In addition to the following cautions, users should see other limitations relating to specific sources of data (see Sources of Data).
- In this surveillance system, every reasonable attempt has been made with the available resources to present comprehensive data on health outcomes and exposures of relevance to work-related lung diseases. The data are drawn from existing major databases. However, other data may exist that would improve the completeness and reliability of the findings.
- Statistics in many tables and figures are based on small numbers. Users are cautioned that these can be unstable. Hence, inferences should be drawn with care, and should take the numerical basis into account.
- A decedent's or survey respondent's usual or current industry and occupation are not always indicative of the industry and occupation associated with the exposure responsible for that individual's work-related disease. Users are therefore cautioned not to make definitive causative inferences about industries and occupations based solely on the various mortality and morbidity tables.
- Work-related respiratory diseases are typically, though not always, chronic and may also have long latencies. As reflected in median ages at death for the pneumoconioses, many affected individuals live to or even beyond average life expectancy. The fact that many affected individuals do not die as a direct result of their work-related respiratory disease led to a decision to consider all causes of death, underlying and contributing, in the development of the summary tables and figures of mortality data. In the absence of national incidence and prevalence morbidity data specific to occupational diseases, the intent is to provide a better assessment of disease occurrence and distribution than would be possible if consideration were restricted to underlying causes of death.
- Certifying physicians typically do not list all of a decedent's diseases on the death certificate. Therefore, even though contributing causes of death are considered, the mortality data probably underestimate the total occurrence of pneumoconioses and other diseases.
- As with any analysis based on death certificate data, there is undoubtedly some misclassification of cause of death. A treating physician may not correctly diagnose a particular disease during a patient's life or, as mentioned above, a certifying physician may fail to list a correctly diagnosed disease on the death certificate, particularly if another disease was directly responsible for the decedent's death. In addition, the diagnoses listed on the death certificate are sometimes miscoded.
- Data that depend, either directly or indirectly, on physician reporting or recording of occupational disease diagnoses can be influenced significantly by the physician's ability or willingness to suspect and evaluate a relationship between work and health. These, in turn, are influenced by evolving medical/scientific information, and by the legal, political, and social environment. Some factors may lead to increased diagnosis and recording/reporting (e.g., the Coal Mine Health and Safety Act of 1969 increasing recognition and recording of coal workers' pneumoconiosis), while other factors may reduce occupational disease recognition or reporting by physicians (e.g., long latency between a work exposure and disease development, or concern about involvement in litigation).
- Byssinosis and asthma lack the characteristic fibrosis and associated radiographic appearance commonly observed in mineral dust pneumoconioses. In addition, advanced stages of asthma and byssinosis may be difficult to distinguish from other chronic obstructive pulmonary diseases, including those due solely to cigarette smoking. For both these reasons, under-diagnosis may be more likely for byssinosis and work-related asthma than for the radiographically apparent pneumoconioses.
- Categorization of lung diseases for which mortality data are presented is limited by the International Classification of Diseases (ICD) coding system used for the National Center for Health Statistics (NCHS) multiple cause-of-death data. Also, ICD-8, ICD-9, and ICD-10 disease rubrics differ somewhat for all types of pneumoconioses (International Classification of Diseases (ICD) Codes). However, the effect of ICD changes is not substantial for most of the diseases under consideration (e.g., there is no indication of any changes in the yearly trend in national silicosis mortality related to changes in the rubrics for the ICD code related to silicosis).
- A general assumption of work-relatedness for pneumoconiosis deaths is reasonable for surveillance purposes. However, a very small proportion of pneumoconiosis decedents may have developed their disease as a result of non-occupational (e.g., avocational) exposure to pneumoconiotic agents.
- Although respiratory diseases other than the pneumoconioses can be caused by occupational exposure to respiratory hazards, the diagnosis alone does not indicate an occupational etiology. As a result, users will note that the types of mortality tables presented differ depending on the specific disease. More comprehensive tables are presented for those diseases that are highly specific for occupational etiology, while a more limited approach is used for diseases that are less likely to be caused solely by occupational exposure.
- Individuals affected by chronic diseases with long latency have much more time to change residences prior to death than individuals affected by acute diseases with short latency. Thus, state of residence at death does not necessarily represent the location of a decedent's occupational exposure, even for a death that results directly from occupational respiratory disease.
- Users are reminded that only about half the states provide data on usual industry and occupation of decedents which meet the National Center for Health Statistics' quality criteria for the national death data files used to develop many of the tables (see States (and Years) for which Industry and Occupation Codes from Death Certificates Met NCHS Quality Criteria, 1985-1999).
- Apparent differences in mortality rates may reflect, in part or in whole, geographical as well as temporal changes in employment patterns affecting the number of workers at risk to various respiratory hazards. Denominators used to calculate mortality rates are based on general population estimates for the location (e.g., national, state, or county) and for the years in which the deaths occurred. The resulting rates have clear public health significance. However, as suggested by some very high proportionate mortality ratios for specific industrial and occupational groups, national and state-specific rates typically represent a dilution of very high mortality among exposed groups of workers by very low mortality within the general population that is not significantly exposed.
- To comply with current Centers for Disease Control and Prevention (CDC) policy, population based mortality rates have been adjusted to the U.S. Year 2000 Standard Population. Updates prior to December 2005 show rates adjusted to the 1940 standard population. Users are cautioned that rates currently presented are not directly comparable with those shown in these earlier updates.
