What event had an enormous effect on US workplace safety?

It was a worker strike. Which lead to many strikes of the labor union. Because of this the Heath Act was formed. It went into effect April 28th, 1971.

In March of 1911, the Triangle Shirtwaist Factory had a massive fire. A lot of young women that were new to the United States worked there. The fire burned so fast and hot that 146 people died. The managers had locked the doors to "prevent theft" - so a lot of dead bodies were found piled by the locked doors. It's pretty interesting, worth googling.