Appendix 7 

Acute Impacts Refined Analysis Figures 
Refined Acute Modeling Approach 

 Initial acute screening risk calculations were performed with the HEM-3 model. 

HEM-3 estimates acute (1-hour) impacts at both polar and census block receptors. It is 

assumed for this short period of time that an exposed individual could be located at any 

off-site location. The lack of readily available detailed property boundary information 

for many of the facilities evaluated made it difficult to determine whether receptors were 

on- or off-site. In the absence of such information, the first ring of polar receptors was 

placed 100 meters from the plant center for many facilities. However, these polar rings 

often transected on-site locations, restricting public access to exposures at these levels 

and thereby overestimating exposures. 

 The screening approach used by HEM-3 to estimate maximum 1-hour exposures 

also likely overestimates. To estimate maximum 1-hour concentrations at each receptor, 

HEM-3 sums the maximum concentrations attributed to each source, regardless of 

whether those maximum concentrations occurred during the same hour. In other words, 

HEM-3 assumes that the maximum impact from each source at each receptor occurs at 

the same time. In actuality, maximum impacts from different sources may occur at 

different times. 

 This appendix includes the results of refined acute assessments performed for 

facilities that exceeded short-term health benchmarks using the HEM-3 approach. The 

refinements can address both areas of conservatism described above. The simplest 

refinement is to plot the HEM-3 polar grid results on aerial photographs of the facilities; 

This allows the assessment of off-site locations that may be accessible to the public (e.g.,

roadways and public buildings.). In cases where this refinement results in lower acute 

risks no further refinement was conducted. However, if acute risk still remain at or near 

levels of concern a further refinement is conducted. This second refinement includes 

remodeling with the AERMOD model directly (instead of using HEM) to estimate 

maximum 1-hour concentrations at each receptor by pairing individual source impacts in 

time and space. The AERMOD results are then plotted on aerial photographs as noted 

above to determine off-site impacts. Modeling with AERMOD also allows the 

estimation of the frequency of higher impacts at or above the health levels of concern. 

The modeled 1-hour concentration of a pollutant divided by its short-term health 

benchmark) at the polar receptors near the facility was below a HQ of 1. Refer to below 

for a brief summary; 

• NEI23077; Acute Risk Screening HQ was equal to 4.2 for glycol ethers, initial 

evaluation of the map indicated the HQ was on-site, with an off-site HQ of 2. 

This site required a refined assessment with AERMOD, in which the off-site HQ 

was less than 1. The acute REL value for ethylene glycol methyl ether was used 

as a surrogate for all glycol ethers. 
Shipbuilding and Ship Repair 

(Refined Acute Risk Asses

• Figure 1 – NEI23077; maximum 

acute HQ(REL) for glycol ethers le

North of the facility
Figure 1 – NEI23077 Acute Glycol Ethers HQ