Silica
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 any masks or breathing equipment to use while mining, despite the fact that management wore such equipment during inspection visits. As a result of the exposure to silica dust, many workers developed silicosis, a debilitating lung disease. A large number of the workers eventually died from silicosis, in some cases as quickly as within a year.
There are no definitive statistics as to the death toll from the Hawks Nest disaster. According to a historical marker on site, there were 109 admitted deaths. A Congressional hearing placed the death toll at 476. Other sources range from 700 to over 1,000 deaths amongst the 3,000 workers. Many of the workers at the site were African-Americans from the southern United States who returned home or left the region after becoming sick, making it difficult to calculate an accurate total.
Read more about this topic: Hawks Nest Tunnel Disaster