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Environmental Health Criteria 151

SELECTED SYNTHETIC ORGANIC FIBRES

(World Health Organization, Geneva 1993)

(Chapter 1 and Chapter 9)

1.  Summary
  1.1  Identity, physical and chemical properties
  1.2  Sources of human and environmental exposure
  1.3  Environmental levels and human exposure
  1.4  Deposition, clearance, retention, durability and translocation
  1.5  Effects on experimental animals and in vitro test systems
  1.6  Effects on humans
  1.7  Summary of evaluation

9. Conclusions and Recommendations for the Protection of Human Health

(* Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organisation, and the World Health Organisation)

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1. SUMMARY

1.1 Identity, physical and chemical properties

Carbon/graphite fibres are filamentary forms of carbon produced by high-temperature processing of one of three precursor materials: rayon (regenerated cellulose), pitch (coal tar or petroleum residue) or polyacrylonitrile (PAN). Nominal diameters of carbon fibres range between 5 and 15 µm. Carbon fibres are flexible and electrically and thermally conductive, and in high performance varieties have high Young's modulus (cooefficient of elasticity measuring the softness or stiffness of the material) and tensile strength. They are corrosion resistant, lightweight, refractile and chemically inert (except to oxidation), and have a high degree of stability to traction forces, low thermal expansion and density, and high abrasion and wear resistance.

Aramid fibres are formed by the reaction of aromatic diamines and aromatic diacid chlorides. They are produced as continuous filaments, staple and pulp. There are two main types of aramid fibres, para - and meta - aramid, both with a nominal diameter of 12-15 µm. Para-aramid fibres can have fine-curled, tangled fibrils within the respirable size range (< 1 µm in diameter) attached to the surface of the core fibre. These fibrils may be detached and become airborne upon abrasion during manufacture or use. Generally, aramid fibres exhibit medium to very high tensile strength, medium to low elongation and moderate to very high Young's modulus. They are resistant to heat, chemicals and abrasion.

Polyolefin fibres are long-chain polymers composed of at least 85 % by weight of ethylene, propylene or other olefin units; polyethylene and polypropylene are used commercially. Except for some types such as microfibre, the nominal diameters of most classes of polyolefin fibres are sufficiently large that they are within the respirable size range.

Polyolefins are extremely hydrophobic and unreactive. Their tensile strengths are considerably less than those of carbon or aramid fibres and they are relatively flammable, melting at temperatures between 100 and 200°C.

Methods developed for counting mineral fibres have been used for industrial hygiene monitoring of synthetic organic fibres. However, these methods have not been validated for this purpose. Factors such as electrostatic properties, solubility in mounting media and refractive index may cause difficulties when using such methods.

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1.2 Sources of human and environmental exposure

The estimated worldwide production of carbon and graphite fibres was in excess of 4000 tonnes in 1984. For aramid it was more than 30,000 tonnes in 1989, and for polyolefin fibres more than 182,000 tonnes (U.S.A. only). Carbon and aramid fibres are used principally in advanced composite materials in aerospace, military and other industries to improve strength, stiffness, durability, electrical conductivity or heat resistance. Polyolefin fibres are typically used in textile applications.

Exposures to synthetic organic fibres have been documented in the occupational environment. Synthetic organic fibres can be released into the environment during production, processing or combustion of composites and during disposal. Very few data are available concerning actual releases of these materials into the environment.

Available data on the transport, distribution and transformation of organic fibres in the environment are restricted to identification of products of municipal incineration of refuse from carbon fibre-containing composites and pyrolysis decomposition products of carbon fibre and aramids. During simulation of municipal incineration, both the diameters and lengths of carbon fibres were reduced. Principal pyrolysis decomposition products of carbon and aramid fibres include aromatic hydrocarbons, carbon dioxide, carbon monoxide and cyanides.

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1.3 Environmental levels and human exposure

Synthetic organic fibre dusts are released in the workplace during operations such as fibre forming, winding, chopping, weaving, cutting, machining and composite formation and handling.

