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

In the first half of the twentieth century epidemiologic evidence linked el evated incidences of pulmonary fibrosis and cancer with inhalation of chrys otile and crocidolite asbestos, a family of naturally occurring inorganic f ibrous materials. As the serpentine and amphibole forms of asbestos were ph ased out, synthetic vitreous fibers (SVFs; fiber glass, mineral wool, and r efractory fiber) became increasingly utilized, and concerns were raised tha t they too might cause adverse health effects. Extensive toxicological rese arch on SVFs has demonstrated that their pulmonary effects are directly rel ated 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 inhal ation studies, asbestos was determined to be 7- to 10-fold more biopersiste nt in the lung than SVFs. Other than its effect on biopersistence, fiber co mposition did not appear to play a direct role in the biological activity o f 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 p ulmonary effects of MMOFs is relatively limited, because traditionally MMOF s were manufactured in diameters too thick to be respirable (inhalable into the lower lung). However, new developments in the MMOF industry have resul ted in the production of increasingly fine-diameter fibers for special appl ications, and certain post-manufacturing processes (e.g., chopping) generat e respirable-sized MMOF dust. Until the mid-1990s, there was no consistent evidence of human health affects attributed to occupational exposure to MMO Fs. Very recently, however, a unique form of interstitial lung disease has been reported in nylon flock workers in three different plants, and respira ble-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 determi ned. It is also unknown whether the biological mechanisms that determine th e respirability and toxicity of SVFs apply to MMOFs. Thus, it is appropriat e and timely to review the current data regarding MMOF workplace exposure a nd pulmonary health effects, including the database on epidemiological, exp osure assessment, and toxicology studies.