Clearly, ancient Egyptians didn't get the memo about lead poisoning. Their eye makeup was full of the stuff. Although today we know that lead can cause brain damage and miscarriages, the Egyptians believed that lead-based cosmetics protected against eye diseases. Now, new research suggests that they may have been on to something.

Previous work indicates that the Egyptians added lead to their cosmetics on purpose. When analytical chemist Philippe Walter and colleagues at CNRS and the Louvre Museum in Paris analyzed the composition of several samples of the Egyptians' famous bold, black eyeliner in the Louvre's collection, they identified two types of lead salt not found in nature. That means that ancient Egyptians must have synthesized them. But making lead salt is a tricky, delicate process that requires tending for weeks--and unlike other common makeup components, the salts are not glossy. So why did they bother?

Ancient manuscripts gave the scientists a clue. It turns out that in those days, people made lead salts and used them as treatments for eye ailments, scars, and discolorations. When Walter told analytical chemist Christian Amatore of the Ecole Normale Supérieure in Paris about the findings, Amatore says he was intrigued because lead is now known to have so many toxic effects.

To see if the lead might confer any health benefits, Amatore, Walter, and colleagues added lead salts to human skin cells called keratinocytes, which were grown in the lab. The researchers hypothesized that the lead would stress the cells and cause them to make hydrogen peroxide, nitric oxide, and other compounds involved in the body's immune response. And indeed, cells treated with lead began pumping out more nitric oxide than did control cells, the team reports online in Analytical Chemistry.

Amatore says that nitric oxide sets off a series of biochemical processes in the body that ultimately send immune cells called macrophages to the site of infection, where they engulf invading organisms. That's probably not what's happening in keratinocytes, says immunologist Martin Olivier of McGill University in Montreal, Canada, who was not involved in the study. It's unlikely that macrophages or other immune cells would exit the body and burst through the skin to fight off infectious agents at the surface, he notes. Instead, nitric oxide released by keratinocytes could directly kill eye-disease-causing bacteria on the skin or near the eye by breaking down a bacterium's structure or DNA. Another plausible scenario, says Olivier, is that lead itself could directly stimulate immune cells already present in the eyelid.

This potential benefit of lead is contrary to everything we know about the substance, but it could fit the model of hormesis, says epidemiologist Jennifer Weuve of Rush University Medical Center in Chicago, Illinois. "The premise behind hormesis is that, for certain exposures, there might be a window where the exposure is harmful but also one where it's helpful," she explains.

Still, Weuve cautions against adding lead to the eyeliner in your makeup case. Modern people live a lot longer than did the ancient Egyptians--many of whom died in their 30s--and the dangers of prolonged lead exposure outweigh any antimicrobial benefit, she says. Indeed, the Egyptians' eyeliner strategy would have backfired on them if they had lived long enough, she notes, as long-term exposure to lead may increase the risk of developing cataracts.