London CNN  — 

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