Citations for safety and health violations by operators of underground coal mines have increased by nearly a third since 2006, with federal mine inspectors focusing about equally on three main types of mining hazards. An NPR analysis of nearly 80,000 citations written last year found that an accumulation of combustible coal dust was the most frequently cited violation overall, accounting for more than one in 10 citations. The analysis found that three categories of problems — fire and coal dust issues; ventilation problems; and electrical issues — each made up about a quarter of the violations found by inspectors nationwide. At Upper Big Branch Mine in Montcoal, W.Va., federal inspectors spent 180 days last year enforcing safety and health standards. Still, they could not prevent the April 5 explosion that killed 29 miners.