# Foreword
Since its inception in 1970 the National Institute for Occupational Safety and Health (NIOSH) has extensively investigated and assessed coal miner morbidity and m or tality. This history of research encompasses epidemiology; medical surveillance; laboratory-based toxicology, biochemistry, physiology, and pathology; exposure assessment; disease prevention approaches; and methods development. The experi ence gained in those activities, together with knowledge from external publications and reports, was brought together in 1995 in a major NIOSH review and report of recommendations, entitled Criteria for a Recommended Standard-Occupational Exposure to Respirable Coal Mine Dust. This document had the following major recommendations:
1. Exposures to respirable coal mine dust should be limited to 1 mg/m3 as a time-weighted average concentration for up to a 10 hour day during a 40 hour work week; 2. Exposures to respirable crystalline silica should be limited to 0.05 mg/m3 as a time-weighted average concentration for up to a 10 hour day during a 40 hour work week; 3. The periodic medical examination for coal miners should include spirometry; 4. Periodic medical examinations should include a standardized respiratory symptom questionnaire; 5. Surface coal miners should be added to and included in the periodic medical monitoring. This Current Intelligence Bulletin (CIB) updates the information on coal mine dust exposures and associated health effects from 1995 to the present. A principal intent is to determine whether the 1995 recommendations remain valid in the light of the new findings, and whether they need to be updated or supplemented. The report does not deal with issues of sampling and analytical feasibility nor technical feasibil ity in achieving compliance.

# Executive Summary
Information relating to occupational pulmonary disease morbidity and mortality of coal miners available up to 1995 was reviewed in the NIOSH publication: Criteria for a Recommended Standard-Occupational Exposure to Respirable Coal Mine Dust, or Coal Criteria Document (CCD). This led to the following principal conclusions concerning health effects associated with coal mining:
1. Exposure to coal mine dust causes various pulmonary diseases, including coal workers' pneumoconiosis (CWP) and chronic obstructive pulmonary disease (COPD). 2. Coal miners are also exposed to crystalline silica dust, which causes silicosis, COPD, and other diseases. 3. These lung diseases can bring about impairment, disability and premature death. This Current Intelligence Bulletin updates the previously published review with re spect to findings relevant to the health of U.S. coal miners published since 1995. The main conclusions are:
1. After a long period of declining CWP prevalence, recent surveillance data indicate that the prevalence is rising. 2. Coal miners are developing severe CWP at relatively young ages (<50 years). 3. There is some indication that early development of CWP is being manifested as premature mortality. 4. The above individuals would have been employed all of their working lives in environmental conditions mandated by the 1969 Coal Mine Health and Safety Act. 5. The increase in CWP occurrence appears to be concentrated in hot spots of disease mostly concentrated in the central Appalachian region of southern West Virginia, eastern Kentucky, and western Virginia. 6. The cause of this resurgence in disease is likely multifactorial. Possible expla nations include excessive exposure due to increases in coal mine dust levels and duration of exposure (longer working hours), and increases in crystalline silica exposure (see below). As indicated by data on disease prevalence and severity, workers in smaller mines may be at special risk. 7. Given that the more productive seams of coal are being mined out, a transi tion by the industry to mining thinner coal seams and those with more rock    1968-87. Figure 3. Probability that an individual starting with no pneumoconiosis (cate gory 0/0) will be classified as 2/1 or greater after 35 years of exposure to various concentrations of coal mine dust. Figure 4. Predicted prevalence of PMF among British coal miners after a 35-year working lifetime by mean concentration of respirable coal mine dust. Figure 5. Prevalence of simple CWP category 1 or greater among U.S. coal miners by estimated cumulative dust exposure and coal rank. Figure 6. Prercentage of evaluated miners with rapidly progressive coal workers' pneumoconiosis by county. Figure 7. Percentage of miners in the NIOSH Coal Workers' X-ray Surveillance Program with coal workers' pneumoconiosis (category 1 or greater) from 1970-2009, by tenure in coal mining. Figure 8. Percentage of miners examined in the NIOSH Coal Workers' X-ray Sur veillance Program with progressive massive fibrosis (PMF) from 1970 2009, by tenure in mining. Figure 9. Observed and predicted prevalences (%) of CWP category 1 or greater by age group and MSHA District. Figure 10

# List of Figures

# List of Tables

# Aerodynamic diameter:
The diameter of a sphere with a density of 1 g/cm3 and with the same stopping time as the particle. Particles of a given aerodynamic diameter move within the air spaces of the respiratory system identically, regardless of density or shape.

