Harvard prof: People who are 'out for a jog with no one around' should wear a 'two- or three-layer cloth mask'

In a pitch to get more Americans to wear more effective masks or double up on masks, one Harvard professor is urging people to really go all in on wearing multiple-layer face coverings.

Even if you're outside jogging ... and you're alone ... with no one around.

What's that now?

America is currently seeing a surge in calls by health experts to double up on masks.

If one mask is good, then two would be even better, according to the thinking.

In fact, if two is good, why not three? — as CNBC actually asked this week.

With each layer, the science says, the effectiveness grows. According to CNBC, doubling up increases efficacy from 55% to 70%. Tripling increases efficacy to 90%.

But that still falls short of N95 masks, which have an efficacy rate of 95%.

In an op-ed for the Washington Post on Tuesday, Joseph Allen, a Harvard University associate professor and director of the Healthy Buildings program at Harvard's T.H. Chan School of Public Health, urged all Americans to adjust their thinking on masks and take extra precautions.

First, he wrote, every American should be using N95s. There has been a shortage of those masks, for which the professor blames the U.S. government since it did not invoke the Defense Production Act and force manufactures to revamp their factories to create the masks.

If you can't get N95s, then go for KF94s or KN95s.

And if those options aren't possible, then double up, he wrote, "with a surgical mask and a cloth mask."

Outdoor warning

Then he had a warning for people who are outdoors. Don't skip out on multiple-layered face coverings just because you're outside.

In fact, even if "you're out for a jog with no one around," the good doctor wrote, you should wear a "two- or three-layer cloth mask":

Many people ask if they need an N95 on their morning runs or while sitting on a park bench. The answer to both is no. Choose masks based on the level of risk for that activity. If you're out for a jog with no one around or on a walk outside with a friend, a simple two- or three-layer cloth mask is fine. But use a hi-fi mask or double-mask if you head indoors. If you're an essential worker, a hi-fi mask is critical.

The doctor did not go into detail about whether, if lone joggers outdoors with no one around should be masked up, there is ever a time or situation where a person should feel OK with not having a mask on their face.