Why wear a mask when you go out in public? Because you may have the virus and not know it, and it's very contagious

President Trump wore a mask on a private tour of a Ford plant Thursday but refuses to do so in public.

Wearing a mask when out in public is even more important as state economies reopen, medical professionals advise. Many Americans are skeptical.

"Some people have objected to masks, and the challenging part about that is you can object to a mask on your own personal health, but it is not your own personal health that it is going to impact," Kentucky Gov. Andy Beshear said May 19. "It is other people's health, so it is more about your willingness to protect other people if you are wearing or not wearing one."

"The governor regularly points out that masks are recommended by the White House and the Centers for Disease Control and Prevention," Melissa Patrick reports for Kentucky Health News. "At one point he said they shouldn't be a partisan issue."

A Kaiser Family Foundation Poll released May 22 found that Democrats are about twice as likely as Republicans to say they wear a mask every time they leave their house: 70% and 37%, respectively. Majorities of each party said they wear a mask "most of the time."

Kaiser Family Foundation chart; click on it to enlarge. More data and poll questions are here. 

The poll also found that while 72% of Americans think President Trump should wear a mask when meeting with others, only about half of Republicans, 48%, agree. The partisan difference in largely driven by Republican men.

Trump says he doesn't need to wear a mask because he is tested daily for the virus, but critics say he is missing an opportunity to set an example that would save lives. Thursday, he refused to wear a mask in front of news cameras while touring a Ford plant in Michigan, saying "I didn't want to give the press the pleasure of seeing it." He wore a mask when news cameras weren't around, but someone on the tour took a picture of him and it was widely circulated.

So, why should you wear a mask? "The simple answer is because the virus is primarily spread by tiny droplets from infected people, not just coughing and sneezing, but from talking and breathing," Patrick writes. "A mask can stop the spread of those droplets, especially from people who have the virus but don't know that they do." Kentucky Health Commissioner Stevn Stack says about one in four people with the virus have no symptoms.

Patrick writes that the value of wearing a mask is illustrated by a new study, published in The Proceedings of the National Academy of Sciences. It shows how normal speaking can launch thousands of droplets that can remain suspended in the air for eight to 14 minutes, allowing them to be inhaled by others. "There is a substantial probability that normal speaking causes airborne virus transmission in confined environments," the study report says.

The researchers said in a letter to The New England Journal of Medicine that the same experiment, using scattered laser light, found that use of a cloth face mask blocked nearly all droplets emitted when talking. They posted a video, the last part if it in slow motion, to show their finding.

Science supporting mask wearing is so strong that more than 100 prominent health experts have asked governors to require them. They write that the research "strongly suggests that requiring fabric mask use in public places could be amongst the most powerful tools to stop the community spread of covid-19."

So why did the CDC recommend not wearing a mask two months ago? "The Mayo Clinic says face masks were not recommended at the start of the pandemic because experts didn't know the extent to which people with covid-19 could spread the virus before symptoms appear, nor was it known that some people have covid-19 but don't have symptoms," Patrick reports. "The CDC now recommends the use of reusable cloth masks so that surgical masks and N95 respirator masks, which continue to be in short supply, can be saved for health-care workers."

The CDC guidance says cloth masks should: fit snugly but comfortably against the side of the face; be secured with ties or ear loops; include multiple layers of fabric; allow breathing without restriction; and be able to be laundered and machine-dried without damage or change to shape. It also cautions that they should not be placed on children under age 2, anyone who has trouble breathing, or anyone who would have trouble removing the mask without help. The Mayo Clinic recommends that cloth face coverings be washed after every day of use.