Passage: The Hawks Nest Tunnel disaster took place between 1927 and 1932 in a West Virginia tunnel project led by Union Carbide. During the construction of the tunnel, workers found the mineral silica and were asked to mine it for use in electroprocessing steel. The workers were not given masks or breathing equipment to use while mining. Due to silica dust exposure, many workers developed silicosis, a debilitating lung disease. According to a marker on site, there were 109 admitted deaths. A congressional hearing placed the death toll at 476.

Question: What caused the silica dust to affect the mine workers fatally?
workers were not given masks or breathing equipment