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Cases are ticking upwards in parts of Europe, the process of unlocking is paused in the UK, and the Americas are still battling to contain vast Covid-19 outbreaks.

But as the tremors of a potential second wave of infections are starting to be felt, some governments are reaching for a new tool that many public health experts have been touting for months: stricter mask mandates.

In recent days, masks have become mandatory in all public spaces – indoors or outdoors – in Madrid, Greece, Portugal’s Madeira Islands and Hong Kong.

Those moves seemingly contradict the long-held understanding that Covid-19 is more dangerous indoors. The British government, among others, used its first steps out of lockdown to encourage people to meet outdoors; parks, beaches and nature spots around the world have been inundated by guests throughout the pandemic.

But the reasoning behind the decisions is simpler than that: after months of mixed messaging from health authorities on face coverings, governments are opting for blanket rules to help make mask-wearing a cultural norm.

“There’s been a lot of confusion about where people should wear masks, and where there’s confusion, people just disengage and don’t wear them,” Melinda Mills, director of Oxford University’s Leverhulme Centre for Demographic Science, told CNN. “That’s why (some countries) are moving to a broad, blanket policy.

“I’m for clarity in public messaging, and in many countries, I think it’s been a mess,” she added.

What we know now about masks

The science behind airborne transmission of Covid-19 is growing, but experts still agree the risk is usually higher indoors.

Earlier this month a group of 239 scientists wrote an open letter to the World Health Organization, appealing for better recognition of the potential airborne transmission of coronavirus. “A lot of people crowded close together indoors where it is poorly ventilated – that is what drives the pandemic,” a co-organizer of the letter and a professor of environmental health, told CNN at the time.

Passengers wearing face masks at Adolfo Suarez-Barajas international airport in Madrid, where coverings have become mandatory in all public places.

But the outdoors is not Covid-free, and universal mandates on mask-wearing are likely to reduce the spread in many kinds of settings.

Researchers reported Monday that communities that mandated the use of face masks in public saw an ongoing decline in the spread of the coronavirus, but it takes some time.

Once mandates had been in place for about three weeks, the daily growth rate slowed by about 2% on average, researchers reported in the journal Health Affairs.

Their estimates suggest that these percentage decreases could add up. They calculate that between 230,000 and 450,000 Covid-19 cases could have been averted by May 22 by mask mandates.

Other scientists agree that there can be a significant risk of outdoor transmission.

“There is open air, so generally the risk is slightly lower, but it’s spreading outdoors too,” said Abrar Chughtai, an epidemiologist at the University of New South Wales. “Whenever you’re unable to maintain social distancing, you should wear a mask.”

“It’s very low risk in a setting like a park, or a big open space where there aren’t many people,” added Richard Stutt of the University of Cambridge in England, who modeled the impact of mask-wearing in curbing Covid-19 transmission.

“But if you’re walking down a busy high street, there’s quite a large potential for spread there – and defining exactly what constitutes a high-risk outdoors space versus a low-risk outdoors space would be very difficult to do.

“There’s additionally the complication that if people are constantly taking their mask off and putting it back on, then they run the risk of contaminating their hands and passing it on to other people,” he noted.

That scientific knowledge is one factor driving stricter mask mandates, which are beginning to appear in countries like the UK too.

But there’s a sociological element at play as well, and it’s a simple one: mask rules are working, and so governments are going further than they may have expected to.

“There were a few countries holding out, thinking the public would revolt (to mask orders), but the public just doesn’t want to be in quarantine anymore,” said Mills. “Governments will be emboldened a little bit in expanding their policies if they think these things are effective.”

Many researchers are surprised at just how effective they’ve been. In April, an alarmingly low proportion of British people were wearing masks – just 19%, according to a Kings College study published on Thursday. But 70% of Brits now say they have worn one in the last few weeks, the same study found.

“People have very quickly become convinced that they do help,” Bobby Duffy, the director of King’s Policy Institute and the leader of the study, told CNN. “There’s very, very high level of belief in their efficacy, which in itself is quite remarkable given how quickly the advice has shifted.”

That shift is particularly noticeable in a country where the government’s attempts to ease restrictions have been criticized as vague and muddled. A study by University College London last week found that just 45% of British people feel they understand the government’s current rules.

The same has been true elsewhere in Europe; hard-hit places like Italy and Spain saw mask use become a cultural norm almost immediately, and studies indicate that even in the United States – where a culture war has erupted over the use of face coverings – adherence is generally very high.

“You can bring in these rules and people will follow them,” Duffy said. “It’s incredible how quickly people get used to things. You can see that in the lockdown measures, too; there was incredible support for going into lockdown, even when a couple of weeks (prior) that would have seemed unthinkable.”

Are lessons finally being learned?

The benefits of expanded mandates are myriad, but the most significant is that it allows governments to firm up their messaging by setting a strict but simple rule: if you’re in public, you have to wear a mask.

“A blanket rule is much easier to adhere to, and also to enforce,” Stutt said.

But many experts are vexed that it’s taken so long to get here – and are imploring other countries to follow suit and make their mask laws black and white.

Two women wear face masks in Seoul. Countries in which covering one's face is a social norm have generally fared better during the pandemic.

Mills said she was “really surprised and shocked” by the conflicting guidance of the World Health Organization and many governments, which said early in the pandemic that mask use wasn’t necessary in curbing spread.

WHO Director-General Tedros Adhanom Ghebreyesus said last month that the organization switched gears and encouraged mask use after several weeks because evidence had evolved. “Our updated guidance contains new information on the composition of fabric masks, based on academic research requested by WHO,” Tedros said last month.

“It was indeed very frustrating as a scientist, (because) there have been a lot of material studies that show these good, high quality masks can really stop filtration,” Mills said.

By contrast, a number of Asian countries – many of which experienced the worst of the SARS epidemic – quickly encouraged citizens to wear masks.

“Think about Japan: they were really effective and clear, they had the Three Cs,” Mills said, referring to a government slogan ordering masks in closed spaces, crowded places, and close-contact settings.

“They just keep repeating it and repeating it, and they’ve had about 1,000 deaths in a population of 126 million,” she added, noting that Japan’s relative success depended on several other factors.

“It’s not quite as simple as this, but those places that did wear masks from the start have had vastly lower death rates,” added Stutt.

The stop-start uptake in much of the West stood in stark contrast to those countries. But now that governments are feeling confident enough to set stricter rules, experts can envisage mask-wearing to become normal in every public place soon.

“A strong message from governments and organizations like the WHO does have a significant effect on people, and makes it much more socially acceptable to wear a mask and increase adoption rates,” said Stutt.

“That mandatory use of masks is not going to result in huge rebellion from the majority of the population,” added Duffy, citing his research that showed a rapid uptake in the UK.

“There will be fractions who do object really strongly to it – that will get a bit larger and more vociferous, and our attention will be drawn to those people,” he said. “But the reality will be that the majority of the population will adapt and accept it.”

CNN Health’s Naomi Thomas contributed to this report.