Potential pulmonary effects of man-made organic fiber (MMOF) dusts

Our plain-English summary of Potential pulmonary effects of man-made organic fiber (MMOF) dusts, by D.B. Warheit, G.A. Hart, T.W. Hesterberg, et al (Critical Reviews in Toxicology, 2001).

The study

• Authors: D.B. Warheit, G.A. Hart, T.W. Hesterberg, J.J. Collins, W.M. Dyer, G.M. Swaen, V. Castranova, A.I. Soiefer, G.L. Kennedy
• Journal: Critical Reviews in Toxicology
• Published: 2001;31(6):697-736
• Type of article: Review
• Cited by: 55 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

In the first half of the twentieth century epidemiologic evidence linked elevated incidences of pulmonary fibrosis and cancer with inhalation of chrysotile and crocidolite asbestos, a family of naturally occurring inorganic fibrous materials. As the serpentine and amphibole forms of asbestos were phased out, synthetic vitreous fibers (SVFs; fiber glass, mineral wool, and refractory fiber) became increasingly utilized, and concerns were raised that they too might cause adverse health effects. Extensive toxicological research on SVFs has demonstrated that their pulmonary effects are directly related to fiber dose in the lung over time. This is the result of deposition (thin fibers deposit in the lower lung more efficiently than thick fibers) and lung-persistence ("biopersistence" is directly related to fiber length and inversely related to dissolution and fragmentation rates). In rat inhalation studies, asbestos was determined to be 7- to 10-fold more biopersistent in the lung than SVFs. Other than its effect on biopersistence, fiber composition did not appear to play a direct role in the biological activity of SVFs. Recently, the utilization of man-made organic fibers (MMOFs) (also referred to by some as synthetic organic fibers) has increased rapidly for a variety of applications. In contrast to SVFs, research on the potential pulmonary effects of MMOFs is relatively limited, because traditionally MMOFs were manufactured in diameters too thick to be respirable (inhalable into the lower lung). However, new developments in the MMOF industry have resulted in the production of increasingly fine-diameter fibers for special applications, and certain post-manufacturing processes (e.g., chopping) generate respirable-sized MMOF dust. Until the mid-1990s, there was no consistent evidence of human health affects attributed to occupational exposure to MMOFs. Very recently, however, a unique form of interstitial lung disease has been reported in nylon flock workers in three different plants, and respirable-sized nylon shreds (including fibers) were identified in workplace air samples. Whether nylon dust or other occupational exposures are responsible for the development of lung disease in these workers remains to be determined. It is also unknown whether the biological mechanisms that determine the respirability and toxicity of SVFs apply to MMOFs. Thus, it is appropriate and timely to review the current data regarding MMOF workplace exposure and pulmonary health effects, including the database on epidemiological, exposure assessment, and toxicology studies.

In plain English

This article was a review, not a new study. The authors reviewed published research on workplace exposure to man-made organic fibers, such as nylon, para-aramid, polyester, polypropylene and acrylic, and on possible lung effects. It covered exposure measurements, at least 52 epidemiology studies, and animal and laboratory studies. In the reviewed surveys, para-aramid exposures in most operations were generally below 0.1 to 0.2 fibers/ml. Studies of nylon flock workers found 8 cases of interstitial lung disease among 165 workers at a Rhode Island plant. In the largest study of acrylic fiber workers, the relative risk of death from lung cancer was 0.8 (95% confidence interval 0.6 to 1.0). In one reviewed rat study, long chrysotile asbestos fibers persisted in the lung much longer than long para-aramid fibers. The authors concluded that low workplace exposures were not associated with increased lung disease risk. They said nylon flock workers with high exposures may be at increased risk of interstitial lung disease, although other exposures could not be ruled out. They noted that most worker studies did not report exposure levels, and that other exposures, including asbestos, complicated interpretation.

