Letter to the Editor regarding the Donovan et al. (2011) article

To the Editor, We are writing to express concerns regarding Donovan et al.’s (2011) review of bystander exposures to asbestos. The authors’ conclusions are not supported by the reviewed literature, but derive wholly from an indoor air dispersion model that was developed for asbestos litigation defense (Drivas et al., 1996). The authors give primacy to this model despite the fact that actual workplace measurements and workplace simulation studies contradict the results generated by the mathematical model. In fact, their own analysis of workplace and simulation studies found no relationship between distance from an asbestos source and exposure level: “No discernable trends were evident when bystander:worker ratios were plotted independently against distance from the primary worker” (2011, p. 64–65). All the evidence indicates that exposure concentration depends on many complex variables, such as ventilation conditions and task. Distance from a point source or primary worker is an unreliable predictor of bystander exposure. This explains why the ratio of bystander:worker concentrations ranges from 0 to >2.5 in Donovan et al.’s analysis (see Figure 1). Donovan et al. arbitrarily cap the bystander:worker concentration ratios at 1. A ratio greater than 1 means the bystander exposure is higher than the worker’s direct exposure that is the only “source” of the bystander exposure. If Donovan et al.’s analysis is valid, ratios above 1 are impossible because they mean that that bystander exposures are higher than those of the worker “source. ” Realizing that such results discredit their entire analysis, Donovan et al. try to explain them away. The authors attribute ratios greater than 1 to “variability that was due to the high number of ratios based on bystander samples with <limit of detection (LOD) results” (2011, p. 54). In other words, the authors argue that all the ratios greater than 1 are due to bystander values that are <LOD. However, all worker values are >LOD, since worker values <LOD were excluded. Therefore, any bystander:worker ratio that uses a bystander value <LOD will have a worker value >LOD, and the ratio will therefore be less than 1. Donovan et al.’s explanation lacks merit: bystander samples <LOD could not have yielded a bystander:worker ratio greater than 1 (see Figure 2). Capping the ratios at 1 skews the mean bystander:worker ratios shown in Figure 1 towards lower levels and misrepresents the actual data. The one possible exception to the exclusion of worker samples <LOD is the Mowat et al. (2005) study, which averaged groups of worker samples together, and thus could have included worker samples <LOD (Donovan et al. 2011, p.68). However all these samples were within the 5–10 feet distance category, and therefore cannot explain the ratios greater than 1 in the other three distance categories. For example, two results at approximately 40 feet find that the bystander concentrations were almost twice those of the worker directly working with the product (2011, p. 65). Such results undermine Donovan et al.’s entire analysis, which maintains that bystander exposures are a simple function of distance from a worker “source. ” Donovan et al. concluded that “there is virtually no impact of the asbestos emitted from a point source, for OSHA fibers, at distances greater than 50 feet,” (2011, p. 67) based entirely on their model’s results. They claim this is “entirely consistent” with “most field studies” (2011, p. 67), when the field studies show no relationship between distance from a point source and exposure level. This cutoff level of 50 feet was not supported by their field study analysis, it was an assumption the authors made in this analysis: “Distances greater than 50 feet away were considered too great to be significantly influenced by the single point source where the primary worker was engaged” (2011, p. 54). The authors conflate their analysis’ assumptions as data to support their conclusions. This assumption begs the question of how far asbestos point source emissions travel, and as discussed next it is inconsistent with and contrary to the results from field studies. Several of the field studies Donovan et al. analyzed directly contradict the 50 feet cutoff level (2011, p. 57). Reitze et al. (1972) found concentrations of 46 f/cc at 75 feet from spray insulators, which was within the range of mean worker (“point source”) samples (35.2–61 f/cc). Additionally, Lorimer et al. (1976) found a concentration of 0.2 f/cc at 60 feet from a worker grinding brake linings, LETTER TO THE EDITOR