by Daniel Weber, Ph.D.
Life doesn’t exist without chemistry. However, chemistry can harm life as well. This “split personality” can be illustrated by the metabolism of food. In all living things, chemical reactions rearrange food molecules into molecules of hormones, proteins, bones, muscles and nerves. Unused molecules are then eliminated in a form that may be dangerous to other living things. In other words, all living things produce toxic waste. Fortunately, ecological systems utilize most of that toxic waste (i.e., metabolized by other organisms.) Bacteria, worms, maggots, sowbugs, and ants are examples of organisms that keep the toxic deposits of others from accumulating—in fact, they often transform these wastes into useful compounds. Over the eons of time, living systems have been endowed with the ability to detoxify many, but not all, harmful chemicals.
The balance described above depends upon three components: 1) waste products can be metabolized by the cellular machinery of other living organisms, 2) the amount of harmful by-products is only a small fraction of all wastes produced, and 3) the rate of utilization is similar to the rate of production of toxic chemicals. If these components (type, chemical quantities, and rate) are unbalanced, toxic chemicals will accumulate in the environment. The understanding that humans cause the imbalance in ALL of these components helps us understand the damage toxic chemicals cause in the environment, wildlife and ourselves.
At first glance, chemicals with names of polychlorinated biphenyls (PCBs), tetrachlorodibenzo-p-dioxin (TCDD or just dioxin), or dichlorvos (an organophosphate insecticide) can be overwhelming just to pronounce, let alone understand their danger. Even familiar chemicals like DDT, lead, mercury, and chromium may act on living systems in ways that can be harmful. Yet, all these chemicals interact with at least one of four basic biological systems or process: nervous system (neurotoxins), hormones (endocrine disruptors), metabolism (metabolic poisons), or the genetic code (genotoxins). Understanding how toxic chemicals interact with these four fundamental categories (toxic action) enables us to conduct risk assessments for wildlife and humans.
Nervous System (Neurotoxins)
Many inorganic (such as metal salts) and organic chemicals (such as pesticides) can affect the nervous system. Pesticides, such as organochlorines, organophosphates, and carbamates, are purposely released into the environment at concentrations that are lethal to the “pests” being targeted. Since physiology among animals is similar, neurotoxic chemicals affect a wide range of animals, including humans. Some of the most potent pesticides manufactured today are found naturally as plant defense compounds, such as nicotine (a pure drop is lethal to humans). Manufactured metals such as lead, mercury, zinc, and cadmium are potent neurotoxic agents and released into the environment. Recent discoveries by scientists demonstrate that drugs such as Prozac are finding their way into lakes and streams where they are affecting normal frog metamorphosis.
Neurotoxic agents affect the transmission of information within and between nerves or cause behavioral birth defects. For example, Permethrins (synthesized from chrysanthemums) are an effective pesticide because they block the transmission of electrical impulses in the nerve’s axon, the arm of the neuron that extends from the main cell body toward another nerve or target organ such as muscle. An excellent example of how chemicals affect the translation of nerve information the effect of lead, a commonly used metal, on human learning. Learning is a complex process, one part is the way in which certain chemical keys unlock cellular pathways that allow a nerve to “create memories.” Lead blocks the locks and makes learning more difficult.
Hormones (Endocrine Disruptors)
Another way our bodies create lines of communication is through hormones. Hormones affect many biological functions, e.g., metabolism, growth, reproduction, and mood. Chemicals that affect hormone activity are called endocrine disruptors. For example, animals such as insects, spiders, and crabs possess a unique hormone related to their growth and molting of their exoskeletons. Specific pesticides have been developed to prevent molting, thus preventing growth. The animal soon dies because it is no longer protected from desiccation or predators by its exoskeleton. Since all invertebrates have this hormone, all are affected, even those not considered “pests.”
As mentioned above, human activity releases lead, mercury, PCBs, DDT, and certain pharmaceuticals into the environment. All of these agents can alter endocrine systems. PCBs, while no longer produced for electrical transformers and backs of carbonless paper, still exist in large quantities throughout the environment. PCBs block the synthesis of thyroid hormones that are important to metabolism and the development of nerves during embryo development. DDT, an organochlorine developed as a neurotoxic pesticide, blocks the hormones involved in calcium metabolism, thus causing the “thin-shelled egg” syndrome that nearly wiped out the American Bald Eagle. A number of pharmaceuticals, industrial byproducts (e.g., dioxin), and surfactants in detergents (nonylphenols), mimic, for example, the activity of estrogen and are now believed to be in sufficient quantities in rivers and lakes to alter reproductive success of many fish species.
A third category of toxic chemicals is the metabolic poisons. These affect basic cell biochemistry, which can cause organ malfunctions. For example: Dioxin, petroleum-based chemicals (benzene, toluene, polycyclic aromatic hydrocarbons), lead and mercury, may change the way a liver operates so that it fails to utilize carbohydrates correctly. Chemicals such as chlorine (used in swimming pools or wastewater treatment plants) or cyanide (from industrial waste) can affect the cell’s ability to use oxygen, cause the formation of radicals in the cell, or inhibit the critically important process of transferring ions across cell membranes.
Genetic Code (Genotoxins)
Genotoxicity implies that chemicals may alter the basic functioning of our genetic code—DNA, genes, chromosomes, etc. If these changes in gene function occur in the cells that produce sperm or eggs, these changes may be heritable, that is, the next generation will inherit a faulty genetic code. If these changes occur in nonreproductive organs, only that individual will be affected. Scientists are finding that many of the chemicals, including lead, cadmium, polycyclic aromatic hydrocarbons, coal derivatives such as anthracene, fertilizers such as nitrites, and some fungicides released into the environment by human activity are genotoxic. Additionally, genotoxic chemicals sometimes lead to birth defects or cancer.
While large proportions of toxic substances are released, simple individual attempts to minimize the damage of toxic substances are certainly beneficial and serve as role models for others. The following are active suggestions you can take:
- Refer to the Environmental Defense Fund’s Chemical Scorecard at http://www.scorecard.org. Knowing what chemicals you use and which of those are toxic will be the first step to wise use and disposal.
- Contact your municipal public works department for recommended procedures of disposing of hazardous wastes, e.g., paints, batteries (car and electronic devices), light bulbs, and pesticides. Do not place these items in your regular garbage.
- Never dump contaminants in a drain or throw them into your household garbage. In fact, participate in local efforts to mark all storm sewers with a “DO NOT DUMP WASTES” decal.
- Look at ingredients of the items you buy. If you can’t find alternatives to products that contain hazardous materials, then use less of them.
Send your comments to Dr. Daniel Weber at firstname.lastname@example.org.