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Potential Consequences of Incidents Involving the Identified Hazardous Materials 31 Determining how many people might fall within that impact area can be done in several ways. If you have detailed population data in a geographic information system, you can overlay that impact area over the population data and automatically count the population inside it. You should be aware of differences in residential (nighttime) and employment (daytime) population for the specific area. Another approach is to use average population density figures for the area (determined from Census data) and combine that with the area of potential exposure. For example, if there is a population density of 1,500 people per square mile and the protective action area or hazard area is 0.5 mile on a side, then the number of affected people would be 1,500 0.25 mi2, or 375 people. For fires, it is more appropriate to use the initial isolation zone as the impacted area, so in that case the number of affected people would be 1,500 (50 m/1000)2 or 4 individuals. Again, all of these people would not be killed or seriously injured, but this provides an estimate of potentially affected population. Step 11 Use population density estimates or a geographic information system with population data layers to determine the potential population exposure for a potential release for each scenario. Using Table 13, specify the CARVER scale value based on the estimated number of potentially exposed individuals. Counting all the potentially exposed individuals as fatalities is very con- servative, but is appropriate for this type of assessment. Estimating Environmental Consequences Environmental consequences can include property damage as well as land and aquatic contamination and remediation. For most hazmat incidents, the impact on the environment will be measurable but not excessively high. The emergency response planner or planning team may judge that many scenarios pose little environmental risk. These scenarios do not need to be assessed. As with population exposure, environmental consequences are determined through two steps: (1) determining the impact area and (2) determining the consequences within that area. The impact area can be determined in the same manner as human-health consequences, by using the impact distances in the ERG. Appendix C provides additional approaches for obtaining more detailed estimates of environmental exposure areas. Step 12 Determine the affected area for environmental impacts from a potential release for each scenario in the hazardous materials portfolio using the same methods used to estimate the affected area for population impacts. Environmental consequences are divided into two types: property damage and land and aquatic contamination. For most materials, one of these two will be the dominant consequence category. Property damage can be assessed for hazards that have the capability of totally destroying a structure--flash fires, fires (flammable gas clouds should be considered here in addition to flammable liquids and solids), and BLEVEs or explosions--by using per-acre land values that consider the number and type of structures typically found. A fire, if not prevented from spreading, can involve nearby structures and do extensive damage. An explosion or BLEVE can do a lot of structural damage, resulting in replacement of the structure as part of the damage estimate. The hazard distance specified in the NRHM Routing Guidelines (2007) or the protective action distance specified using the ERG would provide too large an area. The initial isolation distance specified in the ERG would be more representative of the damage area from a fire or explosion. The circular area specified by this radius is the suggested area to be used to estimate damage to nearby structures. Alternatively, the dispersion code in the Areal Location of Hazardous Atmospheres model (ALOHA 2007) has an option to estimate the damage radius from fires and BLEVEs. The user only has to specify the material and the quantity present.