# Chronic obstructive pulmonary disease (COPD):
Includes chronic bronchitis (in flammation of the lung airways associated with cough and phlegm production), impaired lung function, and emphysema (destruction of the air spaces where gas transfer occurs). COPD is characterized by irreversible (although sometimes vari able) obstruction of lung airways, and should be considered in any patient who has dyspnea, chronic cough or sputum, and/or a history of exposure to risk factors for COPD. The diagnosis should be confirmed by spirometry.
Coal rank: A classification of coal based on fixed carbon, volatile matter, and heat ing value of the coal. Coal rank indicates the progressive geological alteration (coalification) from lignite to anthracite.

# Coal workers' pneumoconiosis (CWP):
A chronic dust disease of the lung arising from employment in a coal mine. In workers who are or have been exposed to coal mine dust, diagnosis is based on the radiographic classification of the size, shape, profusion, and extent of parenchymal opacities.
Crystalline silica: Silicon dioxide (SiO2). "Crystalline" refers to the orientation of SiO2 molecules in a fixed pattern as opposed to a nonperiodic, random molecular arrangement defined as amorphous. The three most common crystalline forms of free silica encountered in general industry are quartz, tridymite, and cristobalite. The predominant form is quartz.
Excess (exposure-attributable) prevalence: The prevalence (cases/population at risk) attributable to workplace dust exposure (in the case of CWP, the prevalence adjusted for radiographic appearances associated with lung aging).
International Labour Office (ILO) classification system: A standardized method for assessing abnormalities related to the pneumoconioses based substantially on compari son of test with reference radiographs. In the system there are 4 categories of simple pneumoconiosis (categories 0, 1, 2, and 3), with 0 implying no definite abnormality.
Progressive massive fibrosis: Coal workers' complicated pneumoconiosis. Diagno sis is based on determination of the presence of large opacities (1 cm or larger) using radiography or the finding of specific lung pathology on biopsy or autopsy.

# Glossary x ii
Quartz: Crystalline silicon dioxide (SiO2) not chemically combined with other sub stances and having a distinctive physical structure.
Respirable coal mine dust: That portion of airborne dust in coal mines that is ca pable of entering the gas-exchange regions of the lungs if inhaled: by convention, a particle-size-selective fraction of the total airborne dust; includes particles with aerodynamic diameters less than approximately 10 ^m.

# D ivision o f R e sp irato ry Disease Studies (DRDS)
Janet Hale, Eva Suarthana, Mei Lin Wang

# H e a lth Effects L a b o ra to ry D ivision (HELD)
Vincent Castranova, Kimberly Clough Thomas This Current Intelligence Bulletin has undergone substantial internal and external scientific peer review with subsequent revision. A draft version of the document was published on the NIOSH website for public comment for 60 days, with notification of its availability via the Federal Register. All input, both internal and external, has been considered, addressed, and responded to in preparation of this final version.

