As the 20th century dawned, tuberculosis—otherwise known as consumption, “white plague” or “white death”—had become the leading cause of death in the United States. The dreaded lung disease killed an estimated 450 Americans a day, most of them between the ages of 15 and 44.

At the time, tuberculosis was associated with dirty, unhygienic living conditions, which were common for the workers who had packed into the cities of Europe and the United States since the Industrial Revolution. With no effective medicine available (yet), the preferred treatment was the open-air cure, or exposing patients to as much fresh air and sunlight as possible. This led to the proliferation of tuberculosis sanitariums, ranging from luxe spa-like resorts to government-run institutions across Europe and the United States.

Though many of its victims were poor city dwellers, no one was immune to tuberculosis—especially not children. In fact, doctors and educators believed that the crowded classrooms and lack of fresh air in many schools helped spread the disease. To keep kids healthy, they decided to take school outside.

The open-air school movement was launched in Germany in 1904, when Dr. Bernhard Bendix, a leading German pediatrician, and Hermann Neufert, a Berlin school inspector, opened the first Waldschule für kränkliche Kinder (or "forest school for sickly children") in Charlottenburg, near Berlin. True to its name, the school was located in the heart of a nearby forest, with simple wooden buildings used for instruction in cold or rainy weather. Students commuted from the city and most suffered from pre-tuberculosis symptoms such as anemia or swollen glands.

That first Waldschule launched a movement that spread quickly across Europe, with similar experimental schools opening in Belgium, Italy, England, Switzerland and Spain. After World War I, the movement became more formalized. The League for Open Air Education spearheaded the first International Congress, held in Paris in 1922; four more international conferences had taken place by 1956.

The open-air school movement arrived in the United States in 1908, thanks to two doctors from Rhode Island. Mary Packard and Ellen Stone were among the first female graduates of Johns Hopkins School of Medicine, and had helped found Providence’s League for the Suppression of Tuberculosis. After running a summer day camp for tubercular children, they thought they would try the fresh-air method on a larger scale during the school year.

The Providence school board authorized the use of an empty brick schoolhouse building, where a second-floor classroom was remodeled to have floor-to-ceiling windows on one side, which could be opened with a hinge and kept open to the air.

As Mary Korr wrote in the Rhode Island Medical Journal in 2016, the students in the Providence open-air school were children who had been exposed to tuberculosis but weren’t actively sick. Over that first cold New England winter, the children snuggled in wearable blankets known as “Eskimo sitting bags” and placed heated soapstones at their feet. A fire in a cylinder stove helped blunt the chill, but the classroom never reached more than 10 degrees above what it was outside.

By the end of the school’s first year, none of the students had gotten sick, and their health had even improved. Fueled by this success, the idea of open-air school quickly spread, and within two years 65 of them had opened in the United States, including 11 in Providence alone. Some used the open-windows method of the Providence schoolhouse, while others held class outdoors or on the rooftop of their school building.

By 1918, some 130 American cities were operating open-air schools of some kind, according to Neil S. MacDonald, author of a book on the Open-Air School movement published that year. “In many of the Western and Southern states,” wrote MacDonald, “there is practically no temperature problem and no reason why all schools should not be open-air schools all the year round.”

As the popularity of the open-air school movement grew, prominent architects in Europe and the United States began designing permanent school buildings that reflected the ideas and values of the movement.

One of the most famous (and radical) of these, a terraced glass and concrete structure by Jan Duiker, was constructed on a city block in the heart of Amsterdam in 1927. Unlike many open-air schools, it aimed to extend the benefits of fresh-air schooling to healthy schoolchildren, not just those who suffered from tuberculosis. In Los Angeles, architect Richard Neutra’s experimental additions to the Corona Avenue Elementary School included a wall of glass that slid open to an outdoor class area, while each indoor classroom was exposed to light on at least two sides.

Thanks to improved public health and sanitation standards, and especially the discovery of streptomycin and other effective antibiotics, tuberculosis receded as a major health threat after the mid-1940s. Within a decade, the open-air school movement had come to an end as well.