As noted earlier, due to the large numbers of chemicals listed in a database, analysis of site chemistry based on common chemical names can be quite problematic. The first problem with engaging in this process is the fact that at times, a simple report of a chemical class such as PAHs will be entered in numerous ways, for example ‘Polycyclic Aromatic Hydrocarbons (PAHs), non-specified . . . ‘ ‘PAHs, assorted chemicals . . . ‘, ‘PAHs, identities unknown’, etc. The varieties of isomers a particular product might have further complicate this issue, and even though at times these isomers do have distinct chemical differences in vitro, when evaluated in vivo or in utero the overall chemical features can often have similar toxic and carcinogenic effects. With just a few exceptions, therefore, chemicals with similar base molecules and major side groups (regardless of location on the chain or ring) are often grouped together and treated as toxicologically similar (but at times retaining the fact that some are more toxic than others given the right occasions).
Using this method of reclassifying chemicals, and carefully defining the chemical subgroups based on both chemistry and overall group-related toxicology/carcinogenicity features, it is possible to identify as many as several dozen groups. For the purposes of using GIS to depict these groups on a site by site basis, the methods chosen for reclassifying chemical information may often be restricted to methods that produce just 10 to 15 subclasses for these chemicals. Once several methods of reclassifying chemicals at sites are applied and then tested, the best method(s) can be used to evaluate the 22 High Risk sites defined for a part of this study.