# Introduction
The publication of the NIOSH Criteria for a Rec ommended Standard-Occupational Exposure to Respirable Coal Mine Dust or Coal Criteria Document (CCD) in 1995 (1) followed a long period of extensive research activity focused on exposure to coal mine dust and its health effects in coal miners. From this research, substantial information had emerged about the extent and severity of respiratory disease caused by coal mine dust, its quantitative rela tionship with dust exposure, its pathology and toxicology, environmental patterns of relevant exposures, and methodologies for assessing these variables. In particular, the findings dem onstrated that not only was there a consider able burden of coal workers' pneumoconiosis (CWP) in the U.S. and other countries, but that underground coal miners were vulnerable to other lung diseases, notably chronic obstruc tive pulmonary disease (COPD). The evidence came from extensive and well-planned epide miologic and laboratory-based investigations undertaken primarily in the U.S., the United Kingdom, and (West) Germany, with support ing information coming from studies in other European countries and Australia.
The available information was thoroughly sum marized in the CCD. It showed that, in 1995, CWP was in decline in the U.S., with downward trends in prevalence in all tenure groups (Figure 4-2 of the CCD (1), included here as Figure 1). This decline was consistent with reductions in coal mine dust exposure mandated by the 1969 Coal Mine Health and Safety Act (1969 Coal Mine Act) (CCD Figure 4-1 (1); Figure 2). De spite this decline in disease levels, NIOSH con cluded from review of the surveillance data and quantitative risk estimates based on the epide miologic studies that the current dust exposure regulations for U.S. coal mines were not suffi ciently protective. Consequently, it proposed lower dust limits for coal mines, enhanced med ical surveillance, and other requirements.   1967 1969 1971 1973 1975 1977 1979 1981 1983 1985 1987 Y ear F ig u re 2. Trend in reported dust concentrations for continuous miner operators, 1968-87.  (Source: Attfield and Wagner ( 2)).
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The CCD noted that the current U.S. federal dust limit for underground coal mines dated back to the 1969 Coal Mine Act, which m an dated a compliance permissible exposure limit of 2 milligrams per cubic meter (mg/m3) of respirable coal mine dust. This limit was derived from British research, which provided the only quantitative exposure-response relationship available at that time. This exposure-response curve (CCD Figure 7-2 (1); Figure 3) predicted that no cases of CWP as severe as category 2 on the International Labour Office (ILO) classifi cation system (3) would develop among m in ers who worked 35 years at 2 mg/m 3. Similarly at that time, the current information indicated that the disabling form of CWP, progressive massive fibrosis (PMF), was very unlikely to develop from less severe ILO categories (e.g., category 1 CWP). Therefore, adoption of the 2 mg/m3 limit was believed, at that time, to be protective against the risk of disability and pre mature mortality that accompanies PMF.
Subsequent scientific findings, emerging be tween 1969 and 1995, disproved some of the assumptions inherent in the adoption of the 2 mg/m3 standard. Firstly, the assumption that miners with CWP less severe than category 2 were at minimal risk of PMF was found to be incorrect. Moreover, additional findings from the British data (CCD Figure 7-6 and Table 4-6 (1); Figure 4 and Table 1), together with new results on U.S. underground coal miners from research undertaken by NIOSH (CCD Figure 7-4 and Table 4-6 (1); Figure 5 and Table 1) showed that there was no threshold at 2 mg/m3 as had been indicated by the original British study (CCD Figure 7-2 (1); Figure 3). Furthermore, the CCD reviewed findings on other lung diseases and their relationship with coal mine dust exposure. It concluded that coal miners were at additional risk of developing ventilatory function deficits, respiratory symp toms, and emphysema in addition to CWP (CCD Table 4-7 (1); Table 2). (Note that the    results provided here are selected to illustrate the main CCD conclusions. For a full under standing of the complete body of knowledge, please see the CCD (1).)
