Toxicology
Toxicology
Toxicology is the scientific study of poisons (toxins). Major topics in toxicology include the detection and chemical analysis of poisons, the study of the metabolic effects of these substances on organisms, and the investigation of methods for treatment of poisoning.
Physiology is the study how organisms function, and the disruption of biochemical pathways by poisons is a key aspect of toxicology. Poisons affect normal physiology in many ways, but some of the more common mechanisms involve the disabling of enzyme systems, induction of cancers, interference with the regulation of bloodchemistry, and disruption of genetic processes.
Organisms vary greatly in their tolerance of exposure to chemicals, and even within populations of the same species there can be great variations in sensitivity. In rare cases, some individuals may be extremely sensitive to particular chemicals or groups of similar chemicals, a phenomenon known as hypersensitivity.
Organisms are often exposed to a wide variety of potentially toxic chemicals through medicine, food, water, and the atmosphere. Humans are exposed to complex mixtures of chemicals, many of which are synthetic and have been either deliberately or accidentally released into the environment.
In some cases, people actively expose themselves to chemicals that are known to be toxic, such as when smoking cigarettes, drinking alcohol, or taking recreational drugs. Voluntary exposure to chemicals also occurs when people take medicines to deal with illness, or when they choose to work in an occupation that involves routinely dealing with dangerous chemicals. However, most exposures to potentially toxic chemicals are inadvertent, and involve living in an environment that is contaminated with small concentrations of pollutants, for example, those associated with pesticide residues in food, lead from gasoline combustion, or sulfur dioxide and ozone in the urban atmosphere. Exposure to toxin-harboring microorganisms can also be detrimental to people. As one example, drinking water contaminated with a bacterium called Escherichia coli O157:H7 can be a serious, even life-threatening, concern, as the bacterium produces a potent toxin that can damage the intestinal tract and the liver.
Traditionally, the discipline of toxicology has only dealt with the direct effects of poisonous chemicals on organisms, and particularly on humans. Recently, however, ecologists have broadened the scope of toxicological investigations to include the indirect effects of chemicals in the environment, a field known as ecotoxicology. Ecotoxicology could be defined as the study of the ecological effects of toxic chemicals, including the direct effects, but also the indirect consequences caused by changes in the structure of habitats, or in the abundance of food. A herbicide used in forestry may not cause direct, toxic effects to animals at the doses given during the pesticide application, but the pesticide may change the vegetation, and thereby change the ecological conditions upon which animals depend.
Toxicology in practice
The toxicologist employs the tools and methods of science to better understand the consequences of exposure to toxic chemicals. Toxicologists typically assess the relationship between toxic chemicals and environmental health by evaluating a number of factors.
To assess the risk associated with exposure to a toxic substance, the toxicologist first measures the exposure characteristics and then computes the doses that enter the human body. He or she then compares these numbers to derive an estimate of risk, sometimes based on animal studies. In cases where human data exist for a toxic substance, such as benzene, more straightforward correlations with the humans risk of illness or death are possible.
Given recommendations from toxicologists, government agencies sometimes decide to regulate a chemical based on limited evidence from animal and human epidemiological studies that the chemical is toxic. Such decisions may have both ethical and political ramifications. For example, to fail to issue warnings about a “suspect” chemical could leave vulnerable members of the population at risk of contracting an avoidable illness; on the other hand, any restrictions placed on the use of the chemical could place burdensome cleanup costs on private industry.
Toxicologists devise new techniques and develop new applications of existing methods to monitor changes in the health of individuals exposed to toxic substances.
Another way to understand the environmental factors contributing to human illness is to study large populations that have been exposed to substances suspected of being toxic. Scientists then attempt to tie these observations to clinical data. Ecologic studies seek to correlate exposure patterns with a specific outcome. Case-control studies compare groups of persons with a particular illness with similar healthy groups, and such studies seek to identify the degree of exposure required to bring about the illness. Other studies may refine the scope of environmental factor studies, or examine a small group of individuals in which there is a high incidence of a rare disease and a history of exposure to a particular chemical.
When a chemical is nonbiodegradable, it may accumulate in biosystems. The result could be that very high concentrations may accumulate in animals at the top of food chains. Chlorinated pesticides such as dieldrin and DDT, for example, have been found in fish in much greater concentrations than in the water in which they swim.
Arsenic is a common toxin, which is encountered mainly in the workplace, near hazardous waste sites, or in areas with high natural levels of the substance. At high levels of exposure, arsenic can cause death or illness. Lead is another toxin. Toxic exposure usually results from breathing workplace air or dust, or from eating contaminated foods. In the past, when lead-based paints were still used, a common source of lead poisoning was the ingestion of lead-based paint chips by infants. Another chemical toxin is mercury. Toxic exposure results from breathing contaminated air, ingesting contaminated water and food, and possibly having dental and medical treatments. At high levels, mercury damages the brain, kidneys, and developing fetuses. Other chemical toxins exist.
Biological toxins also exist. Bacteria, fungi, and algal species can produce toxins that cause illness and even death.
Bill Freedman
Randall Frost