We're not done with poisoning or harming the cast yet. There's a scene in the film where Dorothy, the Cowardly Lion and the Tin Man fall asleep in a poppy field. Having had a nice nap, they're pleasantly awoken by snow falling on them softly. Because who doesn't like getting woken up by snow on their face? Obviously, with the film being shot in a studio, it wasn't real snow. Unfortunately, the snow was made of asbestos.
In fairness to the people making the movie, the dangers of asbestos weren't known at the time, and it was considered to be a wonder material, often used for simulating snow in movies. It was also a building material used in many homes. It was only many years later, when people who worked with it started getting horribly ill, that people looked closer and realized it was intensely carcinogenic and lethal.