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On the basis of the updated body of evidence on the adverse health effects and an evalua tion of the technological feasibility of reduc ing dust exposures, NIOSH recommended that the federal coal mine dust limit be re duced to 1 m g/m 3. Critical to this decision were computed excess (exposure-attributable) prevalences of CWP derived from two stud ies of U.S. coal miners undertaken by NIOSH (CCD Table 7-2 (1); Table 3). Predictions were derived from each study for a working lifetime (i.e., 45 years) exposure at 2 mg/m3 and 1 mg/ m 3. Another source of information critical to the recommendations was information on pre dicted excess lung function decrements for a lifetime's exposure to 1 mg/m 3 and 2 mg/m3 respectively (CCD Table 7-3 (1); Table 4). De tails of the rationale for, and development of, the risk analyses employed in the CCD were subsequently published separately (8). NIOSH also evaluated information from other epide miologic studies in coming to its recommen dations in the CCD. However, because no oth er studies had quantitative exposure-response information, apart from strengthening the case for more stringent regulation, these ad ditional study results did not provide any nu merical basis for standard setting. For simplic ity, NIOSH recommended one exposure limit for the nation rather than different limits by coal rank, based on technological feasibility of reducing exposures, even though CWP preva lence has been shown to vary according to the rank of the coal in studies of miners in the U.S. and other countries.
In addition to recommending a reduction in the exposure limit for coal mine dust, NIOSH also recommended a change in the exposure limit for crystalline silica dust and the method by which it is enforced. Currently, silica levels are intended to be controlled by a reduction in the level of coal mine dust commensurate with the proportion of the dust that is silica. NIOSH proposed a separate limit for respirable crystal line silica in order to more effectively monitor   (19)(20)(21).
To gain a better understanding of the extent of the problem, NIOSH undertook a systematic analysis of rapidly progressive CWP (22). Sta tistics were derived based on each miner' s ra diographic steps of progression of CWP using the standard ILO categorical scores standard ized to a five-year interval. These data were summarized by county and then plotted to reveal 'hot spots' of rapid disease progression (Figure 6; from Antao et al, 2005 (22)). These tended to be located on the eastern edge of the  on the NIOSH internet site (24). These results showed that the prevalence of CWP appeared to have stopped declining around 1995-1999, and has risen since then. The trend reversal appears most apparent in the longer-tenured miners. Virtually that entire group had spent their whole working life in dust conditions mandated by the 1969 Coal Mine Act. An up dated (unpublished) version of this graphic, taking the data up to 2009, shows the increased CWP prevalence observed over the past de cade (Figure 7).
The upward trend visible in Figure 7 for all pneumoconiosis cases (category 1+) is even more evident for PMF (Figure 8). O f particular concern are the prevalence values for the last three five-year periods (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) for m in ers with <25 years tenure, which are well above those observed in the early 1990s. In 2005-2009 alone, 69 coal miners examined in the CWXSP were determined to have PMF. Of these, 11 had less than 25 years total tenure in coal mining, and the majority (56, or 81%) were working in the central Appalachian region.
Since the data from 2005 in Figures 7 and 8 were derived, in part, from the special NIOSH surveys targeted at hot spot areas, there could be the concern that the recent CWXSP find ings may be upwardly biased, with the implica tion that the apparent rise in prevalence may be an artifact. However, overall prevalences for 2005-2009 for the Enhanced and regular programs, derived from state-specific preva lences weighted by the participation rates for the whole program from the different states gives rise to figures of 3.2% prevalence for data from the targeted surveys (Enhanced pro gram) compared to 3.1% for data from the reg ular program. There is therefore no indication whatsoever of any major bias. Moreover, it is  A number of reports of surveillance informa tion from other countries have emerged since 1995 (26)(27)(28)(29)(30)(31). Although mining conditions differ in these other countries, these studies are supportive of the findings in U.S. coal miners.

