Common Contaminants in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 227

• Asbestos
• Acids
• Bases
• Radioactive compounds
• Dioxins
• Emerging contaminants
We begin by briefly discussing the concept of contaminant toxicity. Understanding the
general concepts of toxicity provides the basis for understanding the types, differences,
and potential effects of exposure to contaminants in urban areas. The chapter concludes
with a closer examination of each contaminant category listed above.
7.2 Contaminant Toxicity
Before discussing each contaminant group, a discussion of the meaning, complexity, and
implications of toxicity posed by contaminants is appropriate. Toxicity or potency is the
degree to which a chemical or substance is able to inflict damage to an exposed organ-
ism (USEPA 1989). Note that toxicity does not equal risk. The difference between toxic-
ity and risk is based primarily upon the length of exposure to a chemical or substance,
and whether the dosage received from this exposure is enough to cause harm. As we
now know, toxic substances surround us and are present at most all locations on Earth.
However, there is only risk if we are exposed to a substance or chemical long enough and
at a high enough dose to cause harm. Toxicity does not assess risk—this determination is
reserved for a risk assessment (Chapter 4).
There are three basic types of toxic categories:
1. Chemical or substance, including inorganic and organic substances such as acids
and bases, flammable liquids, metals, etc.
2. Biological, including bacteria and viruses
3. Physical, including sound and vibration, heat and cold, light, radiation, etc.
For the purposes of this topic, we are primarily concerned with chemical or substance
toxicity.
Toxicity is measured by the effects on a whole organism, an individual organ, or the tis-
sue of even a cell. Populations are most often used to measure toxicity since any one indi-
vidual may have a different level of response to a toxin at a certain dose or concentration.
The most common measure of chemical or substance toxicity is termed the LD 50 ; defined as
the concentration or dose that is lethal to 50% of the population being tested (USEPA 1989).
When direct data are not available, the LD 50 is estimated by comparing the substance to
other similar chemicals and organisms.
Another important measure of toxicity is the lowest-observed-adverse-effect level (LOAEL)
or threshold effect value . The LOAEL is the lowest tested dose of a chemical or substance