Perspective pubs.acs.org/crt
Increased Lung Cancer Mortality in Taconite Mining: The Potential for Disease from Elongate Mineral Particle Exposure Jeffrey H. Mandel,* Gurumurthy Ramachandran, and Bruce H. Alexander University of Minnesota School of Public Health, Division of Environmental Health Sciences, Minneapolis, Minnesota 55455, United States ABSTRACT: Taconite mining involves potential exposure to non-asbestiform amphibole mineral fiber. More recent studies have demonstrated increased mortality from respiratory cancers and heart disease among workers in the taconite industry. This finding is not consistent with recent exposure assessment findings, nor is the toxicology of this mineral suggestive of neoplastic disease. The understanding of respiratory disease in taconite mining is hampered by the lack of exposure data to asbestiform mineral fibers that occurred in the 1950s and 1960s. Other industries with similar mineral exposure have not demonstrated definitive associations with respiratory cancer, although non-malignant respiratory disease is a consistent finding in epidemiological studies.
1. BACKGROUND Taconite iron ore mining is a multibillion dollar industry in northeastern Minnesota and northern Michigan. This industry has provided up to two-thirds of the ore needed for steel production in North America. Mining of this ore has been in existence since the 1950s, when it replaced a more concentrated ore (hematite) that was depleted following World War II. Since it is less concentrated in iron, taconite ore requires more processing and introduces the potential for several different exposures. These include respirable dust containing iron oxides and silica and a unique type of mineral, characterized as non-asbestiform amphibole mineral fibers. This fiber has been classified by the National Institute for Occupational Safety and Health (NIOSH) as an elongate mineral particle (EMP), an umbrella term that refers to any particle with an aspect ratio (length/width) of 3:1 or more that is respirable in size. EMPs may be fibrous, which include the asbestiform minerals, typically longer, thinner fibers with aspect ratios of 20:1 or more. Non-asbestiform EMPs are usually shorter and thicker with lower aspect ratios.1 Non-asbestiform amphibole exposure occurs in several types of mining. Although relatively few studies have been undertaken, exposure to this mineral has been of concern because of the established toxicity of asbestiform amphibole exposure. Other types of non-asbestiform exposure outside of the workplace, such as erionite, a fibrous EMP, have been associated with malignant mesothelioma, raising further questions about risks associated with non-asbestiform exposures.2 In 2007, the Minnesota Department of Health’s Cancer Surveillance System reported a 2-fold elevation in death from mesothelioma among taconite workers. This assessment © XXXX American Chemical Society
involved a comparison of disease rates in all current and former taconite workers identified in a previous investigation, compared to rates for the rest of the state of Minnesota.3 This finding was unique among industries where non-asbestiform amphibole EMP exposures occur, and a related case enumeration suggested a common potential exposure to commercial asbestos.4 Since most of the communities where Minnesota mines are located are contiguous to the mining facilities, the possibility of public exposure existed. Cohort mortality investigations have been conducted within the taconite industry beginning in the 1980s. In earlier investigations, the standardized mortality ratio (SMR) was not found to be higher than expected for any of the measured diseases.5−7 More recent investigations have revealed increasing mortality from lung cancer, as demonstrated in Table 1. Mesothelioma and heart disease mortalities have also been detected at higher than expected rates.8 Cross-sectional surveys of former and current miners have found around 7% of participants with evidence of interstitial lung disease and another 16% with pleural abnormalities.9 This Perspective on Statistical Trends considers exposure, toxicological, and epidemiologic information concerning nonasbestiform amphibole EMPs, relevant to taconite, gold, and talc mining, in order to interpret the above change in mortality over time and to determine the potential for this exposure to cause health risks in humans. Received: November 16, 2015
A
DOI: 10.1021/acs.chemrestox.5b00472 Chem. Res. Toxicol. XXXX, XXX, XXX−XXX
Perspective
Chemical Research in Toxicology Table 1. Comparison of the Standardized Mortality Ratio (SMR) for Taconite Investigations author
publication date
observed cases
Higgins5,a
1983
Cooper6,b
1988
Cooper7,b
1992
Allen8,c
2014
298 15 112 801 41 360 1058 65 454 9094 981 30 2676 64
