• Introduction
• Objectives
• Readings & Articles
• Lecture
• Global View
• Assessment

You may also hear the world toxicity thrown around as you go through the readings.  In terms of environmental health, the word “toxic” doesn’t really mean anything.  Everything is toxic.  It depends on the dosage.  For instance, water can be toxic.  If you were to drink a few gallons of water (and not eat anything) over the course of an hour or two, you would be at risk for hyponatremia, a condition whereby your blood sodium concentration drops to dangerous levels.  Why?  Because the water would dilute your blood of essential electrolytes needed to maintain normal cell and bodily function.  If the condition were not corrected, you would be at risk for brain swelling, coma, and possibly death.  In this case, the dosage of water consumed would be toxic.  While it might take a few gallons for a normal grown person to be affected, it might only take 16 ounces for a baby to exhibit signs of water toxicity.  Therefore, calling something “toxic” doesn’t mean anything if we don’t understand how much of it is needed to cause an adverse effect and to whom this adverse affect was caused:  three gallons of water over two hours might be toxic to an adult, but would be fine for an elephant. (The link in this paragraph is not required reading.)

Then what do we mean by toxic?  When it comes to pollutants, toxicity means that the environmental concentration of the pollutant need not be very high in order for it to cause negative health effects on plants, animals, and/or humans.  If a pollutant causes adverse (negative) health effects at levels in the parts per million range, then it is considered toxic.  This is why we don’t label water as “toxic.”  While water can be toxic, it is not toxic at very low concentrations – which is a good thing since I rather enjoy drinking and bathing.  However, something like methylmercury (which you’ll read about in your textbook), can be incredibly toxic. 

The EPA says that any exposure greater than 0.1 microgram per kilogram of body weight per day is too much to be considered safe for humans.  That’s one part per ten billion!  This guideline is based on the most sensitive population to show adverse effects, which is the developing human fetus.  (If you’re a local fisherman/woman, you may want to take a look at the 2010 Indiana Fish Advisory report; many of our Indiana waters have fish consumption advisories.)  In this case, mercury is toxic because we know that at very low body concentrations of less than 1 part per billion, it can have negative health effects on pregnant women and their developing babies. (The links in this paragraph are not required reading.)

For reference, according to the Agency for toxic Substances and Disease Registry (U.S. Centers for Disease Control and Prevention), the 2007 Priority List of Hazardous Substances lists the following substances in order: (1) Arsenic; (2) Lead; and (3) Mercury. (See full list here.)

Rank	Substance	Maximum safe exposure	Based on
1	Arsenic	10 parts per billion	Drinking water exposure
2	Lead	0 – no safe exposure*	Drinking water exposure.
3	Mercury	2 parts per billion	Drinking water exposure.

(The links in the previous paragraph and this table are not required reading.)
*Your book lists 0.015 parts per million (or 15 parts per billion).  This is actually the “action level” not the “maximum level which is considered safe.” 

Finally, we need to also define bioconcentration, bioaccumulation, and biomagnification.  These processes will help you to understand how toxic pollutants can cause negative or adverse health effects in plants, animals, and humans.

Bioconcentration occurs when pollutants simply stick to the outside of an organism. In other words, the organism need not actually eat the pollutant. For instance, if you were too look at Figure 14.12 (5th edition or Figure 13.8, 4th edition) in your book, there is an arrow called “sedimentation.” Imagine if you were a fish or algae and “dust” from the atmosphere containing mercury were to fall on you. Unless you got dusted off before something bigger came along and ate you, you’d have “concentrated” a little bit of mercury on your body.

Bioaccumulation occurs when an organism actually consumes the pollutant either directly from the environment (e.g., drinking contaminated water) or from eating another organism.  The organism now has the pollutant in its body.  Both bioconcentration and bioaccumulation occur in a single organism.

Biomagnification occurs up a food chain. While a single fish can continue to eat all sorts of smaller fish and drink all sorts of contaminated water, that single fish is bioaccumulating the pollutant. If we look at the entire food chain, we can see that each step up the food chain results in larger concentrations of the pollutant in the organisms of the next food chain tier. Going back to Figure 14.12 (5th edition or Figure 13.8, 4th edition), it shows how the concentration of mercury increases as you move from it being bioconcentrated on a primary producer, an algae (Chlorophyte), then bioaccumulated on a primary consumer, zoopolankton (copepod), then bioaccumulated up through the food chain to a secondary consumer, a small fish, and further bioaccumulated in a tertiary consumer, a larger fish. This process of increasing pollutant concentration up a food chain is called biomagnification; the concentration of the pollutant is magnified (or increases) up the food chain. We can imagine that a copepod isn’t going to eat just one alga and a fish isn’t just going to eat one fish; therefore, larger organisms at the top of the food chain have the greatest body loads of pollutants. In many food chains, humans are at the top, meaning we often have the highest body loads or concentrations of pollutants.