In the early years of ship building there was great risk of workers being struck on the head by tools or materials that were accidently dropped from above decks. To protect themselves, workers would cover their hats with tar and let them bake in the sun, which gave the hats some rigidity. During this era miners usually wore a soft cap made of cloth or canvas. A few years later, industrial operations began using steel hard hats, which were inspired by the helmets provided to troops in the first world war.
By the 1930’s hard hats had become more routine. Approximately 3,500 men worked on the Hoover Dam each day, and each worker was required to wear a steel hard hat. The same mandatory hard hat rule also applied for those working on the Golden Gate Bridge. Soon many other industries began mandating hard hats, the oil well industry being an early adopter.