Toxic substances are materials that are poisonous to living organisms. There are hundreds of thousands of artificial and natural toxic substances, also known as toxins , that are found in solid, liquid, or gaseous form. Toxic substances damage living tissues or organs by interfering with specific functions of cells, membranes, or organs. Some destroy cell membranes, others prevent important cell processes from occurring. Many cause cells to mutate, or make mistakes when they replicate themselves. Some of the most common and dangerous anthropogenic (human-made) toxic substances in our environment are chlorinated hydrocarbons , including DDT (dichlorodiphenyl-trichloroethane ) and polychlorinated biphenyl (PCB). Many of these are produced by pesticide manufacturers and other chemical industries specifically because of their ability to kill pests. Petroleum products, produced and used in oil refining, plastics manufacturing, industrial solvents, and household cleaning agents, are also widespread and highly toxic agents. Heavy metals , including cadmium , chromium, lead , mercury , and nickel , and radioactive substances such as uranium and plutonium are also dangerous toxic agents. Once a toxic substance is released into the environment, plants may absorb it along with water and nutrients through their roots or through pores or tissues in their leaves and stems. Animals, including humans, take up environmental toxic substances by eating, drinking, breathing, absorbing them through the skin, or by direct transmission from mother to egg or fetus.
The toxicity (the potential danger) of all toxic materials depends upon dosage. Some toxins are deadly in very small doses; others can be tolerated at relatively high levels before an observable reaction occurs. All chemical substances can become toxic in high enough concentrations, but even very toxic chemicals may cause no reaction in very small amounts. If you eat an extremely large dose of table salt (sodium chloride), you will suffer severe reactions, possibly even death. On the other hand, in moderate doses table salt is not toxic. On the contrary, it is essential for your body to continue functioning normally. Likewise, more unusual trace elements such as selenium are important to us in extremely minute amounts, but high concentrations have been observed to cause severe birth defects and high mortality among birds.
One of the most important characteristics that determines a substance's toxicity is the way the substance moves through the environment and through our bodies. Most chemicals and minerals move most effectively when they are dissolved by water or by an oil-based liquid. Generally compounds of mineral substances, including sodium chloride, selenium, zinc, copper , lead, and cadmium, dissolve best in water. Organic chemicals (those containing carbon ), including chlorinated hydrocarbons , benzene , toluene , chloroform, and others, dissolve most readily in oily solvents, including gasoline , acetone , and the fatty tissues in our bodies. In the environment, inorganic substances are often mobilized when ground is disturbed and watered, as in the case of irrigated agriculture or mining, or when waste dumps become wet and their soluble contents move with runoff into groundwater or surface water systems. Animals and plants readily take up these substances once they are mobilized and widely distributed in natural water systems. Organic chemicals mainly move through the environment when human activity releases them through pesticide spraying, by allowing aging storage barrels to leak, or by accidental spills. Natural surface and groundwater systems further distribute these compounds. Animals that ingest these compounds also store and distribute toxic organic substances in their bodies.
Once we ingest or breathe a toxic substance, its mobility in our body depends upon its solubility and upon its molecular shape. As they enter our bodies through the tissues lining our lungs or intestines or more rarely through our skin, fat soluble compounds can be picked up and stored by fatty tissues, or lipids, in our cells. Proteins and enzymes in our blood, organs, tissues, and bones recognize and bond with molecules whose shape fits those proteins and enzymes. In most cases our bodies have mechanisms to break down, or metabolize, foreign substances into smaller components. When possible, our bodies metabolize foreign substances and turn them into simpler, water soluble compounds. These are easier for our bodies to eliminate by excretion in feces, urine, sweat, or saliva.
Not all substances are easy to metabolize, however. Some persistent compounds such as DDT simply accumulate in tissues, a process known as bioaccumulation . If gradual accumulation goes on long enough, toxic dosages are reached and the animal will suffer a severe reaction or death. Furthermore, the by-products (metabolites) of some substances are more dangerous than the original toxin and more difficult to excrete. Toxic metabolites accumulate in our tissues along with persistent toxins. Although accumulation can occur in bones, fat reserves, blood, and many organs, the most common locations for toxins to accumulate is in the liver and kidneys. These organs have the primary responsibility of removing foreign substances from the blood stream, so any substance that the body cannot excrete tends to collect here.
The parts of our bodies that are most easily damaged by exposure to toxic substances, however, are areas where cells and tissues are reproducing and growing. Brain cells, bones, and organs in children are especially susceptible, and improper cell replication is magnified as growth proceeds. Serious developmental defects can result—a widespread example is the accumulation of lead in children's brains, causing permanent retardation. In adults, any tissue that reproduces, repairs, or replaces itself regularly is likely to exhibit the effects of toxicity. Linings of the lungs and intestines, bone marrow, and other tissues that regularly reproduce cells can all develop defective growths, including cancer , when exposed to toxic substances. An individual's susceptibility to a toxic substance depends upon exposure—usually workers who handle chemicals are the first to exhibit reactions—and upon the person's genetic resistance , age, size, gender, general health, and previous history of exposures.