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: review of published literature on exposure, epidemiology and toxicology of man-made organic fiber (MMOF) dusts
• Who was studied: published studies of workers with potential exposure to organic fiber dust (at least 52 epidemiology studies), plus workplace exposure surveys and animal studies in rats, hamsters and guinea pigs
• Exposure: dusts of man-made organic fibers such as nylon, para-aramid, polyester, polypropylene, acrylic and polyvinyls, plus related semisynthetic (cellulose acetate) and carbon fibers; chrysotile asbestos appears as a comparison fiber in some animal studies
• Main results:
  • Reviewed para-aramid workplace surveys: exposure concentrations for most operations were generally below 0.1 to 0.2 fibers/ml
  • Reviewed studies of nylon flock workers found 8 cases of interstitial lung disease (ILD) among 165 flock workers (5%) in a Rhode Island plant
  • Blair et al. (1998), workers at three acrylic fiber plants: relative risk of death 0.8 (95% CI 0.6 to 1.0) for lung cancer, 0.8 (95% CI 0.0 to 4.5) for nasal cavity and sinus cancer, and 0.5 (95% CI 0.3 to 0.7) for nonmalignant respiratory disease
  • Goldberg and Theriault (1994), 10,211 workers exposed to cellulose acetate, cellulose triacetate and polypropylene: relative risk for lung cancer 0.8 (95% CI 0.6 to 0.9)
  • Textile and clothing industry studies: a relative risk of nasal cavity and sinus cancer greater than 1 was reported in 9 of 11 studies in Western Europe; the authors pointed to wood dust as a confounder
  • Searl (1997), rats: long (more than 15 µm) chrysotile asbestos fibers were far more biopersistent in the lung than long para-aramid fibers or 475 glass fibers
  • Chronic rat inhalation study of para-aramid (Lee et al., 1988): 6% of female rats, and none of the males, exposed to 100 fibrils/ml developed lesions first called cystic keratinizing squamous cell carcinomas and later reclassified as proliferative keratin cysts
  • 90-day rat inhalation study of polypropylene (Hesterberg et al., 1992): increased lung cellularity and mild bronchiolitis, but no collagen deposition and no tumors
• The authors' conclusion: The authors concluded that low occupational exposures to airborne MMOF dust were not associated with increased risk of pulmonary disease. They also concluded that workers exposed to high levels of nylon fiber dust may be at increased risk of interstitial lung disease, although other exposures cannot be ruled out as a cause.
• Limitations the authors state:
  • Most reviewed epidemiology studies did not specify the level of fiber dust exposure
  • Other workplace exposures, including asbestos and hardwood dust, make it difficult to attribute findings to synthetic fibers
  • Possible reporting bias: studies showing relative risks below 1.0 may be underrepresented in the published literature
  • In animal studies, the mechanical processing needed to make respirable test fibers could alter the fibers' biological activity
  • Apart from para-aramid, there is little toxicological information on other MMOF dusts

Topics

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

Medical terms in this study, explained in our Lexicon: Pulmonary Fibrosis, Chrysotile, Crocidolite, Serpentine Group, Amphibole Group, Vitreous, Deposition, biopersistence.

Related studies

• Man-made respirable-sized organic fibers: what do we know about their toxicological profiles? — D.B. Warheit, K.L. Reed, et al. (2001)
• Environmental measurements of total dust and fiber concentration in manufacturer and user of man-made mineral fibers — J.H. Kim, H.S. Chang, et al. (1999)
• Pulmonary effects in rats inhaling size-separated chrysotile asbestos fibres or p-aramid fibrils: differences in cellular proliferative responses — D.B. Warheit, M.A. Hartsky, et al. (1996)
• Lung proliferative and clearance responses to inhaled para-aramid RFP in exposed hamsters and rats: comparisons with chrysotile asbestos fibers — D.B. Warheit, S.I. Snajdr, et al. (1997)
• Determinants of the pathogenicity of man-made vitreous fibers (MMVF) — G. Oberdörster (2000)
• Biopersistences of man-made vitreous fibers and crocidolite fibers in rat lungs following short-term exposures — R.P. Musselman, W.C. Miiller, et al. (1994)

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