A Critical Review of Coal Workers Pneumoconiosis (CWP) and Coal Rank for Evaluation of Safe Exposure Levels in Coal Mining

Abstract

The US coal mine dust ("CMD") standard is among the lowest in the world. Lowering exposures to <2 milligrams per cubic meter ("mg/m 3 ") markedly reduced prevalence of coal workers pneumoconiosis ("CWP"). Lowering the standard to 1 mg/m 3 is proposed because of a recent increase in rapidly progressive coal workers pneumoconiosis. The question addressed in this review is whether coal rank should be considered in setting a new standard. In 1949 the first British coal dust standard for anthracite was 650 particles/cm 3 compared to 850 particles/cm 3 for lower ranked coals. The TLV for anthracite is 0.4 mg/m 3. Exposure-response analyses of CMD and category ≥2 CWP show strong associations for high rank coal (coal rank 5 or anthracite and rank 4) with increased prevalence below the current standard. There were no apparent increases in CWP ≥2 for low rank coals 1-3 at exposures below the current 2.0 mg/m³ standard. All studies show higher prevalence of CWP at higher ranks compared to lower ranks without regard to dust concentration. Assuming a background prevalence of 5% among non-dust exposed workers, the evidence suggests that below 2.0 mg/m 3 there is no excess CWP ≥2 for coal ranks 3-5 (low-medium ranks) in the US. Exposure-response of CMD and mortality shows a strong association with nonmalignant respiratory diseases ("NMRD"), but no associations with chronic bronchitis, emphysema, lung cancer or stomach cancer. When stratified by rank, the excess NMRD mortality is confined entirely to miners exposed to anthracite. This review confirms the important role of rank in development of CWP and indicates a substantial increased pulmonary fibrogenicity when exposed to high vs. low rank coals. These data are suggestive that the current standard may not be protective for the highest rank coals. Recent evidence suggests further exposure-response analyses are needed using more specific exposure metrics including iron. and increased mining of low coal seams surrounded by high silica-containing rock producing coal with large quantities of quartz (e.g., 30-40%). Coal rank 1 is an important risk factor for CWP as exposure-response and toxicity are related to rank. Low rank coal dust contains a lower proportion of uncoated silica 2 particles than dusts of high rank coal [2], so part of the toxicity may be related to quartz. Freshly crushed high rank coal contains a greater concentration of oxygen radicals [3,4] and smaller particle sizes [5] than lower rank coals. "Black lung" was recognized as a disease in British coal miners in the mid-17 th century and called a pneumoconiosis ("dusty lung") in 1874. Silica was thought to be causative agent until CWP was found to occur when there was minimal silica in the CMD. CWP is detected via chest radiographs with severity measured by profusion of opacities. Stages of CWP are defined by the International Labor Office ("ILO") Guidelines for the Classification of Radiographs of Pneumoconioses and divided into major categories of normal (category 0) simple CWP