ABSTRACT
Risk assessments often deal with single agents such as a new chemical, an exotic species, or a
harvesting method. However, organisms and ecosystems are always exposed to mixtures of
chemicals and to diverse and multiple hazardous agents. Often, things that are treated as
single agents for regulatory purposes such as crude oil, polychlorinated biphenyls (PCBs), and
pesticide formulations are actually mixtures. This chapter begins with a discussion of methods
for addressing chemical mixtures and then presents an expansion of those methods to address
combined risks from multiple activities and agents at a site.
