Yet, the 200 employees who worked there had all been spared from cholera outbreaks that hit the city in 1832, 1849, and 1852. When Burq learned that 400 to 500 copper workers on the same street had also mysteriously dodged cholera, he concluded that something about their professions—and copper—had made them immune to the highly infectious disease. He launched a detailed investigation into other people who worked with copper, in Paris and cities around the world.

In 1852, physician Victor Burq visited a copper smelter in Paris's 3rd arrondissement, where they used heat and chemicals to extract the reddish-brown metal. It was a dirty and dangerous job. Burq found the facility to be "in poor condition," along with the housing and the hygiene of the smelters. Normally, their mortality rates were "pitiful," he observed.

Since then, studies have shown that copper is able to destroy the microbes that most threaten our lives. It has been shown to kill a long list of microbes , including norovirus, MRSA, a staph bacteria that has become resistant to antibiotics, virulent strains of E. coli that cause food-borne illness, and coronaviruses—possibly including the novel strain currently causing the COVID-19 pandemic.

Today, we have insight into why a person handling copper day in and day out would have protection from a bacterial threat: Copper is antimicrobial. It kills bacteria and viruses, sometimes within minutes. In the 19th century, exposure to copper would have been an early version of constantly sanitizing one's hands.

After visiting 400 different businesses and factories in Paris, all of which used copper, and collecting reports from England, Sweden, and Russia on more than 200,000 people, he concluded to the French Academies of Science and Medicine in 1867 that “copper or its alloys, brass and bronze, applied literally and pregnantly to the skin in the cholera epidemic are effective means of prevention which should not be neglected.”

In the 1865 Paris epidemic, 6,176 people died of cholera, out of a population of 1,677,000 people—that’s 3.7 people out of every 1,000. But of the 30,000 who worked in different copper industries, only 45 died—an average of around 0.5 per 1,000.

In the 1854 to 1855 cholera epidemic, Burq could not find any deaths of jewellers, goldsmiths, or boilermakers—all those who worked with copper. In people in the army, he found that musicians who played brass instruments (brass is partly copper) were also protected.

On copper surfaces, bacteria and viruses die. When a microbe lands on a copper surface, the copper releases ions, which are electrically charged particles. Those copper ions blast through the outer membranes and destroy the whole cell, including the DNA or RNA inside. Because their DNA and RNA are destroyed, it also means a bacteria or virus can’t mutate and become resistant to the copper, or pass on genes (like for antibiotic resistance) to other microbes.

Many of the microbes that make us sick can live on hard surfaces for up to four or five days. When we touch those surfaces, the microbes can make it into our bodies through our nose, mouth, or eyes, and infect us.

“What happened is our own arrogance and our love of plastic and other materials took over,” Schmidt said of the cheaper products more frequently used. “We moved away from copper beds, copper railings, and copper door knobs to stainless steel, plastic, and aluminum.”

If copper were more frequently used in hospitals, where 1 in 31 people get healthcare-acquired infections (HAI), or in high-traffic areas, where many people touch surfaces teeming with microbial life—it could play an invaluable role in public health, said Michael Schmidt, a professor of microbiology and immunology at the Medical University of South Carolina, who studies copper. And yet, it is woefully absent from our public spaces, healthcare settings, and homes.

Before people even knew what bacteria and viruses were, they knew that copper could—somehow—ward off infection. The first recorded medical use of copper is from one of the oldest-known books, the Smith Papyrus, written between 2600 and 2200 B.C. It said that copper was used to sterilize chest wounds and drinking water. Egyptian and Babylonian soldiers would similarly put the shavings from their bronze swords (made from copper and tin) into their open wounds to reduce infections. A more contemporary use of copper: In New York City’s Grand Central Station, the grand staircase is flanked by copper handrails. “Those are actually anti-microbial,” Schmidt said.

The copper smelters were, ostensibly, exposed to less of the cholera bacterium because their surroundings included a lot of copper that bacteria couldn't live on. That and they potentially were covered in copper particles. If metallurgy doesn't call to you, there are now some products that are advertised as "copper hand sanitizers," but they work only if you can expose every surface of your hands to the copper for at least a full minute—essentially transferring any microbes to the copper surface to be killed. It could be difficult to get to every part of your skin's surface, so having copper surfaces in your environment paired with handwashing would be the ideal combination.