CDC Updates Close Contact Guidelines: 6 Feet Isnt Enough | Elemental – Elemental

Last Updated on October 23, 2020 by

The coronavirus ignores this outdated social distancing measure

Robert Roy Britt

The U.S. Centers for Disease Control and Prevention (CDC) just updated its definition of “close contact” with a person infected with Covid-19 — wording that should influence how all of us think about social distancing, and which will change who gets traced and notified when someone tests positive. “Close contact” had been defined as 15 continuous minutes or more within six feet of an infectious individual, whether or not either person wore a mask and regardless of whether the contact occurred indoors or outside, though the CDC says those are important factors that affect transmission.

The new definition: 15 minutes or more of cumulative exposure to infectious individuals, within six feet, in a 24-hour period — such as three five-minute exposures — still regardless of masking and whether the contact was inside or outdoors.

The new recommendation was announced October 21, the same day the CDC published a case study of a correctional officer in Vermont thought to have caught the coronavirus via 22 brief encounters, within six feet of various infected inmates, all adding up to 17 minutes of exposure during an eight-hour shift.

Tom Frieden, former CDC director, calls the new definition a “sensible change.” “Makes a big difference for contact tracing efforts,” tweeted Syra Madad, PhD, an epidemiologist and director of the special pathogens program at NYC Health + Hospitals, a public health system. State and local health agencies, with varying degrees of effort and effectiveness, aim to trace and track down “close contacts” and urge them to quarantine. The federal government has no formal tracing plan.

But as several leading infectious-disease experts have been shouting into the void since springtime, the coronavirus does not obey this six-foot rule. Not even close.

“I have never believed that six feet is enough indoors without everyone wearing a mask,” says Richard Corsi, PhD, dean of the College of Engineering and Computer Science at Portland State University and an expert on indoor air quality. “I also do not buy that there is a ‘threshold’ dose that such guidance implies.”

A person could absolutely be infected with less than 15 minutes of total exposure, Corsi tells Elemental.

“Given the growing evidence on the importance of aerosol transmission, close contact (and tracing) should be expanded to include anyone in the same room breathing/sharing the air… not just within six feet.”

Out the mouths of infected people with every breath or spoken word come thousands upon thousands of virus particles, all hitching rides inside respiratory droplets so small they’re invisible, like the individual particles in a plume of smoke. These aerosols, as they’re called, can keep viable virus particles aloft for minutes or even hours. Indoors, and particularly in poorly ventilated areas, the bad air can build up and potentially cause infections throughout a room.

Well-fitted masks can stop most, but not all of these aerosols, which is why the CDC and all leading infectious-disease experts advise a layered approach to prevention: avoid crowds, mask up, keep distance, and wash hands often.

Kimberly Prather, PhD, director of the Center for Aerosol Impacts on Chemistry of the Environment in the Scripps Institution of Oceanography at the University of California, San Diego, had this to say about the new CDC definition:

“Given the growing evidence on the importance of aerosol transmission, close contact (and tracing) should be expanded to include anyone in the same room breathing/sharing the air… not just within six feet.”

Such thinking should apply to bars and restaurants, gyms, offices, and schools — any indoor space where people share air for extended periods — scientists have been saying.

Prather, knowing what she knows about coronavirus transmission in aerosols, told me previously that she wears a mask even outside and keeps a good 10 feet from other people when outdoors. She admits she’s very cautious, but that illustrates what those in the know think about the ability of this virus to get around.

The six-foot “rule” was developed and adopted by the CDC and other health agencies long ago, based on studies of coughs and sneezes done mostly before WWII, when technology could not detect aerosols and considered only larger respiratory droplets that were found to mostly fall to the ground within about six feet, Prather and colleagues noted in a perspective published in May in the journal Science.

A review of the research on virus transmission by respiratory droplets, dating back to the 1800s and up through this year, was published in the British Medical Journal this August. The researchers concluded that the six-foot rule fails to take into account a host of factors now well understood by scientists, including the very existence of aerosols. “Current rules on safe physical distancing are based on outdated science,” the researchers concluded.

In explaining what’s known now, Corsi pushes back not only on the CDC’s arbitrary thresholds, but also on close contact being defined only by time and distance.

Whether someone catches Covid-19, and how severe their symptoms become, depends ultimately on the dose they inhale and perhaps each individual’s biological susceptibility, scientists say. While the exact dose of viral particles required for an infection is not known, and may vary by individual, there are several factors that determine the dose a person inhales.

Among them:

  • Concentration of virus emitted by an infected person
  • Proximity to the infected person
  • Dosage inhaled per minute
  • Duration of exposure

But wait, there’s more math to the equation, as Corsi outlines in his blog. The amount of virus a person emits, measured as “respiratory minute volume,” is affected greatly by activity.

“If someone is doing aerobic exercise in a gym or dancing, their respiratory minute volume can be as high as 10 to 15 times what it would be at rest,” Corsi says. Likewise, the amount of virus another person inhales can vary greatly by their own activity and breathing level. And both emission and inhalation are tremendously affected by mask-wearing and other physical circumstances.

“Six feet is not enough to protect yourself from exposure to aerosols, those small particles that remain airborne inside for a long time.”

Ventilation is a major factor in reducing the spread of Covid-19, and it doesn’t get enough respect, experts say.

A good cross-flow created by open doors or windows brings in outside air to dilute any virus-laden air, Corsi explains. An air conditioning system with high-quality filtration can also reduce the concentration of viral particles.

Those are two of four ways virus particles will be cleared out of a room, explains Joseph Allen, assistant professor of exposure assessment science at Harvard T.H. Chan School of Public Health: Some particles will settle out on surfaces. What remains is “diluted out of the air through ventilation, it’s cleaned out of the air through filtration, or it’s deposited in the lungs. And of course, we’re trying to avoid that fourth one.”

Scientists don’t wish to entirely distance themselves from the six-foot guideline. They just want policymakers, public health officials, and everyone else to understand the nuances. Shelly Miller, PhD, an environmental engineer at the University of Colorado, Boulder, who studies the indoor transmission of infectious diseases, explains:

“Six feet is not enough to protect yourself from exposure to aerosols, those small particles that remain airborne inside for a long time,” Miller tells me. “However, six feet helps get you out of the personal cloud that is around an infected person that contains high concentrations of virus, and removes you from the high-velocity plume that flies out of their mouth when they talk.”

While no scientist can pin down an exact distance that’s safe for all situations, Allen offers this view:

“Six feet is good,” he advised back in May. “But 10 feet is better.”

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