Detecting Dioxins in Food and Water

As a consequence of their widespread occurrence in the environment and their tendency to dissolve in fatty matter, dioxins bioaccumulate in the food chain. More than 90% of human exposure to dioxins is attributable to the food we eat, particularly meat, fish, and dairy products. Typically, dioxins and furans (a group of compounds resembling the dioxins in structure, which we'll discuss later) are present in fish and meat at levels of tens or hundreds of picograms (pg, or 10^12 gram) per gram of the food; in other words, they occur at levels of tens or hundreds of parts per trillion. However, the bulk of dioxins and furans in nature are not present in biological systems: Attachments to soil and to sediments of rivers, lakes, and oceans are their most common sinks.

The ability of chemists to detect TCDD and other organochlorines in environmental samples has improved by orders of magnitude over the past few decades. In the early 1960s, when Carson's Silent Spring was published, the lower limit for analysis of DDT and other such compounds was the parts-per-million level. Ten years later, detecting such substances at the parts-perbillion level was possible but not common or easy. By 1990, parts-per-trillion detection was possible in soil or biota samples, and parts-per-quadrillion was possible for water samples. Today, a few labs detect some substances at limits up to 1000 times lower than these! At these latter levels, many organochlorines are found in every environmental sample, no matter how "clean" an environment it came from. By the late 1990s, researchers at the Centers for Disease Control in Atlanta were able to detect as little as 10~16 grams of TCDD in human serum samples.

The potential impact on human health of exposure to dioxins is documented later, following a discussion of the properties of PCBs and furans, two types of chemicals with which dioxins share many properties.

PROBLEM 11-5

Given its formula and Avogadro's constant (6.02 X 1023 molecules/mol), deduce how many molecules are present in 10-16 grams of TCDD.

PCBs

The well-known acronym PCBs stands for polychlorinated biphenyls, a group of industrial organochlorine chemicals that became a major environmental concern in the 1980s and 1990s. Although not pesticides, they found a wide variety of applications in modem society because of certain other properties they possess. Since the late 1950s, over 1 million metric tons of PCBs have been produced, about half in the United States and the rest mainly in France, Japan, and the former Eastern bloc nations. Like many other organochlorines, they ate very persistent in the environment and they bioaccumulate in living systems. As a result of careless disposal practices, they have become a major environmental pollutant in many areas of the world. More than 95% of the entire U.S. population has detectable concentrations of PCBs in their bodies. Due both to their own toxicity and to that of their "furan" contaminants, PCBs in the environment have become a cause for concern because of their potential impact on human health, particularly with regard to growth and development.

In the following sections, we consider what PCBs are, how they are made, what they are used for, and how they become contaminated and released into the environment.

Continue reading here: The Structure of PCB Molecules

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