"This is a, I would say, senseless dividing line," said Doug Burgum, governor of North Dakota,
his voice catching as he talked of the rows that have broken out in his state over the wearing of face-coverings.
There are similar spats elsewhere in America, for masks have become the latest aspect of the culture war that has emerged there over how to deal with covid-19.
Some shops refuse entry to maskwearers and Mike DeWine, the governor of Ohio,
has rescinded an order requiring people to wear them, saying that he "went too far".
Elsewhere in the world, by contrast, there is increasing acceptance that mask-wearing is a good thing.
On May 5th, for example, the Royal Society, Britain's top science academy,
concluded that masks "could be an important tool for managing community transmission".
This is not so much because they protect the wearer--the normal reason people may put them on in times of pestilence--
but rather because they stop the wearer infecting others.
In this context covid-19's particular peculiarlty--that people who test positive for it often do not have symptoms--is important.
Research published last month in Nature Medicine, by Xi He of Guangzhou Medical University and Eric Lau of Hong Kong University,
上个月，由广州医科大学的Xi He以及香港大学的Eric Lau发表于《自然医学》杂志的研究表明，
suggests that 44% of cases are caused by transmission from people without symptoms at the time of transmission.
Those who do have symptoms should not, of course, be out and about at all. In their case masks are irrelevant.
But to break the chain, it behoves even the symptomless to assume that they might be infected.
Covid-19 is transmitted, above all, by virus-laden droplets of spit. Experiments show that face-coverings as simple as tea-towels are effective.
One study found that a tea-towel worn around the face captured 60% of droplets.
At 75%, a surgical mask did better, but not overwhelmingly so.
Governments are beginning to take this on board. As part of the loosening their lockdown,
the Dutch are required to wear face-coverings on public transport--but not ones of medical grade,
which should be reserved for professionals. This encourages people to make their own.
Neither laboratory studies nor the data on asymptomatic transmission provide watertight evidence of the efficacy of masks.
That would need randomised controlled trials, in which one group wore masks and the other did not.
This would be ethically tricky, since it might condemn one of the groups to a higher death rate.
Hamsters, which are susceptible to covid-19, are the next best thing to people.
So researchers at Hong Kong University put cages of healthy hamsters next to cages of infected ones,
with a fan in between drawing air from the infected to the healthy cage. They sometimes also placed a stretched-out face mask in the air stream.
With no interposed mask, two-thirds of the healthy hamsters were infected within a week.
With a mask interposed close to the healthy hamsters (the equivalent of a healthy person wearing a mask), one-third were.
With the mask close to the infected hamsters, only a sixth were.