In the case of carbon/graphite fibres, respirable fibre concentrations are generally less than 0.1 fibres/ml but concentrations of up to 0.3 fibres/ml have been measured close to chopping or winding operations. Fibres may also be released during machining (drilling, sawing, etc.) of carbon fibre composites, although most of the respirable material thus produced is non-fibrous.

Average airborne concentrations of para-aramid fibrils in the workplace are reported to be less than 0.1 fibrils/ml in filament operations and less than 0.2 fibrils/ml in floc cutting and pulp operations. During staple yarn processing, average airborne fibril concentrations are typically below 0.5 fibrils/ml, but levels as high as approximately 2.0 fibrils/ml have been reported. Other end-use workplace exposures are typically below 0.1 fibrils/ml on average with peak exposures of 0.3 fibrils/ml. Special potential for exposure was demonstrated for waterjet cutting of composites, levels being as high as 2.91 fibrils/ml. Particles of mean aerodynamic diameter of 0.21 µm have been generated during laser cutting of epoxy plastics reinforced with aramid fibres, but the fibre content of the dust was not reported. Certain volatile organic compounds (including benzene, toluene, benzonitrile and styrene) and other gases (hydrogen cyanide, carbon monoxide and nitrogen dioxide) are also produced during such operations.

Limited air monitoring data from a facility producing polypropylene fibres indicate maximum airborne levels for fibres longer than 5 µm of 0.5 fibres/ml, with most values being less than 0.1 fibres/ml. Scanning electron microscopy showed that airborne fibre sizes range from 0.25 to 3.5 µm in diameter and 1.7 to 69µm in length. In a single ambient sample collected near a carbon fibre weaving plant, a concentration of 0.0003 fibres/ml was detected. The average dimensions of the fibres were 706 µm by 3.9 µm. The release of carbon fibre at the crash site of two military aircraft, following combustion of carbon fibre composite used in construction, has also been reported. No other relevant information on concentrations in the environment was identified.

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1.4 Deposition, clearance, retention, durability and translocation

Few data on specific synthetic organic fibres have been identified. The data on para-aramid fibres (Kevlar) indicate that, when inhaled, these fibres are deposited at alveolar duct bifurcations. There is also evidence of translocation to the tracheobronchial lymph nodes.

1.5 Effects on experimental animals and in vitro test systems

For the synthetic organic fibre types reviewed here, there is a dearth of good quality data from relevant experimental studies.

There are no adequate studies in which the fibrogenic or carcinogenic potential of carbon/graphite fibres has been examined. Effects following short-term inhalation exposure (days) of rats to respirable-size pitch-based fibres included inflammatory responses, increased parenchymal cell turnover and minimal type II alveolar cell hyperplasia. Available data from intratracheal instillation and an intraperitoneal injection study are considered inadequate for assessment owing to the lack of characterization of the test materials and lack of adequate documentation of protocol and results. A mouse skin painting study on four carbon fibre types suspended in benzene was inadequate for the evaluation of carcinogenic activity.

In the case of para-aramid fibres, the majority of data is derived from experiments on Kevlar. Short-term (2 week) inhalation studies of Kevlar dust have resulted in a pulmonary macrophage response which decreased in severity after exposure ceased. Short-term studies of ultrafine Kevlar fibrils have shown a similar macrophage reaction and patchy thickening of the alveolar ducts. Both lesions again decreased after exposure, but a minimal amount of fibrosis was present 3-6 months later. A two-year inhalation study of Kevlar fibrils in rats induced exposure-related lung fibrosis (at > 25 fibres/ml) and lung neoplasms (11% at 400 fibres/ml and 6% at 100 fibres/ml in female rats; 3 % at 400 fibres/ml in male rats) of an unusual type (cystic keratinizing squamous cell carcinoma). Increased mortality due to lung toxicity was observed at the highest concentration, indicating that the Maximum Tolerated Dose had been exceeded. There is considerable debate concerning the biological potential of these lesions and their relevance in humans. The full carcinogenic potential of the fibrils may not have been revealed in this study because it was terminated at 24 months.

