The San Francisco Chronicle today published what attempts to be the umpteenth definitive guide on different mask types, how well they protect against COVID, and when they should be replaced — in light of everyone's heightened anxieties in the latest of surges. But the information is based on earlier variants and should not be taken as gospel.

The Centers for Disease Control this week is likely to issue new guidance encouraging more use of N95 and KN95 masks amid the Omicron surge, as there is evidence to suggest that cloth masks no longer cut it with this variant. And capitalizing on the moment, the Chronicle had a new front-page piece all about N95s, how long you should use the same one, and how well they protect you against infection.

Within the piece is a recreation of the chart below, originally created early last year by Dr. Lisa Brosseau, a retired professor in the University of Illinois Chicago’s school of public health. Versions of the chart have been circulating widely on social media in the last week as people's mask vigilance has once again been increasing, and as everyone is again worried about getting infected while many people around them are testing positive. But even Dr. Brosseau cautions that people shouldn't be looking at this chart for any kind of accurate current guidance. First and foremost in the caveats, it was based on outdated CDC data collected prior to the Delta variant's arrival, and way before the wily Omicron variant.

"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," Dr. Brosseau says, speaking to CBS affiliate WNCN.

Chart via American Conference of Governmental Industrial Hygienists

The chart purports to estimate how long a COVID-negative "receiver" would be safe in close contact with a COVID-positive source while wearing different types of masks. The "N95 FFR (10%)" refers to a N95 mask with 10% leakage, while the "1%" row refers to 1% leakage, depending on how tightly fitted it is.

Under this thinking, a person wearing a somewhat leaky N95 mask would be safe on a 5-hour plane ride next to a COVID-positive person wearing a surgical mask, but not really as safe if that infected person were wearing just a cloth mask — and if both people were wearing cloth masks, transmission could likely occur in 40 minutes or less.

Brosseau created the chart in the spring of 2021, after being asked to head a COVID task force for the American Conference for Governmental Industrial Hygienists, a nonprofit occupational health association. And Brosseau says, "Even industrial hygienists have been sort of trying to make it into something more than it is."

And all of us should be skeptical about such metrics given how many variables are involved — including how recently vaccinated a person is, which COVID variant we're talking about, how much the infected person is "shedding" based on how recent the infection was, and what constitutes close contact.

"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,” says Dr. Emily Sickbert-Bennett, an associate professor of infectious diseases at the University of North Carolina School of Medicine, speaking to WNCN. But, Dr. Sickbert-Bennett cautions, "The numbers that are presented in that table are presented in a very concrete way. And this is a very abstract concept. There’s a lot of things that go into exposure that could make it a much shorter time to an infectious dose."

Brosseau wants everyone to know that this was meant to be an easy-to-understand illustration based on 2020 data, not some definitive guide based on current CDC expertise — as it's being presented by the Chronicle.

"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 tells the station. “So rather than be complicated, I just relied on the 15 minutes as a baseline to try and illustrate the differences."

Brosseau said she would likely revise the 15 minutes to something lower for the more infectious Omicron variant, except there isn't enough data and she wouldn't know what number to assign at this point.

"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," Brosseau says.

The basic idea here is: For the next few weeks as the virus is everywhere, don't spend long periods in indoor spaces with unmasked strangers if you don't want to be infected! Or even unmasked friends who aren't being particularly cautious — unless the risk is worth it to you and you're not worried about getting sick, or you've already had the Omicron. And if you keep a N95 mask on the whole time you're probably pretty safe, but no one is ever perfectly safe.

Related: UCSF's Dr. Bob Wachter Details His Son's Recent COVID Infection, His Own Worry