# Epidem iology
The CWXSP finding of increased prevalence of CWP has prompted a series of analyses by NIOSH aimed at identifying factors that might be causing the increase. In an analysis under taken as part of this series (32), predicted risk of CWP was derived for each individual who participated in the CWXSP from 2005-2009 using published exposure-response models (7). The models were coal-rank specific, and used both age and cumulative coal mine dust exposure as predictors. The resulting indi vidual risks (lying between 0 and 1) were then summed over subsets of the data and com pared with the observed prevalences. (Further models, published later, were also available, but the early relationships were preferred because they were based on greater numbers of ob servations and had more specific adjustment for coal rank. The later predicted prevalences were somewhat higher than those presented here.) The results tabulated by MSHA region and age are shown here in Figure 9. It is clear that CWP prevalence is less than expected in some regions (observed  predicted). That is, in the north ern Appalachian region and the mid-west and western coal fields the observed prevalences are generally below those predicted in all age groups. However, in the southern WV, eastern and central KY, Tennessee, and VA MSHA re gions the observed prevalences are 2-4 times greater than predicted from cumulative coal mine dust exposure and age. Clearly, some fac tor or factors must be acting differently across the regions to cause this regional pattern.
At least three environmental factors impact the central Appalachian region in this respect. These are: thin seams, small mines, and, for VA, high coal rank. The mining of thin coal seams, which often involves the deliberate cut ting and extraction of substantial amounts of (often siliceous) rock overlying or underlying the coal seam, is particularly prevalent in Ap palachia (33). NIOSH has been investigating the health implications of possible excessive crystalline silica exposure arising from the cutting of the rock adjacent to the coal seams (34). This analysis used the presence of r-type pneumoconiotic opacities on the chest X-ray as an indicator of crystalline silica exposure. This type of opacity is a radiographic manifes tation of nodules in the lung having a typology often associated with excessive exposure to sil ica dust. An increase in the prevalence of such opacities could then well indicate that miners are more frequently being exposed to crystal line silica dust, or are experiencing exposure to higher levels of silica dust. Increased exposure to crystalline silica dust may well be arising from industry trends, whereby there is greater focus on mining thinner seams of coal as the more productive thicker seams are mined out.
The findings from this study indicated that the proportion of radiographs showing r-type opacities increased during the 1990s, and particularly after 1999, in KY, VA, and WV, compared to the 1980s. They could potentially be explained by an increase in the frequency and/or intensity of silica exposure among un derground coal miners. This hypothesis was confirmed by evidence from dust sampling in mines in that region indicating that excessive silica exposures are occurring (35). In the CCD, NIOSH not only recommended that compli ance procedures for crystalline silica be made more effective, but that the exposure limit be reduced. In the light of these epidemiological findings (34), therefore, this recommendation remains appropriate and even more urgent. A report on British coal miners also associated an increase in CWP prevalence with rock cut ting (36).
In another NIOSH analysis (19) trends in CWP prevalence were examined by mine size (i.e., employment). The hypothesis investigated was that smaller mines lack resources in many ar eas for the full protection of workers, including dust suppression and up-to-date knowledge of means to prevent disease development. (It may also be that smaller mines tend to be those m in ing the thinner coal seams.) The results show that CWP prevalence is increasing in mines of all sizes, but the trend is much more obvious and much greater among miners employed in smaller coal mines. a contributory causative factor for CWP, work ing longer hours leads to the inhalation of more dust into the lungs. For example, working 12 hours leads to 50% more dust entering the lung compared to a regular 8-hour shift, assuming all other factors are equal (e.g., exposure con centration and breathing rates). Additionally, working longer workshifts reduces the time available between workshifts for the process of clearing the dust deposited in the lungs. Unfor tunately, the available information on working hours in U.S. coal miners is not miner-specific but rather by coal mine, substantially reducing the validity of a formal analysis of this hypoth esis. A report on British miners concluded that longer working hours were a factor in causing an increase in CWP prevalence at two mines (36). In the CCD, NIOSH recommended re ducing dust exposures below the 1 mg/m 3 REL for work shifts exceeding 40 hr/week (using the method of Brief and Scala (37)). This ap proach has also been recommended for British coal mines (38).
Finally, productivity per hour worked also increased from 1978-2000, although it has since declined (Figure 11). Of course, these increases in productivity (and, presumably, in creased potential for dust generation) should have been met by commensurate increases in dust prevention measures (e.g., ventilation and water sprays) in order to maintain compliance with the permissible exposure limit. Superfi cially, the current data appear to confirm this, in that airborne dust levels have apparently not risen during that period (Figure 12). However, the veracity of coal mine dust data has been challenged in the past (39). Moreover, the dis covery of abnormal white centers in the dust sampling filters prompted a special inspection    (42). Not only has the YPLL been increasing in younger CWP decedents (<65 years old), but the YPLL per CWP dece dent has also been increasing over those same years (Figure 15; Figure 2 from CDC (42)). This may be related to the observed increase in CWP prevalence observed in recent years as noted earlier.
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The post-1995 period saw the publication of a number of mortality analyses that augmented the earlier mortality findings on coal miners.
program (40) that showed that dust levels from operator samples consistently were lower than those from MSHA inspector samples, and that these differences were greater the smaller the mine (Figure 13). As with hours worked, there is a lack of reliable productivity data linkable with the health outcome data in order to inves tigate this issue further.

