Review of the Air Quality Criteria Document for Particulate Matter

of particles due to respective changes in local pH. After dissolution or leaching of somecomponents from a particle these can be binding of solutes (metals) to macromolecules; animportant pathway also is transport via caveolae across the epithelium as well as theendothelium. The importance of differences between epithelial vs. endothelial pore sizes forlower molecular weight solutes could also be addressed here.Page 7-38, line 1: Snipes and Clem used 3, 9, and 15 µm particles and found only the 3µm to be translocated, did Takahashi really see 5 and 9 µm particles being translocated?Lines 4-6: One has to be very careful when drawing conclusions with respect tolymphatic transport of particles based on intratracheal instillation studies: In such studies highdoses are instilled as a bolus leading to local overload which messes up the normal clearancesignificantly and easily can result in lymphatic translocation which will not occur under normalconditions. Also the statement that particles >5 µm have significant deposition within thealveolar region is not correct for the rat. In the context of species differences related tolymphatic clearance, studies by Thomas et al. (1971) could be cited here showing differencesbetween rodents and dogs, accumulation of particles in local lymph nodes being much greater indogs.Page 7-42, line 29: A most important feature of Morrow's hypothesis is that a volumetricoverloading of alveolar macrophages occurs which eventually impairs its clearance function.Page 7-43, line 11: I am not sure I understand why the slower alveolar macrophagemediatedclearance in humans compared to rats (it is always slower in humans) would cloud theoverload relevance for humans: Humans also live about 25 times longer than rats.Lines 14-15: It is hard to imagine how under normal environmental exposureconditions, overload will occur in compromised lungs. What compromised lungs would that be?Line 26: Although it is generally assumed that intratracheal instillation deliversan "exact" dose to the lung, this does not mean that this dose is really found there shortly afterthe instillation because some of the material is rapidly cleared out by the following exhalations.The amount of this loss depends highly on the instilled volume as well as the instillationtechnique, i.e., synchronizing with respiration or not.Page 7-44, line 9: It is not clear what is said here, the amount that is deposited in thelower airways by instillation can be adjusted, it is not due to by-passing the nose. Probably whatis meant is that the distribution of material is different between the two techniques.Page 7-45, line 11: It is unclear what is meant by percentage retention of particles: Isthat the intercept of the retention curve with the ordinate, or is that the retention halftime? If theretention halftime is meant here that would be explainable since normally by instillation highdoses are delivered which result in overloaded areas with retarded clearance. Thus, it might bebetter to compare inhalation and instillation-associated retention kinetics by describing therespective retention halftimes.Line 18: The bulk of the instilled material certainly goes beyond the terminalbronchioles, otherwise you would see all of it being cleared in a short time by the mucociliaryescalator. Of course, the very periphery of the lung is not well dosed, and as mentioned before,the coverage depends also on the instillation technique, i.e., synchronization with breathing ornot.Line 29: Disposition of particles is only one factor determining their biologicaleffects.Page 7-50, line 1-6: For a discussion of "human equivalent concentration (HEC)" EPA'sRfC document should also be quoted here. Furthermore, earlier in this section, emphasis was onthe lung burden expressed as per unit lung surface area as being more appropriate, whereas herethe amount per gram of lung is indicated. This might be confusing for the reader.Lines 13-19: The Asgharian 2000 reference is missing in the reference list, is thata publication describing the MPPDep model which should be mentioned here as well?As a general comment on this section, it should also be stated in a concludingsummarizing paragraph that all models are just that: models. They have inherent uncertainties,which can be large and differences between model results can probably most of the time beexplained by these uncertainties.A - 67