SMR all-cause resp. cancer cardiovascular all-cause resp. cancer circulatory all-cause resp. cancer circulatory all-cause resp. cancer mesothelioma circulatory hypertensive heart disease
95% confidence limits 87 84 90 88 59 89 91 97 91 104 116 277 110 181
77−97 47−138 74−109 82−94 43−82 80−98 85−97 75−123 83−100 102−104 109−124 187−396 106−114 139−233
a
Observed rate for those who worked in taconite jobs for 1 year or more compared to expected rate for the white male population of Minnesota. Observed rate for those who worked in taconite jobs for 3 months or more compared to expected rate for the white male population of Minnesota. c Observed rate, adjusted for age, calendar period, and gender, for those who worked in taconite jobs for 1 year or more compared to expected rate for the white male population of Minnesota. b
Table 2. Comparison of Asbestiform and Non-asbestiform Elongate Mineral Particlesa length width aspect ratio physical properties persistence exposure setting animal toxicity associated human pathology a
asbestiform
non-asbestiform
>5 μm 3:1 (may be much higher, with some over 20:1) occur in bundles of fibers that split along cleavage planes; high tensile strength; flexible; strong surface resistance high during industrial application of commercial asbestos; mining and processing of raw fibrous asbestos material high lung cancer; mesothelioma; pleural plaques; interstitial lung disease
0.25 μm >3:1 (generally not much higher) do not separate along cleavage planes; reduced tensile strength; brittle; weaker surface resistance low during mining and processing of raw nonfibrous material low pathology not well-established
As measured by phase contrast and transmission electron microscopy.
2. EXPOSURE ISSUES FOR NON-ASBESTIFORM EMPS Non-asbestiform amphibole EMPs have been studied infrequently. There have been occupational studies in two other mining industries with similar exposures. One is the Homestake gold mining industry of South Dakota. This has an identical type of EMP exposure (cummingtonite−grunerite mineral series) and has been studied on several occasions.10−12 The other industry involves talc mining and processing. It, too, has been studied in the United States and Europe. Although it is in the same non-asbestiform EMP classification, talc typically involves the anthophyllite−tremolite mineral series. The identification of EMP in the work or ambient environment says little about its potential to cause harm. One problem in the interpretation of EMP exposure data is the way measurements of EMPs are done. The traditional way is to count EMPs with the use of phase contrast microscopy (PCM). PCM has limitations in visualizing structures that are less than 0.25 μm wide. Since EMPs can exist with smaller widths than this, PCM may undercount thinner structures. PCM is also not able to distinguish asbestiform from non-asbestiform minerals, nor is it able to distinguish amphibole from nonamphibole types. Nevertheless, it is the approach recommended by U.S. federal agencies for counting EMPs. Other techniques, for example, transmission electron microscopy (TEM), are better suited to more accurate identification of EMP dimensions. TEM in combination with additional methods (electron
diffraction X-ray analysis, ISO 13794) is needed to identify asbestiform amphibole minerals. A naturally occurring EMP associated with the taconite ore body is found in northeastern Minnesota. It is in the cummingtonite−grunerite mineral series and is the nonasbestiform amphibole analogue of fibrous amphibole cummingtonite−grunerite, commercially known as amosite asbestos. It is non-asbestiform because its fibers are not arranged in parallel bundles and are not friable. On this basis, it has a lower exposure potential in the natural, unharmed state. This EMP is thought to be less pathogenic, in part because most nonasbestiform EMPs are shorter in length and wider.1,13 It is the longer fibers of the asbestiform variety that are thought to be more pathogenic, related to the difficulty the body has in clearing these materials and to their tensile strength, as shown in Table 2. Work within Minnesota’s taconite industry involves potential exposure to non-asbestiform amphibole EMPs, silica, and respirable dust.14,15 The amphibole EMPs are specifically located in the eastern Mesabi Range, where, in one major portion, the Peter Mitchell pit, it has been determined that less than 1% of the total rock mass contains asbestiform amphibole EMPs.16 An even smaller amount of the fibrous mineral is estimated to enter the ambient air during mining and processing. Despite the low potential for exposure, the toxicity of asbestiform EMP is well-established and, in the case of B
DOI: 10.1021/acs.chemrestox.5b00472 Chem. Res. Toxicol. XXXX, XXX, XXX−XXX
Perspective
Chemical Research in Toxicology
decreased residence time in the body. Results of multiple epidemiologic, animal, and in vitro studies suggest that short fibers (