# M o rta lity
A report on temporal patterns in pneumoconio sis mortality in the U.S. showed a substantial de cline in numbers of deaths from CWP between 1968 and 2000 (41). This decline is consistent with the reductions in dust level mandated by the 1969 Coal Mine Act. A major additional factor contributing to the declining number of CWP deaths is the diminishing coal mining Most of those studies outside of the United States and United Kingdom did not have quantitative measurements of dust exposure. However, they do support previous findings concerning the overall increased mortality of coal miners and the additional risk imposed by the development of CWP (43)(44)(45). Studies using quantitative exposures showed that m or tality from CWP increased with increasing cumulative exposure to coal mine dust (46). The British study (47) included exposure esti mates for respirable quartz; cumulative expo sure to respirable coal mine dust and respirable quartz were each highly significant predictors of pneumoconiosis mortality, although the relationship was stronger with coal dust than quartz. Respirable quartz exposure was asso ciated with a small but statistically significant relative risk for lung cancer mortality (47).

# Toxicology
Although coal mine dust and crystalline sili ca dust remain the two exposures of primary concern for environmental control, the post-1995 period has seen the publication of results from analyses aimed at eliciting information on what constituents of coal mine dust predict CWP development. These include: 1) free radi cals, in which particles from freshly-fractured siliceous rock have been found to be more fibrogenic than aged particles (48); 2) particle occlusion, in which clay present in the rock strata can surround the silica particles and render them less toxic (49); and 3) bioavailable iron, which has been found to predict coal mine dust toxicity (50,51).
McCunney et al. (52), favored the third expla nation (bioavailable iron) and downplayed the role of quartz in the etiology of coal workers' pneumoconiosis. However, in an analysis of lung inflammatory cell counts from bronchoalveolar lavage in coal miners and non-miners, Kuempel et al. (53) showed that quartz dust (as ei ther cumulative exposure or estimated lung burden) was a significant predictor of pulmo nary inflammation and radiographic category of simple CWP. Cumulative coal dust exposure did not significantly add to those predictions, which may have been due to the high correla tion between the coal and quartz cumulative exposures, such that separate effects for these two dusts could not be clearly demonstrated.
Against this, epidemiologic research has not demonstrated a strong effect of crystalline sil ica on CWP development in situations where silica levels are low. Rather, the level of coal mine dust, per se, has been the strongest pre dictor of CWP. However, the work of Laney et al. (34), as noted above, showed clear evidence of an increase in r-type radiographic opacities (typically associated with silicosis) and rapid progression of pneumoconiosis among U.S. coal miners in Kentucky, Virginia, and West Virginia, suggesting that they were exposed to excessive levels of respirable crystalline silica, and were thus at risk of silicosis. As noted pre viously, dust sampling results support this hy pothesis (23). There is, therefore, the clear need to minimize exposure to silica dust, especial ly for those jobs involving drilling or cutting sandstone and other siliceous rock. Moreover, as noted above, this is particularly pertinent because changing mining conditions might be leading to an increase in the potential for expo sure to silica dust.
Page and Organiscak (54) linked the issue of coal rank, a known risk factor for CWP devel opment in the U.S., Britain, and Germany, with the potential for higher levels of free radicals to be encountered where such coals are mined, and noted above by Dalal (48) and others to have greater levels of cytotoxicity.

