a generator of these wastes, you're bound by state and federal regulations to dispose of them properly. (Violate the rules and you may be liable for fines up to $25,000 per incident, per day. You may also damage your public reputation and incur long-term liability.)
