Short and long-term health effects from exposure to organophosphorus (OP) military and insecticidal nerve agents are evaluated based on the abundant scientific literature published over five decades on health effects in humans (from human experimentation and occupational exposures) and in laboratory animals. Four distinct health effects are identified: acute cholinergic toxicity; organophosphate-induced delayed neuropathy (OPIDN); subtle long-term neuropsychological and neurophysiological effects; and a reversible muscular weakness called 'intermediate syndrome'. Some effects are subtle and difficult to differentiate from health effects caused by other diseases or occupational exposures. Each effect has data suggesting threshold exposure levels below which it is unlikely to be clinically detectable. Therefore, meaningful interpretation of human and animal studies requires rigid exposure characterization. Because precise exposure levels are often difficult to reconstruct, a system for characterizing exposure is proposed based upon observed initial acute signs and symptoms, as high-level (definitive cholinergic poisoning); intermediate-level (threshold cholinergic effects including miosis, rhinorrhea or clinically measurable depression of cholinesterase); and low-level (no immediate clinical signs or symptoms) exposure. Threshold exposure levels for known long-term effects from OP nerve agent are at or above intermediate-level exposure. Long-term health effects seen at intermediate-level exposures or in many survivors of high-level exposure are subtle, detectable in exposed populations but not individuals, and not reported in individuals experiencing low-level exposure alone. Co-exposure to other pharmaceutical agents may promote or protect against health effects from OP nerve agents, but qualitatively they are the same effects seen with OP nerve agents alone. Thus, the system for characterizing exposure based on initial acute effects is also useful for evaluating health outcomes from co-exposure to OP nerve and other agents.