Foundational aspects of the concept of chemical activity

Philipp Mayer

DTU-Environment, Technical University of Denmark, Denmark

The chemical activity of an organic chemical quantifies its potential for spontaneous physicochemical processes, such as diffusion, sorption, and partitioning. For instance, the chemical activity of a sediment contaminant determines its equilibrium partitioning concentration in sediment-dwelling organisms and differences in chemical activity determine the direction and extent of diffusion between environmental compartments (Di Toro et al., 1991; Reichenberg and Mayer, 2006). This makes chemical activity a meaningful and well-defined exposure parameter that is closely linked to fugacity and freely dissolved concentration (Reichenberg and Mayer, 2006). The conversion of published toxicity data (i.e. effective concentrations, EC50) to chemical activity space (i.e. effective chemical activities, Ea50) provided the first indication that narcosis for different chemicals, organisms and exposure media occurs within a relatively narrow range of chemical activity (Reichenberg and Mayer, 2006; Meyer, 1899; Overton, 1901; Ferguson, 1939; Mayer and Holmstrup, 2008) (Figure 3.1.1). During the last 10 years several studies have confirmed this for the “baseline toxicity” of non-polar organic chemicals and their mixtures (Mayer and Holmstrup, 2008; Mackay et al., 2011; Smith et al., 2013; Thomas et al., 2015).

The first part of the presentation emphasised the physical meaning of chemical activity, by clarifying that “mass concentration” and “chemical activity” are two complementary dimensions for chemicals in the environment, exactly like we know from other areas (heat content versus temperature; water content versus water activity). The second part focused on how chemical activity can be measured and controlled in environmental research and testing, which has the very important implication that chemical activity goes beyond modelling and re-calculations of mass based data. The third part summarised recent experimental studies that link toxicity to chemical activity.

This presentation initiated the discussions at the workshop of why and how chemical activity can help to:

1. predict baseline toxicity,
2. link exposure between media,
3. compare sensitivities between species,
4. assess excess toxicity of chemicals and
5. add up the exposure for mixtures?

   Figure 3.1.1. Three series of toxicity data with different organisms, compounds, and means of administration plotted as the median effective concentration (EC50) and the median effective activity (Ea50). The EC50 values span several orders of magnitude, whereas the Ea50 values are relatively constant for a wide range of agents, targets, and routes of exposure (Figure taken from Reichenberg and Mayer, 2006).