RALEIGH, N.C. (WNCN) — A chart making the rounds this week on social media breaks down how long it takes to catch COVID-19 from an infected person based on what — if any — masks both people are wearing.

The doctor who made that table is concerned that people might be reading it the wrong way.

This chart shows how long it could take to transmit an infectious dose of COVID-19 based on what masks, if any, are being worn by both the infected and uninfected person. (Source: American Conference of Governmental Industrial Hygienists.)

“I worry a lot about people taking the numbers and seeing them as sort of a bright line between when you’re safe and when you’re not safe,” said Dr. Lisa Brosseau, a retired professor in the University of Illinois Chicago’s school of public health.

Brosseau made the chart after she was asked to head a COVID task force for the American Conference for Governmental Industrial Hygenists — a nonprofit occupational health association.

It was designed to be an easy-to-understand guideline to help workers stay safe during the pandemic.

On social media this week, it took on a life of its own.

“People are looking for this type of information, obviously, because they’re concerned about the transmission with the omicron variant that we’re seeing now,” said Dr. Emily Sickbert-Bennett, an associate professor of infectious diseases at the University of North Carolina School of Medicine.

Said Brosseau: “Even industrial hygienists have been sort of trying to make it into something more than it is.”

The chart lays out the rough differences between the types of masks, saying two unmasked people could transmit the virus in as little as 15 minutes while it would take 2,500 hours for COVID to spread between two people wearing the highest grade of N95 masks.

Sickbert-Bennett, who has published research on mask effectiveness, said the chart “is showing … a representation of different types of masks on a very average sense and their filtration ability.

“They based it off a few important assumptions that people should be aware of,” she said.

First, the chart was published in Spring 2021 — before not only the current omicron surge but also the delta wave that came last summer and fall.

And those times aren’t nearly as firm as they appear.

“The numbers that are presented in that table are presented in a very concrete way,” Sickbert-Bennett said. “And this is a very abstract concept.”

Brosseau makes the same point: “That 15 minutes obviously isn’t cast in stone,” she said.

She says they’re based on estimates from the CDC.

“There’s a lot of things that go into exposure that could make it a much shorter time to an infectious dose,” she said. “Those things are really hard to know what their effects will be, and they’re hard to measure. 

“You don’t know who’s infectious. You don’t know how many people are infectious. You don’t know the ventilation in a space, and those things are hard to measure and predict,” Brosseau added. “So rather than be complicated, I just relied on the 15 minutes as a as a baseline to try and illustrate the differences.”

Brousseau said those approximations make it extremely difficult to adjust for omicron.

“I tell people I’m really not in favor of changing that 15-minute contact-tracing time to anything else — even if we know that omicron is more transmissible — because I don’t know what to change it to,” she said.