Comments on Gilmour et al., “Comparative Toxicity of Size-Fractionated Airborne Particulate Matter”

Gilmour et al. (2007) have reported an analysis of the toxicity of a water-dilute acid extract of airborne particles in four different geographical areas, South Bronx, NY (SB), Sterling Forest, NY (SF), Salt Lake City, UT (SL), and Seattle, WA (S). All but one (SF) of the locations is urban in character. The extracts were instilled in mouse lungs; the animals were subsequently euthanized, for certain toxicological tests. The goal of the study was to differentiate the toxicological reactions to particles by size and chemical composition from widely different locations. Sampling was done for 4 wk at each location to obtain material for extraction and use in mouse exposures. (The calendar dates by location were not listed.). The study provides insight into the apparent toxicity of waterweak nitric acid-soluble extracts from ambient particles, and the apparent source of these differences. For example, it is of interest to observe that coarse particles (PM10−2.5), particles sampled between 2.5 and 10 μm aerodynamic diameter, from SB appear to be more toxic than either of the other size fractions (ultrafine [PM0.1—particles less than 0.1 μm aerodynamic diameter] or fine [PM2.5—particles less than 2.5 μm aerodynamic diameter] sampled in the study) from SB, based on polymorphonuclear leukocyte (PMN) values and macrophage inhibitory protein (MIP)-2 in bronchoalveolar lavage (BAL) fluid and creatine kinase in plasma. These results were not seen at other study locations. Two other potentially insightful results were described as well: The BAL measurements from the SB ultrafine PM extracts at a higher dose level were significantly more harmful than at the lower dose level (results not found for the other size fractions in SB or at other sites). The coarse PM extracts from SL were associated with elevated protein in BAL. Thus, in SB the PM extract seemingly produced a more toxic response than other sites, with most of the toxicity indexes due to the coarse fraction. Unfortunately, the significance of these findings is obscured by incomplete reporting of (a) the sampling and extraction methods, (b) the analytical results of the extracted PM chemical composition, and (c) the source apportionment reported for each site. The study adopted a nonstandard method for collecting particles compared with conventional Teflon and quartz filter-based sampling used in U.S. air monitoring. Their method used a high-volume staged impactor-filter sampler, with collection of coarse and fine particles in a polyurethane foam, and ultrafine particles on a polypropylene filter. The particle size cut points of the sampler are not reported, and no reference is given to the comparability of this sampling method with the standard filter-based method currently used the national sampling networks. The brief description of the analytical chemistry using waterdilute acid extraction of the foam and the filters glosses over the fact that this method will not dissolve much of the refractory portion of the collected PM, including many metal oxides, and tarry black or elemental carbon. The authors state that their recovery is about 80% except for iron, based on comparison with a water-dilute acid extract of a reference standard. This sidesteps this issue of the insoluble material that would make up the gravimetric mass concentration observed using direct filter analysis. Thus, it is not possible to compare the results of the toxicity experiments with complete filter-based chemical composition and gravimetric mass concentration as reported in contemporary air monitoring data. The chemical composition of the aggregated samples of different particle size from each of the sites is not tabulated with a material balance for the reader to inspect, and only a small portion of the complete composition data expected from the instrumental analysis of the extracts is discussed in the study. Additionally, a comparison of particle composition reported at the same locations by the conventional filter-based air monitoring chemistry is not included. This prevents the reader from evaluating the similarities and differences in the PM extracts, which could lead to enhanced interpretation of the toxicity data. The identification of PM source contributions using chemical mass balance (CMB) calculations modeling also is only summarized briefly (CMB only was applied to fine particles, because the coarse particle analysis was “unreliable” [reasons not stated]), and source profiles are “unavailable” for ultrafine particles. The CMB methodology is referred to an unpublished paper for more details (Duvall et al., 2007). Unfortunately, this manuscript as currently available from its authors (Fall of 2007) is no more explanatory of this CMB application than Gilmour et al. Application of CMB requires that source profiles are chosen for each of the sites, based on chemical composition and knowledge of sources from emission inventories. This is not discussed; nor is it clear if the profiles were used uniformly for all sites. It is well known that each of the locations used has pollution from different local and regional sources, so that the preselection of relevant sources is important for the credibility of the results.