The girl in this image was one of about 2 million child workers in the United States in the early 1900s. She and other young laborers worked long hours in coal mines, textile mills, and other places in dangerous conditions for little pay.

Lewis Hine, a teacher turned photographer, snapped this photo—and many others like it—to document those situations. He hoped his work would inspire new laws to protect young workers.

Child labor became common in the U.S. during the Industrial Revolution, when workers were in high demand. Pay was so low that families often needed their kids’ wages to get by. By 1900, nearly 20 percent of American workers were under the age of 16.

Children often worked 12 or more hours a day, six days a week. They rarely received much schooling. But Hine’s images—along with decades of work by labor rights advocates—helped change things.

In 1938, Congress passed the Fair Labor Standards Act. The law set limits on how kids could be employed. It also improved job conditions for workers of all ages.