The first formal process for curbing the spread of infection by detaining travelers from an affected region until their health was proved was instituted in what is now Dubrovnik, Croatia, in 1377, against the bubonic plague. (This temporal buffer was originally 30 days, but when that proved too short, it was extended to 40 days, or quaranta giorni, from which we derive the word “quarantine.”)

Mail disinfection soon followed, as the then Republic of Venice extended and formalized the quarantine process to include cargo. Items that were considered particularly susceptible, including textiles and letters, were also subject to fumigation: dipped in or sprinkled with vinegar, then often exposed to smoke from aromatic substances, from rosemary to, in later years, chlorine. Once the items were treated, a distinctive wax seal or cancellation was usually applied to them, so the recipient would know where and when the disinfection had been carried out. (Such marks often provide the only remaining evidence of the ebb and flow of disease; some minor outbreaks of plague or typhus in remote areas of medieval Europe, for example, would have been lost to history without their postal traces.)

The diseases changed, but for centuries mail disinfection techniques remained largely the same. As recently as 1900, during a plague outbreak in Honolulu, letters were routinely disinfected by clipping off the two opposite corners of each envelope and then spreading a batch of mail out in an airtight room filled with sulfur fumes for three hours.

A representative for the U.S. Postal Service was unwilling to discuss current sanitization protocols. But the agency’s website reports that the only mail items receiving treatment are letters and parcels sent to ZIP codes beginning in 202, 203, 204 and 205, which serve federal government agencies in Washington, D.C. In a process that began shortly after the 2001 anthrax attacks, the Postal Service sends mail destined for those ZIP codes to New Jersey, where they are put on a conveyor belt and passed under a high-energy beam of ionizing radiation that kills bacteria and viruses. The letters and packages are then “aired out” for a while, before being forwarded to their destinations. The paper is left slightly faded and somewhat crispy, but sterile.

Should mail irradiation be extended beyond these exclusive ZIP codes, to help prevent the spread of the coronavirus? On CBS News’ “Face the Nation” on Sunday morning, Scott Gottlieb, the former commissioner of the Food and Drug Administration, warned that SARS-CoV-2 could potentially be transmitted by contaminated objects. “This is a sticky virus,” he said. The structure of the coronavirus’ protective envelope helps it bond tightly to certain surfaces: skin in particular, as well as fabric and wood, but also plastic and steel.

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Some government agencies seem concerned that all of that circulating paper might be a potential vector. In early February, when COVID-19 was still just “the coronavirus” — and, for most Americans, still someone else’s problem — China’s central bank announced that it would quarantine the country’s cash, to prevent the disease from spreading from one person to another on money. The government collected bank notes from Hubei, the worst-hit province, and then sanitized the stacks of bills, either by baking them at a high temperature or bathing them in ultraviolet rays. The newly laundered cash was then kept in isolation for seven to 14 days before being rereleased into the banking system.

A few weeks later, the U.S. Federal Reserve began quarantining dollar bills repatriated from Asia, holding them for seven to 10 days before allowing them to re-enter the domestic financial system. Bank notes are made of cotton pulp, not wood fiber, but still: Why sanitize money and not mail?

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Representatives of the big three package deliverers in the United States — UPS, FedEx and the Postal Service — insisted there is no need. “The CDC has advised that there is a low risk of transmission on packages,” said Matthew O’Conner, a spokesman for UPS. FedEx, in a statement, said, “The guidance from the WHO is that the likelihood of an infected person contaminating commercial goods is low, and the risk of catching the virus that causes COVID-19 from a package that has been moved, traveled, and exposed to different conditions and temperature is also low.”

David Partenheimer, a spokesman for the Postal Service, noted that the surgeon general, Dr. Jerome M. Adams, along with the Centers for Disease Control and the World Health Organization, has “indicated that there is currently no evidence that COVID-19 is being spread through the mail.”

This is because many scientists think it is quite unlikely that you can catch the coronavirus by touching a surface that has the virus on it and subsequently touching your own mouth or nose. (One review of scientific publications on the subject concluded that hand-washing seems to cut the risk of respiratory infection by a mere 16% — but added that the studies examined were of poor quality and more research was urgently needed.)

The German Federal Institute for Risk Assessment — Germany’s equivalent of the FDA — advises that while the virus could, theoretically, be transmitted through this kind of “smear” infection, as opposed to the standard “droplet” infection, there have been no known cases in which individuals have caught the coronavirus by touching a contaminated surface and then transferring the virus to their mouth or nose. Then again, contact transmission is notoriously difficult to study and document.

A paper published in the New England Journal of Medicine last week shed more light on the subject. A group of researchers from the National Institutes of Health, the CDC, Princeton University and the University of California, Los Angeles, misted virus particles into a rotating drum and studied how long the floating particles survived on various surfaces. They found that the SARS-CoV-2 virus survived for up to 24 hours on cardboard — three times longer than its cousin, the original SARS.

“In that light, you might expect the virus to remain viable for hours but probably not days on mail,” said James Lloyd-Smith, one of the study’s authors. “But there are important caveats.”

Among these: The study specifically looked at aerosolized virus particles, rather than the fine droplets that infected people emit with each cough or sneeze. The line between aerosols and droplets is fuzzy, but, broadly, droplets are bigger and settle more quickly, while aerosols are smaller and float for longer.

“Little is known about the rate at which infected people generate such aerosolized viruses,” Lloyd-Smith said, although some medical procedures — including intubation to assist with breathing — are known to create aerosols. In addition, the experiment was designed to simulate indoor air; viruses like this one are expected to be stable for longer at lower temperatures and lower humidity.

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In an experiment that might better mimic the effect of sneezing on a letter, researchers from Hong Kong University pipetted droplets of SARS-CoV-2 onto various surfaces. Their results, which have not yet been peer reviewed and do not include details on temperature and humidity conditions, found no infectious virus left on paper after three hours — although, alarmingly, a significant level could be detected on the outer layer of a surgical mask after seven days.

“The bottom line is that there is some hypothetical risk of viable viruses surviving on mail,” Lloyd-Smith said. “But given the time periods involved, this seems like a pretty minimal risk to the general public.”

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Ben Chapman, a food safety specialist at North Carolina State University who studies the interaction of pathogens and surfaces, agreed that although the virus can persist on packages, these have not been identified as a risk factor for transmission. Nonetheless, he said, “I’d just wash my hands after handling,” rather than spray with Lysol or wipe with bleach. “I want to preserve the good sanitizers for risky things, and hand-washing works just as well as spraying.”

For the delivery workers pulling 12-hour shifts to keep up with demand, and struggling to access the sanitizing supplies and hand-washing facilities they need, the situation is potentially riskier. “I would be more worried about the mail sorters and mail carriers, who are exposed to many more pieces of mail,” Lloyd-Smith said.

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Delivery is deemed an “essential function” by the Department of Homeland Security, and the volume of packages is expected only to increase as more Americans are required to shelter in place. Irradiating all mail is both overkill and likely impossible, given the volumes in question and the resources required. But making sure that the people who are keeping mail moving during this pandemic are protected also seems essential, not to mention ethical.

“The safety and well-being of U.S. Postal Service employees and customers is our highest priority,” said Partenheimer, who added that the agency is closely monitoring the situation to implement CDC and public health department advice.

This article originally appeared in The New York Times .