Intratracheal instillation of a single dose of shredded Nomex paper (2.5 mg) containing fibres with diameters of 2 to 30 µm resulted in a non-specific inflammatory response. A granulomatous reaction developed two years post-exposure. Intratracheal instillation of a single dose of 25 mg Kevlar resulted in a non-specific inflammatory response which subsided within about one week. A granulomatous reaction and a minimal amount of fibrosis were observed later.

In three studies, intraperitoneal injection of Kevlar fibres (up to 25 mg/kg) resulted in a granulomatous response but no significantly increased incidence of neoplasms. It was suggested by the authors of these investigations that the lack of neoplastic response was possibly due to the agglomeration of the Kevlar fibrils in the peritoneal cavity.

There are no adequate studies in which the fibrogenic or carcinogenic potential of polyolefin fibres has been examined. A 90-day inhalation experiment in rats with respirable (46 % < 1 µm) polypropylene fibres (up to 50 fibres/ml) indicated dose- and duration-dependent changes characterized by increased cellularity and bronhiolitis. No relevant data on intratracheal instillation are available. In intraperitoneal injection studies on polypropylene fibres or dust in rats, there was no significant increase in peritoneal tumours.

There are inadequate data on which to make an assessment of the in vitro toxicity and genotoxicity of synthetic organic fibres. For aramids, studies have shown that short and fine para-aramid fibrils have cytotoxic properties. With regard to polyolefin fibres, there is some evidence of cytotoxicity of polypropylene fibres. Mutagenicity tests on extracts of polyethylene granules gave negative results.

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1.6 Effects on humans

In a cross-sectional study of 88 out of 110 workers in a PAN-based continuous filament carbon fibre production facility, there were no adverse respiratory effects, as assessed by radiographic and spirometric examination and administration of questionnaires on respiratory symptoms. In other less well documented studies, adverse effects have been reported in workers involved in the production of both carbon and polyamide fibres, data presented in the published accounts of these investigations, however, were insufficient to assess the validity of the reported associations.

1.7 Summary of evaluation

Data concerning the exposure levels of most synthetic organic fibres are limited. Those data available generally indicate low levels of exposure in the occupational environment. There is a possibility of higher exposures in future applications and uses. Virtually no data are available with respect to environmental fate, distribution and general population exposures.

On the basis of limited toxicological data in laboratory animals, it can be concluded that there is a possibility of potential adverse health effects following inhalation exposure to these synthetic organic fibres in the occupational environment. The potential health risk associated with exposure to these synthetic organic fibres in the general environment is unknown at this time, but is likely to be very low.

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9. CONCLUSIONS AND RECOMMENDATIONS FOR PROTECTION OF HUMAN HEALTH

The data reviewed in this report support the conclusion that respirable, durable organic fibres are of potential health concern. The following actions are suggested for protection of human health.

To the maximum extent possible, the organic fibres that are produced should be non-inspirable or at least non-respirable. Respirable fibres should not be produced by splitting or abrading during subsequent processing, use and disposal.
If small-diameter respirable fibres are necessary for specific products or applications, these fibres should not be biopersistent or exhibit other toxic effects.
All fibres that are respirable and biopersistent must undergo testing for toxicity and carcinogenicity. Exposures to these fibres should be controlled to the same degree as that required for asbestos until data supporting a lesser degree of control become available. The available data suggest that para-aramid fibres fall within this category. Furthermore, other respirable organic fibres should be considered to fall within this category until data indicating a lesser degree of hazard become available.
Populations potentially exposed to respirable organic fibres should have their exposure monitored in order to evaluate exposure levels and the possible need for additional control measures.
Populations identified as being those most exposed to respirable organic fibres should be enrolled in preventive medicine programmes that focus on the respiratory system. These data should be reviewed periodically for any early signs of adverse health effects.

Reference:

This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Program, the International Labour Organization, or the World Health Organization.

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