Washed Cotton in The 1978 OSHA Cotton Dust Standard

Current Intelligence Bulletin 56

Recent Evaluations of Mildly Washed Cotton

Initial Studies of Modern Batch Kier Washing

As mentioned above, the earlier batch kier washing trials had been performed on systems involving hand loading of cotton fiber without prior mechanical opening or prewetting. This resulted in incomplete wetting of cotton fibers during the washing process, which was probably the cause of the inconsistent results observed in the earlier studies of batch kier washing. In 1988, Task Force investigators visited two companies utilizing batch kier processes with automated systems for mechanically opening and thoroughly wetting cotton fiber during the kier-loading process. To evaluate the effectiveness of batch kier washing using this state-of-the-art opening and wetting technology, arrangements were made to wash cotton on one of these commercial systems for comparison with the same cotton washed using a continuous process as partially exempted in the 1985 revised standard. The study [Perkins and Berni 1991] is summarized in the following paragraphs.

A blend of Midsouth cotton of predominantly grade code 52 (i.e., color grade code 52 and leaf grade code 5 according to the current classification system [USDA 1993a]) was selected as a worst-case test. This is the lowest grade that, following a continuous process wash, would be exempted under the 1985 revised standard from all provisions except for medical surveillance and medical recordkeeping. Furthermore, over a recent 5-year period, less than 10% of the cotton crop was lower than this grade [USDA 1989, 1990, 1991, 1992, 1993b, 1994]. Part of the blended cotton was left unwashed, part was mildly washed using a continuous-wash system according to process criteria stipulated in the 1985 revised OSHA standard, and part was mildly washed in the modern batch kier system—two kiers each of two different washing conditions. One of the batch kier wash conditions involved temperature and water-to-fiber ratio as specified in the 1985 revised standard for continuous wash systems. The other wash condition—lower water-to-fiber ratio and shorter wash time, but with a higher temperature—was considered a priori to be more commercially feasible.

Compared with the unwashed cotton, all three mild washings resulted in substantial and statistically significant reduction of card-generated airborne cotton dust—in each case by at least 50%. In addition, the three different mild wash treatments were highly effective and statistically equivalent in reducing the endotoxin content of card-generated airborne elutriated dust. As a result, the concentration of airborne endotoxin—a more specific indicator of the respiratory hazard contained in cotton dust than gravimetrically measured dust concentration (see Appendix)—was very effectively reduced by all three washings, from more than 300 ng/m³ for the unwashed cotton (at a dust level of 1.98 mg/m³) to less than 10 ng/m³ for each of the washed cottons (at dust levels ranging from 0.35 mg/m³ to 0.89 mg/m³). These low airborne endotoxin levels generated during card processing of the washed cottons were all below a relative "threshold" for acute airway response in humans described previously by NIOSH investigators in this same setting [Castellan et al. 1987]. Given the well-recognized interlaboratory variability associated with measurement of endotoxin activity in dust samples [Perkins 1992; Chun and Perkins 1994], it is important to note that the endotoxin assays were from the same NIOSH laboratory using the same standards that were used in the earlier studies.

In sum, the results of this research clearly demonstrated that the modern batch kier system is capable of mildly washing cotton as effectively as the already partially exempted continuous batt process. However, after reviewing the research results and obtaining input from selected Task Force member organizations, OSHA was disinclined to provide any new exemption for batch kier washed cotton without first having "additional data from washing trials involving a range of cotton blends at more than one washing facility," as well as more information on implications for chronic respiratory disease potential of mildly washed cotton [Adkins 1990]. Over the past several years, research findings have been generated that provide this additional information.

Additional Studies of Modern Batch Kier Washing

To further assess the effectiveness of mildly washing cotton in modern batch kier systems, another blend of predominantly color grade code 52 and leaf grade code 5 cotton (this time grown in Texas) was washed on a batch kier system operated by another company [Jacobs et al. 1993; Perkins and Olenchock 1995]. Washing, done at 60°C and a 40:1 water-to-fiber ratio as stipulated in the 1985 revised standard for continuous wash systems (or, alternatively, at 93°C and 17:1 water-to-fiber ratio), resulted in a reduction of at least 50% in dust- generating capacity (compared with the unwashed cotton) under identical carding rates and ventilation conditions. In addition, this batch kier washing resulted in a statistically significant 19- to 55-fold reduction of endotoxin concentration in card-generated elutriated dust (compared with dust from the unwashed cotton), based on blinded endotoxin assays from the NIOSH laboratory, which has demonstrated reproducibility of its carefully standardized endotoxin assay procedures over the period covered by all the relevant studies [Perkins and Olenchock 1995]. These endotoxin assay results indicated that airborne levels of endotoxin for the washed cotton were less than half the relative "threshold" of 9 ng/m³ defined by that same NIOSH laboratory [Castellan et al. 1987]. (Jacobs and colleagues independently reported an airborne concentration of endotoxin on the order of 50 ng/m³ —a 12-fold reduction for the washed cotton [Jacobs et al. 1993]. Such between-laboratory discrepancies in endotoxin results are common and may arise from differences in extraction and assay procedures.)

On the basis of human ventilatory responses to experimental exposures to dust from this washed cotton, Jacobs and colleagues concluded that their results "suggest that modern batch-kier systems can effectively remove the acute pulmonary toxicity of cottons washed at 60°C and a 40:1 water-to-fiber ratio" [Jacobs et al. 1993]. The endotoxin assay results from the NIOSH laboratory on card-generated airborne dust samples support this overall conclusion. Current evidence offers a basis for accepting the measurements of acute human airway response and airborne endotoxin made during the washing studies—not only as indicators of potential acute respiratory hazard, but also as surrogates for the chronic respiratory hazard of cotton dust (see Appendix).