Prevalence of r-type opacities among participants in the NIOSH Coal Workers' X-ray Surveillance Program by small opacity profusion category, 1980–2008, any tenure
| Year | Small opacity profusion category | ||
| 0 | 1 | 2 or 3 | |
| 1980–1989 | 0.02% (8/45444) | 3.5% (45/1303) | 16.5% (31/188) |
| 1990–1999 | 0.02% (3/15106) | 7.2% (29/403) | 30.6% (19/62) |
| 2000–2008 | 0.08% (23/27493) | 12.4% (95/765) | 43.0% (68/158) |
Category 0 includes borderline abnormality (ie, 0/1).
NIOSH, National Institute for Occupational Safety and Health.