British Prime Minister Boris Johnson visits Ruislip High School in Uxbridge, west London, on Sept. 28. (Stefan Rousseau/PA/AP)

LONDON ­— With winter on its way and coronavirus guidelines advising teachers to keep classroom windows open, students across Northern Europe are preparing for the chill by packing blankets in their school bags and layering up in warm clothing.

There is increasingly a consensus among experts that good ventilation is one of the best ways to prevent the virus from spreading. Anthony Costello, a former director at the World Health Organization, said last month that children “can survive a bit of cold, and they’re going to have to, because ventilation is so important.”

With temperatures in Germany frequently dropping to freezing, children in the city of Bochum are bracing for a crisp learning environment as officials advise teachers to open the windows for fresh air every 20 minutes. Children have been told to bring blankets and wrap up.

Critics have called the advice a threat to the health of students, with Finn Wandhoff, chairman of the Student Union of Germany, accusing the government of failing the education system by not opting for online learning.

In Scotland, where First Minister Nicola Sturgeon says keeping schools open is a priority, Edinburgh’s chief education officer, Andy Gray, wrote a letter to parents urging them to ensure that their children wear extra layers of clothing when they return to school on Oct. 26 after a midterm break, local media reported.

In guidance issued in August, the Scottish government said: “The opening of doors and windows, where it is safe to do so, should be encouraged to increase natural ventilation and also to reduce contact with door handles.”

British health expert Susan Michie pointed out that many schools have windows that do not open and, therefore, need government funding to boost ventilation.

“I think also pupils will have to get used to — and staff — coming in wearing more clothes,” Michie said.

Britain remains the worst-hit country in Europe, with almost 43,000 lives lost to the coronavirus.

Pablo del Pozo, a music teacher in Spain’s southern Cádiz province, told El Pais newspaper that students were being forced to sit by wet window sills during lessons, leading to complaints.

“We are being told that it’s better for a child to catch pneumonia than covid-19,” he said.

Although some schools struggle with the advice that the cold air needs to be brought in, schools in Denmark and other Nordic education systems are taking lessons — and young students — outside.

More than half of about 200 Norwegian schools surveyed in a poll by researchers Ulrich Dettweiler and Gabriele Lauterbach last month said they were holding more classes outdoors — a move some already had planned on that was further propelled by the pandemic.

In Denmark, the forest is the new classroom

At Samso Frie Skole, a private school on the Danish island of Samso, young children bike or walk to a nearby forest, where they sit on logs to study and shelter in farmhouses from bad weather.

Being outdoors, staff members say, has had positive effects on many of the students, who use stones to work out during physical-education classes and hunch over crawling insects during biodiversity lessons.

But in the United States, the debate isn’t just about the weather.

Although the Centers for Disease Control and Prevention calls to open windows and doors for fresh air to flow through classrooms, teachers say they must tread a fine line between protecting students from the global health crisis and from the threat of a school shooting.

“Do I keep my classroom door open to improve air circulation or close it to protect my students from an active shooter? This is just one of the impossible questions teachers are facing this year,” high school English teacher Tiffany Mumm tweeted in August.

Her question soon went viral, with thousands joining her as she attempted to weigh which would be a greater threat to students.

Other teachers from across the United States joined the conversation, with some saying active-shooter policies meant that they had to keep their doors locked — despite the coronavirus-related recommendations.

Want to buy schools time? Open the windows.

In August, experts on healthy buildings from Harvard University urged school officials to ensure that they had adequate ventilation systems and plans in place, saying that although the advice to open windows and doors during class may sound “too simple to be true,” it is “the simplest and quickest way to increase the air-exchange rates” in the classroom.

“Yes, I’m telling you to open up the window,” Anthony S. Fauci, the nation’s top infectious-disease expert, said in response to a question during a forum at the Harvard T.H. Chan School of Public Health about the airborne spread of the virus.

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