Risk assessment due to environmental exposures to fibrous particulates associated with taconite ore

Our plain-English summary of Risk assessment due to environmental exposures to fibrous particulates associated with taconite ore, by R. Wilson, E.E. McConnell, M. Ross, et al (Regulatory Toxicology and Pharmacology, 2008).

The study

• Authors: R. Wilson, E.E. McConnell, M. Ross, C.W. Axten, R.P. Nolan
• Journal: Regulatory Toxicology and Pharmacology
• Published: 2008;52(1 Suppl):S232-45
• Cited by: 20 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

In the early 1970s, it became a concern that exposure to the mineral fibers associated taconite ore processed in Silver Bay, Minnesota would cause asbestos-related disease including gastrointestinal cancer. At that time data gaps existed which have now been significantly reduced by further research. To further our understanding of the types of airborne fibers in Silver Bay we undertook a geological survey of their source the Peter Mitchell Pit, and found that there are no primary asbestos minerals at a detectable level. However we identified two non-asbestos types of fibrous minerals in very limited geological locales. Air sampling useful for risk assessment was done to determine the type, concentrations and size distribution of the population of airborne fibers around Silver Bay. Approximately 80% of the airborne fibers have elemental compositions consistent with cummingtonite-grunerite and the remaining 20% have elemental compositions in the tremolite-actinolite series. The mean airborne concentration of both fiber types is less than 0.00014 fibers per milliliter that is within the background level reported by the World Health Organization. We calculate the risk of asbestos-related mesothelioma and lung cancer using a variety of different pessimistic assumptions. (i) that all the non-asbestos fibers are as potent as asbestos fibers used in the EPA-IRIS listing for asbestos; with a calculated risk of asbestos-related cancer for environmental exposure at Silver Bay of 1 excess cancer in 28,500 lifetimes (or 35 excess cancers per 1,000,000 lifetimes) and secondly that taconite associated fibers are as potent as chrysotile the least potent form of asbestos. The calculated risk is less than 0.77 excess cancer case in 1,000,000 lifetimes. Finally, we briefly review the epidemiology studies of grunerite asbestos (amosite) focusing on the exposure conditions associated with increased risk of human mesothelioma.

