Benchmarks and pollutant rankings are sought:
Rankings of environmental releases and assessments of the environmental performance of facilities that are based solely on quantities of compounds that are released are misleading. Emission quantities alone are not sufficient because some compounds that are released are highly detrimental to the environment or to human health while others are relatively benign. In addition, effects range across a wide variety of environmental impact categories, including smog formation, human carcinogenicity, aquatic toxicity, terrestrial toxicity, climate change, acidification, and stratospheric ozone depletion. Also, some compounds persist in the environment for much longer than others, increasing their potential to cause harm. Finally, where the compound is found in the environment -- in air, water, or soil -- and the population of living things that exist where the compound is found have an impact on the harm it can cause.
In a simple scheme for weighting the releases of specific compounds according to their threat to the environment, information about the relative potency of the compounds might be combined with release estimates of those compounds. If this is done, the emissions of compounds that are highly potent are given more weight than compounds that have little effect. As an example, for a human health impact category, reference doses or threshold limit values might be used to determine potency factors.
While having a sense of a compound's potency of effect in the environment adds greatly to the understanding of the potential impact of releases of that compound, developing potency factors that account for the important variables in complex natural system is not a straightforward task. For example, toxicity values are difficult to determine with any accuracy, even under laboratory conditions where all but one variable is controlled. Also, global conditions such as climate change and stratospheric ozone depletion are notoriously complex, which makes determining accurate potency of effect values for emissions associated with these environmental effects problematic.
Some schemes for assessing environmental impacts rate chemicals based only on human toxicity, while others rate chemicals for a large number of diverse environmental impact categories. Some impact categories might apply to biological species found only in specific habitats (for example, aquatic toxicity values) while other impact categories are global (for example, stratospheric ozone depletion and climate change). When more than one impact category is evaluated, there is no generally accepted method for combining the evaluations of different impact categories to obtain a single aggregated environmental impact score. Instead, ratings for the different impact categories are generally considered separately.
While different impact categories cannot be aggregated, it would be impractical to have a separate impact category for each biological species. Some impact assessment schemes have separate impact factors for aquatic and terrestrial life, but within those broad categories the response of different species to the same dose of a compound is very different. In most impact assessment schemes, toxicity values are aggregated across species. Sometimes this is done by letting the most sensitive species determine the overall toxicity evaluation. Another way of aggregating toxicity values is to set them at a level where a given percentage of species are unharmed.
Impact assessment schemes differ in the way they handle releases to different environmental media. The environment is typically modeled as a group of "compartments." Figure 1 shows some of the environmental compartments in which pollutants appear in the environment: air, water, soil, sediment, and living things. Sometimes it is appropriate to evaluate only releases to one environmental compartment or to one environmental compartment per impact category, or to use a separate category of potency factors for releases to each compartment.
When a pollutant is released into the environment, it does not necessarily remain in the compartment to which it was released. Emissions to surface waters of a highly volatile compound will transport mostly to the air compartment, for example. Figure 2 shows a diagram of this process. The vapor pressure, solubility of the compound, and the temperature of the environment are some of the variables that effect the extent to which this transfer occurs, and the temperature and degree of mixing in the system affect the rate at which it occurs.
Besides transporting into different compartments, compounds can react with other compounds once they are released. Highly reactive compounds do not remain in the environment for long periods of time. When assessing environmental impacts, sometimes it is desirable to take the environmental fate and transport of emissions into account, perhaps by incorporating the results of fate and transport models.