- Current proportionate mortality ratios (PMRs) are not directly comparable to those presented in earlier updates (prior to December 2005) because they have been adjusted for age (in five-year categories), sex, and race, whereas previous PMRs were adjusted only for age (in 20-year categories). Users are also reminded that, because of the lack of smoking information in the national death files, PMRs have not been adjusted for smoking.
- Over the period covered by data presented, median ages at death have generally increased for all pneumoconioses. The user is cautioned to realize that this increase is the result of many factors, only one of which may be a general reduction of disease severity (e.g., due to enhanced diagnostic sensitivity and fewer severe cases). Another possible factor is a reduced number of younger workers at risk due to changing employment patterns. Reduced mortality from other causes of death is undoubtedly another important factor.
- Data from the Coal Workers' Health Surveillance Program (CWHSP) have a number of limitations. The program is restricted to currently employed miners and participation rates in many states are less than 50%. Missing or inaccurate work history information may affect tenure calculations. Pathologic disease in some individuals may not be detected radiographically and, although rare among working populations, various non-occupational conditions may result in radiographic abnormalities consistent with pneumoconiosis.
- The main usefulness of the Bureau of Labor Statistics (BLS) Annual Survey of Injuries and Illnesses is to assess occupational injuries, because work-attribution of traumatic injuries is typically quite clear to the employers. In contrast, work-related diseases are generally under-recognized and under-reported by employers.
- The Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA) exposure data presented should be considered provisional and subject to revision. The samples were collected for regulatory compliance purposes, rather than for the surveillance of worker exposures, and therefore may not represent exposures typically experienced by workers. Nonetheless, these data provide the best available national exposure information for industries in the United States.
- MSHA and OSHA data for similar agents are presented in a parallel format. The user is cautioned that MSHA and OSHA are separate agencies with separate regulatory jurisdictions over different industries. The number of compliance samples collected by an agency depends upon many factors, including the size and nature of an industry, congressional actions, and regulatory policies.
- To identify pneumoconiotic agents included in the MSHA and OSHA data systems, the following documents were reviewed: Documentation of TLVs® and BEIs®, 6th edition (American Conference of Governmental Industrial Hygienists); Occupational Respiratory Diseases (NIOSH Pub.No. 86-102); the Pocket Guide to Chemical Hazards (NIOSH Pub No. 97-140); and various NIOSH Criteria Documents. The resulting list of pneumoconiotic agents represents those agents associated with the most prevalent types of pneumoconiosis, but is not intended to be a complete listing of all agents that may cause pneumoconiosis.
- Many of the geometric mean exposures include samples that could not be quantified with the sampling and analytical methods used. Rather than assume the values of these samples were zero, estimates of the sample results were used to calculate the geometric mean. The methods for estimating the sample result are described in the Exposure Methods and users should keep in mind this uncertainty underlying the geometric mean concentrations.
- Although OSHA adopted permissible exposure limits (PELs) of 0.1 mg/m3 for quartz and 0.05 mg/m3 for cristobalite that were enforced from March 1, 1989 through March 22, 1993, neither OSHA nor MSHA currently has a PEL specific to any form of crystalline silica. Instead, the relevant PELs are for respirable dust containing crystalline silica. These PELs take the form of formulas in which the PEL for respirable dust is reduced as the crystalline silica content of the dust increases. The PEL formulas vary with the agency and the industry, but, with all of them, the effective allowable exposure to quartz is less than or equal to 0.1 mg/m3 and the effective allowable exposure to cristobalite is less than or equal to 0.05 mg/m3, regardless of silica content. Thus, the percentage of OSHA samples exceeding the PEL is greater in the years when the formula PEL is applied (all years except 1989 through 1993) than it would be if a 0.1 mg/m3 quartz or 0.05 mg/m3 cristobalite PEL had been applied for these years. Users should keep the preceding explanation in mind when considering data showing apparent temporal discontinuities in the annual percentage of OSHA silica samples exceeding the PEL.
- The percentage of respirable coal mine dust samples exceeding the PEL was calculated using the MSHA PEL of 2 mg/m3 Mining Research Establishment (MRE) for respirable coal mine dust containing no more than 5% quartz. Because the quartz content could not be reliably identified for most of the respirable coal mine dust samples, no attempt was made to use the MSHA formula for reducing the PEL when the quartz content exceeded 5%. Thus, the percentage of respirable coal mine dust samples exceeding the PEL is a lower limit, and the actual percentage exceeding the PEL is very likely higher than reported.
- In addition to samples in which quartz was identified, the respirable quartz data
include MSHA samples identified as:
- Nuisance dust, respirable fraction, less than 1% quartz;
- Unlisted particulate, respirable fraction, less than 1% quartz; and
- Respirable dust (not analyzed or below detection limit) from metal/nonmetal mines because, although the samples did not indicate quartz exposure, they were collected, in part, to assess exposure to quartz. This provides a more accurate estimate of the geometric mean exposures and the percentage of exposures that exceed a PEL or recommended exposure limit (REL).
- For available exposure data for agents associated with each type of pneumoconiosis, users are reminded that the time period over which the exposure data were collected does not necessarily correspond to the time period during which most of the decedents represented in the mortality data acquired their disease. For most pneumoconiosis deaths, there is a latency period of at least several years between first occupational exposure and onset of disease. Subsequent death typically occurs many years after disease onset.