In the film, snowfall triggered by Glinda the Good Witch awakens Dorothy and the Lion from their poppy-induced sleep. Unfortunately, this scene was having the complete opposite effect on the actors in real life, as all four were covered asbestos. Per Asbestos.com, this material is the broken-down form of six different minerals, which produce hairlike fragments when crushed. While asbestos was used in the ancient world for many purposes, from 1866 onward it became a quintessential insulation material for buildings with fire-resistant properties (via the Mesothelioma Help website). The fire resistance it provided came at a deadly cost, however, as the fibers remain trapped within the lungs once inhaled. 

Multiple conditions, including lung and ovarian cancers, have been linked to asbestos exposure, and on the 1939 set of “The Wizard of Oz” the product was more than plentiful. Medical professionals at the time had begun to acknowledge the health risks that came with exposure, yet a lack of legal push-back (to this day America has not fully banned asbestos) allowed such excessive use (via Kazan Law). It was only in the latter half of the 20th century that safer fake-snow materials began to see use by production companies, including the “revolutionary” concept of using actual snow (via Tedium).