From the late nineteenth and early twentieth centuries, child labor grew exponentially in every corner of urban working-class America. The 1870 U.S. census recorded 750,00 children under the age of fifteen working in mines, factories, and other mass industries. By 1905, the number more than doubled, including children as young as five years old. Employers subjected children to long hours, low wages, and hazardous environments, which often resulted in injuries, illness, and mental health conditions.