What is the most prevalent occupational illness for miners?

Noise-Induced Hearing Loss (NIHL), despite 25 years of regulation.

Answer Extra: 

The use of heavy equipment, the drilling of rock, and the confined work environment all contribute to high levels of noise exposure in mining. Every day, 80% of miners work where the time-weighted average (TWA) exceeds 85 dBA. Moreover, 25% are exposed to a TWA noise level that exceeds 90 dBA - the permissible exposure limit (PEL). As a result, as many as 70-90% of all miners have NIHL significant enough to be considered a disability.

source: 
Studies by the National Institute for Occupational Safety and Health (NIOSH)