Christmas light enthusiasts have always known not to drop what they are hanging. A broken bulb can spoil everything. Now they have another reason to be careful. Despite its growing popularity as a green lighting alternative, light-emitting diodes (LEDs) may not be as eco-friendly as advertised.

According to researchers in California, the efficient LEDs are not so safe after all. Scientists from the University of California campuses at Irvine and Davis conducted a series of tests on LEDs used for Christmas lights, traffic lights, and automobile headlights and brake lights that determined that they contain unsafe amounts of numerous toxic compounds.

The studies found that those LEDs contain lead, arsenic and a dozen other potentially hazardous substances. They reached their conclusions by crunching, leaching and measuring different colored LEDs sold in Christmas light strands. They also compared the results for different colored LEDs. The results were notable.

In general, high-intensity, brighter bulbs had more contaminants, but low-intensity red lights still contained up to eight times the amount of lead allowed under California state law. White bulbs contained the least amount of lead but had high levels of nickel.

Many of the elements, including lead and arsenic, have been linked to cancer and various other ailments. Copper is also considered an environmental hazard.

Of course, breaking one light is not necessarily a disaster. However, taken collectively—when considering the hazards posed from these toxic elements in the production of new lights, cleanup and disposal of broken or old and worn out lights, and the risk of small children swallowing the lights-—the dangers are real.

The authors of the studies have forwarded their findings to state and federal officials in the hopes that new testing requirements will lead to the replacement of these toxic chemicals in the manufacturing of LEDs.

There is no word on the effect these findings will have on the greater lighting industry, but if further research produces similar results, they could hamper LED adoption and affect which energy-efficient technology reigns supreme in the future.