In plain English

This article assessed risk from airborne fibers around a taconite facility in Silver Bay, Minnesota. A geological survey of the Peter Mitchell Pit found no primary asbestos minerals. Twelve air samples near the residential area in late 1998 gave nine values. Among fibers 5 micrometers or longer, 15 (79%) had elemental compositions consistent with cummingtonite-grunerite and 4 (21%) with tremolite-actinolite. The reported mean was less than 0.00014 fibers per milliliter, at the low end of World Health Organization asbestos background levels. Comparing 387 airborne fibers with 288 amosite fibers, the authors judged the Silver Bay fibers mostly non-asbestos. Using a linear no-threshold model, they pessimistically assumed all fibers had the asbestos potency in EPA-IRIS. On that basis they calculated, rather than observed, 35 asbestos-related cancer deaths per 1,000,000 lifetimes. Hodgson and Darnton figures gave 51 per 1,000,000 if the fibers were as potent as amosite, or 0.77 if as potent as chrysotile. The authors concluded that moving waste rock disposal onto land sufficiently reduced the risk. They noted that results depend on the linear model.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Quantitative risk assessment using a linear (no-threshold) dose-response model, based on the authors' own air sampling, a fiber size comparison and a summarized geological survey. It is not an epidemiological study of residents.
• Who was studied: Ambient air at the sampling station closest to the residential area of Silver Bay, Minnesota: twelve air samples collected October 24 to December 9, 1998 (three in duplicate, giving nine values). The survey also used 49 bulk rock samples from the Peter Mitchell Pit near Babbitt, Minnesota. The risk calculations assumed a Silver Bay population of about 2500 residents with lifetime exposure.
• Exposure: Environmental (non-occupational) airborne fibers 5 micrometers or longer associated with taconite ore. Only two fiber types with an elemental composition similar to asbestos minerals were found. 15 fibers (79%) had compositions consistent with cummingtonite-grunerite and 4 fibers (21%) with tremolite-actinolite. For the calculations the fibers were assumed to be as potent as asbestos in general (EPA-IRIS), as amosite or amphibole asbestos, or as chrysotile.
• Main results:
  • Geological survey of the Peter Mitchell Pit: no primary asbestos minerals were found.
  • Geological survey: less than 1% of the total rock mass in the pit was fibrous.
  • Silver Bay air: the mean concentration of fibers 5 micrometers or longer was reported in Table 1 as less than 0.00014 f/mL (n = 9). Several individual samples were below the detection limit. The text describes this level as about 35% of the level EPA-IRIS predicts would cause one cancer death in 10,000 lifetimes.
  • The authors reported that the Silver Bay fiber concentration was at the low end of World Health Organization background levels for airborne asbestos.
  • The size distribution of 387 Silver Bay airborne fibers was compared with 288 respirable amosite fibers. The aspect ratio of the Silver Bay fibers did not change with length, while that of amosite increased with length. The authors took this to mean the exposures were predominantly non-asbestos fibers.
  • The EPA-IRIS calculation treated the Silver Bay fibers as if they all had the average potency of asbestos. The authors described their assumptions as pessimistic.
  • EPA-IRIS model estimate for Silver Bay residents with lifetime exposure: about 1 excess asbestos-related cancer death in 28,500 lifetimes, or 35 per 1,000,000 lifetimes. This was calculated, not observed.
  • Hodgson and Darnton model, mesothelioma: 24 cases per 1,000,000 lifetimes assuming the fibers are as potent as amosite (alternate 1).
  • Hodgson and Darnton model, mesothelioma: 0.24 cases per 1,000,000 assuming the fibers are as potent as chrysotile (alternate 2).
  • Hodgson and Darnton model, lung cancer: 27 cases per 1,000,000 lifetimes assuming amphibole asbestos potency, and 0.53 per 1,000,000 assuming chrysotile potency.
  • Combined Hodgson and Darnton totals for mesothelioma and lung cancer: 51 asbestos-related cancers per 1,000,000 lifetimes (amosite potency) or 0.77 (chrysotile potency).
  • The authors calculated that the upper limit of fiber concentrations in Silver Bay was six times lower than the post-cleanup level for asbestos fibers that EPA used for residents near the World Trade Center.
• The authors' conclusion: The authors concluded that disposing of the waste rock on land was sufficient to reduce the cancer risk from fiber exposure in the general population around the Silver Bay facility. They put the combined excess risk of lung cancer and mesothelioma at approximately 1/1000 of the background risk. They also concluded that nothing indicated further steps were needed. They stated their belief that non-asbestos fibers are far less potent than asbestos.
• Limitations the authors state:
  • The conclusions rest on a model with a linear dose-response relationship. The authors noted that under a threshold model the predicted effects would be approximately zero.
  • The Hodgson and Darnton risk coefficients used were derived from asbestos and do not address cleavage fragments or other non-asbestos fibers.
  • The authors argued that assuming all fibers were as potent as amosite seriously overestimates the risk for long-term Silver Bay residents.
  • Risks of the calculated size could never be directly proven to exist.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis.

Medical terms in this study, explained in our Lexicon: Environmental Exposure, Taconite, Mineral Fiber, Asbestos, Air Sampling, Cummingtonite-Grunerite, Tremolite-Actinolite Series, Mesothelioma.

Related studies

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• The relationship between various exposure metrics for elongate mineral particles (EMP) in the taconite mining and processing industry — J. Hwang, G. Ramachandran, et al. (2014)
• Comprehensive assessment of exposures to elongate mineral particles in the taconite mining industry — J. Hwang, G. Ramachandran, et al. (2013)
• Occupational exposures and lung cancer risk among Minnesota taconite mining workers — E.M. Allen, B.H. Alexander, et al. (2015)
• An overview of the risk of lung cancer in relation to exposure to asbestos and of taconite miners — G. Berry, G.W. Gibbs (2008)
• A case-control study of mesothelioma in Minnesota iron ore (taconite) miners — C.S. Lambert, B.H. Alexander, et al. (2016)

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