# Risk Analysis
Kuempel et al. (13) describe in more detail the risk analyses provided in the NIOSH CCD, in cluding the excess (exposure-attributable) prev alence of CWP and PMF in underground coal miners exposed to various levels of coal mine dust for a working lifetime (as shown in the CCD and also presented here in Table 1). More recent risk estimates have been provided from research on British coal miners (Figure 16, from Figure 1 of Soutar et al. (55)). The latter apply to coal composed of 86.2% carbon (coal rank) and to underground coal miners who work 40 years at the designated coal mine dust level. Risks of PMF range from 0.8% at 1.5 mg/m3 to about 5% at 6 mg/m3, while risks of category 2 or greater CWP range from about 1.5% at 1.5 mg/m3 to about 9% at 6 mg/m3. Note that due to the different ways in which the risk estimates are derived, these are not directly comparable with those from U.S. studies shown in Table 1. How ever, the findings are consistent with those from U.S. studies in indicating that even at the lower coal mine dust levels recommended by NIOSH, and as noted in the CCD, some incidence of CWP would still be expected, especially among miners of higher rank coal. Soutar et al. (55) also provide information on the risk of silicosis in underground coal miners.
Their findings were developed from observa tions at one mine in which unusually high con centrations of crystalline silica dust occurred periodically (56). In their analysis, the authors chose to divide the analysis between exposures  2 mg/m 3. This dichotomy, in the authors' presentation, was associated with more rapid development of silicosis in the > 2 mg/m3 exposure range compared to chronic sil icosis development at exposures < 2 mg/m3. The findings indicate that short excursions to high silica dust intensities are considerably more hazardous than the same level of cumulative exposure at a lower intensity. They therefore demonstrate that mining situations involving the cutting of rock should be avoided if at all possible, or if necessary, that all precautions should be taken to minimize dust exposures.
The findings for <2 mg/m3 (which apply to most coal mining environments that do not involve direct rock cutting) are given in Figure 17 (Figure 3  3 Other Respiratory Disease Outcomes Coggon and Taylor (57), in an extensive re view, concluded that the "...balance of evi dence points overwhelmingly to an impair ment of lung function from exposure to coal mine dust, and this is consistent with the in creased mortality from COPD that has been observed in coal miners." Findings on COPD and related outcomes in coal miners since 1995 (58)(59)(60)(61)(62)(63)(64) have continued to support their con clusion (which was largely based on pre-1995 information). The findings have also identified other risk factors in coal mining for pulmo nary disease development. These include work in roof bolting, exposure to explosive blasting fumes, and exposure to dust control spray wa ter previously stored in holding tanks (65).
The post-1995 findings have also elucidated patterns of lung function decline in coal m in ers, indicating that new miners tend to suffer more severe declines on starting work, after which the declines attenuate somewhat. This finding, derived initially from the analysis by Seixas et al. (66), was explored further by Hen neberger and Attfield (58), who confirmed that the temporal pattern of lung function decline was different in newly-hired coal miners as compared to experienced miners. A possible reason for this could be a healthy worker sur vival effect. A study to explore this issue fur ther, undertaken on new Chinese coal miners, confirmed that starting work in coal mining led to large initial drops in lung function, after which lung function declined at a lesser rate (67). In a follow-up analysis, the researchers reported that the development of respiratory symptoms consistent with bronchitis contrib uted to the early declines in lung function (68).
A recently published mortality study from the United States ( 46) comprised a longer followup of a study on the same cohort of under ground coal miners published in 1995 (69).
It showed that mortality from chronic airway obstruction (CAO) was elevated. Smoking, pneumoconiosis, coal rank region, and cumu lative coal mine dust exposure were all predic tors of mortality from CAO. Dust exposure ef fects were observed within the never-smoker subset of the cohort. The observed dust-related relative risks for CAO were similar to those for pneumoconiosis. The findings showed dustrelated effects for chronic bronchitis and em physema as well as CAO. A recent British study re-affirmed that mortality from COPD was re lated to coal mine dust exposure (47). Finally, the implications of COPD (due to coal mine dust exposure as well as smoking) in causing increased mortality was explored by examin ing mortality risk in relation to rates of ventila tory function decline in coal miners (70). Rates of ventilatory decline 2-3 times the normal age-related decline were associated with dis tinct increases in subsequent mortality.
Past pathologic studies have shown that em physema severity in coal miners is related to dust exposure. Recent studies on South African and U.S. coal miners confirmed these findings (71,72). Important additional information on this topic, using quantitative estimates of both coal mine dust exposure and smoking amount, has been recently published by Kuempel et al. (73). These authors found a highly significant relationship between cumulative exposure to respirable coal mine dust and emphysema severity at autopsy, controlling for effects of smoking, age, and other variables. The effect of dust exposure was similar in magnitude to that of smoking, and was seen in the never-smok ing subgroup. In a further analysis, Kuempel et al. established that exposure to coal mine dust can produce clinically important levels of em physema in coal miners (74).
The above findings support the CCD's recom mendation to reduce the permissible coal mine dust exposure limit in underground coal mines to prevent the development of COPD, the as sociated severe declines in lung function, and the ensuing premature mortality.
There have been several reports of interstitial disease associated with exposure to coal mine dust, perhaps representing a manifestation of CWP, although little systematic research on this topic has been undertaken (75, 76).

# Risk Analysis
Kuempel et al. (13) describe in more detail the risk analyses summarized in the NIOSH CCD, including the excess (exposure-attributable) prevalence of lung function deficits in under ground coal miners exposed to various levels of coal mine dust for a working lifetime (CCD Table 4-7 (1); Table 2). More recent risk esti mates from research on U.K. coal miners have been published (Figure 15; Figure 1 of Soutar et al. ( 55)). They apply to coal composed of 86.2% carbon (coal rank) and to underground coal miners who work 35 years at specified coal mine dust levels ranging from 1 to 6 mg/m3. Risks of a deficit of approximately 1 liter in forced expira tory volume in 1 second (FEV1) among never smokers range from 10% at zero dust expo sure to about 19% at 6 mg/m3. The concomi tant risks for smokers range from about 22% to 36%, respectively. Note that due to the differ ent ways in which the risk estimates have been derived, these are not directly comparable with those shown from U.S. studies shown in Table 2. However, they are consistent with findings from U.S. studies in that even at the 1 mg/m3 coal mine dust exposure limit recommended by the CCD, some occupational effect on ven tilatory function is expected. ). Overall, levels of both coal mine dust and crystalline silica dust were re ported to be higher in underground mines than in surface mines.

