recovery occurs. Examples are acute toxicity from exposure to a toxicant and acute gastroenteritis. Chronic effects or diseases usually result from long-term or repeated exposures, may have longer latency periods, and have longer duration. Examples include cancer, neurotoxicity, and infections associated with chronic diseases such as hepatitis A and liver disease, and coxsackie viruses and diabetes. Developmental effects and reproductive toxicity, while conditions of a long-term nature, may result from short-term exposure to harmful agents.
Two principles guide the evaluation of human stressors: 1) the dose makes the poison and 2) there is specificity between agent and effects. Other issues that must be considered are latency (time between exposure and effect); possibility of secondary spread (i.e., from person to person); and the possibility of additivity, synergism, or antagonism between multiple exposures. All of these factors are used in developing risk assessment models to extrapolate from high to low doses in humans and to extrapolate between animal species.
In coastal urban wastewater and urban runoff, the two major classes of contaminants that are of potential concern to human health are hazardous chemicals and infectious agents. These include metals and organic chemicals that may pose varying risks depending on the method of disposal and ultimate environmental fate (i.e., disposal in the ocean, land disposal, or incineration). The toxicity of metals may vary by route of exposure, and by physical and chemical form such as valence state, whether in organic or inorganic state, whether sorbed or dissolved, and whether hydrated or complexed. For example, inorganic, but not organic, arsenic is a carcinogen (Gibb and Chen 1989). Cadmium is considered to be carcinogenic by the inhalation but not the oral route (Life Systems, Inc. 1989; IRIS 1993). Hazardous organic chemicals have entered coastal waters from a number of sources, most of which are due to industrial and agricultural activities. Many of these, like DDT and PCBs, have since been banned but continue to be present in sediments and the tissues of aquatic organisms and water fowl.
Infectious agents of concern include bacteria (e.g., campylobacter, salmonellae, v. cholerae), viruses (e.g., poliovirus, coxsackie, echovirus, adenovirus, and hepatitis A), and parasites (e.g., cryptosporidium, giardia, and entamoeba). Further information on infectious agents is contained in Appendix B. Exposure takes place while swimming in contaminated waters or eating contaminated shellfish. Several diseases can result. These range from subclinical infection to acute, self-limited respiratory, gastrointestinal, skin, or ear infections to extreme gastrointestinal and liver disease (e.g., cholera and viral hepatitis) and other potentially terminal diseases.
Most of these pathogens are derived from human feces; their presence in the environment is often associated with a source of domestic wastewater (e.g., septic tanks, combined sewer overflows, or sewage treatment plant