Appendix 7 

 Dispersion Model Receptor Revisions and Additions
Dispersion Model Receptor Revisions and Additions for the Seven Chemical Sector 

Source Categories1

To estimate ambient concentrations for evaluating long-term exposures, the HEM-3 

model uses the geographic centroids of census blocks (currently utilizing the 2010 Census) as 

dispersion model receptors. The census block centroids are generally good surrogates for where 

people live within a census block. A census block generally encompasses about 40 people or 10-

15 households. However, in cases where a block centroid is located on industrial property, or 

where a census block is large and the centroid less likely to be representative of the block’s 

residential locations, the block centroid may not be an appropriate surrogate. 

Census block centroids that are on facility property can sometimes be identified by their 

proximity to emission sources. In cases where a census block centroid was within 300 meters of 

any emission source, we viewed aerial images of the facility to determine whether the block 

centroid was likely located on facility property. The selection of the 300-meter distance reflects a 

compromise between too few and too many blocks identified as being potentially on facility 

property. Distances smaller than 300 meters would identify only block centroids near the 

emission sources and could exclude some block centroids that are still within facility boundaries, 

particularly for large facilities. Distances significantly larger than 300 meters would identify 

many block centroids that are outside facility boundaries, particularly for small facilities. Where 

we confirmed a block centroid on facility property, we moved the block centroid to a location 

that best represents the residential locations in the block. 

In addition, census block centroids for blocks with large areas may not be representative 

of residential locations. Risk estimates based on such centroids can be understated if there are 

residences nearer to a facility than the centroid, and overstated if the residences are farther from 

the facility than the centroid. To avoid understating the maximum individual risk associated 

with a facility, in some cases we relocated block centroids, or added dispersion model receptors 

other than the block centroid. We examined aerial images of all large census blocks within one 

kilometer of any emission source. Experience from previous risks characterizations indicates 

that in most cases the MIR is generally located within 1 km of the facility boundary. If the block 

centroid did not represent the residential locations, we relocated it to better represent them. If 

residential locations could not be represented by a single receptor (that is, the residences were 

spread out over the block), we added additional receptors for residences nearer to the facility 

than the centroid. 

For these source categories, the table below contains each census block for which we 

changed the centroid location because it was on facility property or was otherwise not 

representative of the residential locations in the block. The table also contains blocks that were 

                                                            

1 Pesticide Active Ingredient Manufacturing, Polyether Polyols Production, Polymers and Resins IV‐‐ Styrene

Acrylonitrile, Polymers and Resins IV‐‐ Polystyrene, Polymers and Resins IV‐‐ Polyethylene Terephthalate, Polymers

and Resins IV‐‐ Methyl Methacrylate‐Butadiene Styrene, and Polymers and Resins IV‐‐ Acrylic Butadiene Styrene.
removed because there are no apparent residences in the block and, consequently, no locations of 

potential human exposure. Finally, the table also contains the locations of additional receptors 

that were included to represent residential locations nearer to the facility than the block centroid. 

Revised Census Block Centroid Locations and Additional Receptors 

for the Seven Chemical Sector Source Categories 

Source

Category Block ID NEI ID

New

Latitude

New

Longitude Comment

PAI 010970057003073    30.934591 ‐88.004772 Centroid doesn't represent

population

PAI 220479532003021    30.232813 ‐91.106034 On plant property

PAI 220479532003028    Remove ‐ no apparent population

PAI 260639512004004    43.850451 ‐82.652354 On plant property

PAI 291279601002004    39.826082 ‐91.411017 Centroid doesn't represent

population

PAI 480396645012046    28.972839 ‐95.398462 Centroid doesn't represent

population

PAI 540390114021035    38.24671 ‐81.543117 Centroid doesn't represent

population

PP 220050303001069    30.161006 ‐90.989435 Centroid doesn't represent

population

PP 220890627001013    Remove ‐ no apparent population

PP 220890627001028    29.988181 ‐90.424947 Centroid doesn't represent

population

PP 220890627001059    29.987055 ‐90.421922 Centroid doesn't represent

population

PP 261635801001003    42.230185 ‐83.148653 On plant property

PP 420350302004066    41.112823 ‐77.469265 Centroid doesn't represent

population

PP 450830229003131    35.027623 ‐82.073821 On plant property

PP 480570005001030    28.509267 ‐96.736011 Centroid doesn't represent

population

PP 481830105004003    32.40865 ‐94.688783 Centroid doesn't represent

population

PP 482012526003180    29.828717 ‐95.087191 Centroid doesn't represent

population

PP 482030206042068    32.4262 ‐94.646719 Centroid doesn't represent

population

PR4‐ABS 090117012002027    41.433666 ‐72.086528 Centroid doesn't represent

population

PR4‐MBS 211110127011012    38.225737 ‐85.825854 On plant property

PR4‐PET 011030051012033    34.62329 ‐87.060798 Centroid doesn't represent

population

PR4‐PET 370370207022034    35.624683 ‐79.050146 Centroid doesn't represent

population

PR4‐PET 371590519011005    35.713365 ‐80.620958 On plant property
Source

Category Block ID NEI ID

New

Latitude

New

Longitude Comment

PR4‐PET 450179501001002    33.854628 ‐81.003794 Centroid doesn't represent

population

PR4‐PET 510411004031004    37.359812 ‐77.31268 Centroid doesn't represent

population

PR4‐PS 220479532003048    30.218114 ‐91.083684 Centroid doesn't represent

population

PR4‐PS 360010143024020    42.586073 ‐73.854949 Centroid doesn't represent

population

PR4‐PS 391670203001019    39.383083 ‐81.505112 Centroid doesn't represent

population

PAI    22047NEI5992 30.234536 ‐91.11074 Additional receptor

PP    22089NEILA13809 29.989074 ‐90.433287 Additional receptor

PP    42035NEIPA13‐5603 41.114421 ‐77.472729 Additional receptor

PP    48201NEI11319 29.809934 ‐95.109341 Additional receptor

PP    48201NEI11319 29.810593 ‐95.103182 Additional receptor

PP    48291NEI8432 30.000748 ‐94.958395 Additional receptor

PP    48291NEI8432 30.006498 ‐94.953137 Additional receptor

PP    48291NEI8432 29.997325 ‐94.95161 Additional receptor

PR4‐ABS    09011NEI43289 41.440671 ‐72.079033 Additional receptor

PR4‐ABS    09011NEI43289 41.437822 ‐72.079675 Additional receptor

PR4‐ABS    17099NEI51461 41.323494 ‐88.766613 Additional receptor

PR4‐ABS    39061NEI43304 39.126067 ‐84.724342 Additional receptor

PR4‐MBS    21111NEI32966 38.21207 ‐85.838867 Additional receptor

PR4‐MBS    21111NEI32966 38.2068 ‐85.854378 Additional receptor

PR4‐PET    37037NEI44658 35.619895 ‐79.031026 Additional receptor

PR4‐PET    37037NEI44658 35.620662 ‐79.051028 Additional receptor

PR4‐PET    45063NEI41393 33.851709 ‐81.010661 Additional receptor

PR4‐PS    09011NEI43289 41.440671 ‐72.079033 Additional receptor

PR4‐PS    09011NEI43289 41.437822 ‐72.079675 Additional receptor

PR4‐PS    21059NEI32867 37.789791 ‐87.157104 Additional receptor

PR4‐PS    21059NEI32867 37.799681 ‐87.152048 Additional receptor

PR4‐PS    39167NEI40518 39.374087 ‐81.531627 Additional receptor