Toxins that cause an immediate response, usually a health crisis occurring within a few days, are said to have acute effects . Subacute toxic effects appear more gradually, over the course of weeks or months. Chronic effects may begin more subtly, and they may last a lifetime. General classes of toxic substances with chronic effects include the following:
- Neurotoxins, which disable portions of the nervous system, including the brain. Because nerves regulate body functions and because nerve cells are not replaced after an individual reaches maturity, damage is especially critical.
- Mutagens, which cause genetic alterations so that cells are improperly reproduced. These can lead to birth defects or tumors. Compounds that specifically affect embryos are called teratogens.
- Carcinogens cause cancer by altering cell reproduction and causing excessive growth, which becomes a tumor.
- Tumerogens are substances that cause tumors, but if tumors are benign, they are not considered cancerous.
- Irritants, which damage cells on contact and also make them susceptible to infection or other toxic effects.
Natural toxins occur everywhere, and many are extremely toxic. Some, such as peroxides and nitric oxide, even occur naturally within our bodies. Many people avoid potatoes, peppers, tomatoes, and other members of the nightshade family because they are sensitive to the alkaloid solanine that they contain. Mushrooms and molds contain innumerable toxic substances that can be lethal to adults in minute quantities. Ricin, a protein produced by castor beans, can kill a mouse with a dose of just three ten-billionths (3/10,000,000,000) of a gram. Ricin is one of the most toxic organic substance known. Usually we encounter these natural substances in small enough doses that we do not suffer from them, and in many cases our bodies are equipped to metabolize and eliminate them. Except for toxic minerals and heavy metals released by agriculture and mining, most people rarely worry too much about naturally occurring toxic substances, even though they include some of the most toxic agents known.
The most problematic environmental toxic substances are anthropogenic materials that are produced in large quantities for industrial processes and home use. These are dangerous first because most of them are organic and thus bond readily with our tissues and second because their production and distribution occurs rapidly and is poorly controlled. Every year hundreds of new substances are developed, but the testing process is time-consuming and expensive. The National Institute of Safety and Health has listed 99,585 different toxic and hazardous substances, but there are over 700,000 different chemicals in commercial use, and of these only 20% have been thoroughly tested for toxic effects. One-third have not been tested at all. Once these substances are manufactured, their sale, transportation , and especially disposal are often inadequately monitored. Of the more than over 292 million tons (265 million metric) of toxic and hazardous materials produced every year the United States, about 220 million tons (200 million metric) are used or disposed of properly by recycling , chemical conversion to non-toxic substances, incineration , or permanent storage. About 66 million tons (60 million metric) are inappropriately disposed of, mostly in landfills, with non-toxic solid waste . Of course, significant amounts of substances that are properly used also freely enter the environment—including pesticides applied to fields, benzene, gasoline, and other volatile organic compounds that evaporate and enter the atmosphere , and paints and solvents that evaporate or gradually break down after use.
National efforts to control toxic substances in the United States began with the National Environmental Policy Act , passed by Congress in 1969. This act required the establishment of standardized rules governing the identification, testing, and regulation of toxic and hazardous substances. The law setting out those rules finally appeared in the 1976 Resource Conservation and Recovery Act (RCRA). This law sets up standard policy for regulating toxic and hazardous substances from the time of production to disposal. Efforts to deal with new and historic environmental problems associated with toxic substances began with the 1980 Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). This act provided for setting up a multi-billion dollar Superfund to pay clean up costs at the hundreds of abandoned toxic waste sites around the country. Unfortunately, Superfund money is proving woefully inadequate in meeting real cleanup expenses.
International trade in toxic waste remains a dire problem that has not yet been adequately addressed. In many developing countries pesticides, organic solvents, heavy metals associated with mining, and other noxious materials are regularly released to the environment. Systems for monitoring their release are often completely non-existent. Workers handling these substances frequently suffer terrible diseases and pass on genetic disabilities to their children. Because many of the most dangerous toxins are produced in the United States and Europe for sale to developing regions, wealthier nations stand in a position to force some efforts at regulation. Thus far, however, neither regulation nor worker education has been an international priority.
[Mary Ann Cunningham Ph.D. ]
Cunningham, W. P. Environmental Science: A Global Concern. Dubuque, IA: William C. Brown, 1992.
Kamrin, M. A. Toxicology. Chelsea, MI: Lewis Publishers, 1988.