# Dust Exposure Assessment
The primary advance since 1995 in the dust as sessment arena has been the development of a continuously-measuring personal dust monitor (PDM) (81). The PDM enables within-shift as sessment of dust exposures, permitting prompt action to intervene and reduce excessive lev els. Conventional practices that rely on the gravimetric assessment of dust collected on air sample filters preclude speedy remediation be cause the delay in obtaining results from the dust laboratory could mean that miners con tinue to be over-exposed before any indication of a problem is available from the laboratory results. The personal dust monitor is now a commercially available product and, as its use is adopted by mines, more timely and targeted interventions to reduce dust exposures will be possible. In 2010, the Mine Safety and Health Administration published new rules that pro vide for the approval and use of the PDM, in addition to the Coal Mine Dust Personal Sam pler Unit, for determining the concentration of respirable dust in coal mine atmospheres (82).

# Com pliance Policy and Procedures
The federal policies and procedures for regu lating underground coal mine dust levels have been the subject of criticism from their intro duction in 1969. Since 1995 further critiques have been published (83,84). The first publica tion provided a historical review, the basic ar gument being that the problems were intrinsic to a process in which an industry essentially regulates itself (i.e., through performing the airborne sampling upon which citations are based). The second publication revisited an issue that was addressed in the NIOSH CCD, in which it was recommended that MSHA not apply any upward adjustment of the REL for instrument uncertainty. Information indicat ing that reported dust levels from mine opera tor sampling were systematically lower than those obtained by mine inspectors during un announced visits to mines to measure expo sures has been published by MSHA (40).

# Surface Coal Mining
Studies published prior to the NIOSH CCD showed that U.S. surface coal miners (particular ly workers on drill crews) were at risk of develop ing CWP (or silicosis). There was also evidence that ventilatory function was reduced in relation to the number of years worked as drill operators or helpers. Since the CCD, a British study has reported evidence of CWP among workers in the dustier jobs and an association with intensity of exposure (85). Dust exposures were generally <1 mg/m3. In the United States, a relationship between tenure in surface coal mining jobs and prevalence of CWP (ILO category 1/0 or greater, and PMF) was reported (16).

# Summary
A considerable body of literature has been pro duced from studies of coal miners and the coal mining environment since 1995, both in the United States and elsewhere. Many of the new er publications, particularly those from other countries, lack quantitative dust exposure mea surements, prohibiting full and valid examina tion of exposure-response relationships. Never theless, their findings all support early findings on British and U.S. coal miners, reinforcing the generally-accepted understanding that exposure to coal mine dust can give rise to various respira tory diseases, and that those diseases can cause disability and premature mortality. The remain der of the newer publications that do have quan titative exposure data report findings that refine or augment the fundamental exposure-response results summarized in the CCD.
Overall, the following conclusions can be made: 1. No new findings have emerged since 1995 that contradict the basic summarization of the respiratory health effects of coal mine dust and their relationship with dust exposures described in the CCD (1). 2. No new findings have emerged that sub stantially modify the basic understand ing of coal mine dust exposure and its impact on respiratory health described in the CCD. 3. The new findings that have emerged strengthen prior results and also refine or add further knowledge on disease pat terns and etiology described in the CCD. 4. Overall, the logical basis for recommen dations concerning prevention of occu pational respiratory disease among coal miners remains essentially unaffected by the newer findings that have emerged since publication of the CCD.
New findings of particular note are: 1. After a long period of declining CWP prevalence, recent federal surveillance data indicate that the prevalence is rising. 2. Coal miners are developing severe CWP at relatively young ages. 3. There is some indication that the m or tality of younger coal miners from CWP is increasing. These workers would have been employed all of their working lives in environmental conditions mandated by the 1969 Coal Mine Act. 4. The pattern of CWP occurrence across the nation is not uniform; hot spots of disease appear to be concentrated in the central Appalachian region of southern WV, eastern KY, and western VA. 5. The cause of this resurgence in disease is likely multifactorial. Possible explanations include excessive exposure due to increas es in coal mine dust levels and duration of exposure (longer working hours), and in creases in crystalline silica exposure (see below). As indicated by data on disease prevalence and severity, workers in small er mines may be at special risk. 6. Given that the more productive seams of coal are being m ined out, a transi tion by the industry to mining thinner coal seams and those with more rock intrusions is taking place and will likely accelerate in the future. Concomitant with this is the likelihood of increased potential for exposure to crystalline sil ica, and associated increased risk of sili cosis, in coal mining.
In summary, every effort needs to be made to reduce exposures both to respirable coal mine dust and to respirable crystalline silica. As rec ommended in the CCD, the latter task requires establishing a separate compliance standard in order to effectively limit exposure to silica dust.