Poisons and Toxins

Of course, in the modern world, humans are responsible for many of the toxic chemicals that are now being dispersed into the environment. In some cases, humans are causing toxic damages to organisms and ecosystems by emitting large quantities of chemicals that also occur naturally, such as sulfur dioxide, hydrocarbons, and metals. Pollution or poisoning by these chemicals represents an intensification of damages that may already be present naturally, although not to nearly the same degree or extent that results from additional human emissions.

Humans are also, however, synthesizing large quantities of novel chemicals that do not occur naturally, and these are also being dispersed widely into the environment. These synthetic chemicals include thousands of different pesticidal chemicals, medicines, and diverse types of industrial chemicals, all of them occurring in complex mixtures of various forms. Many of these chemicals are directly toxic to humans and to other organisms that are exposed to them, as is the case with many pesticides. Others result in toxicity indirectly, as may occur when chlorofluorocarbons (CFCs), which are normally quite inert chemicals, find their way to the upper atmospheric layer called the stratosphere. There the CFCs degrade into simpler chemicals that consume ozone, resulting in less shielding of Earth's surface from the harmful effects of solar ultraviolet radiation, with subsequent toxic effects such as skin cancers, cataracts, and immune disorders.

As an example of toxicity caused to humans, consider the case of the accidental release in 1984 at Bhopal, India, of about 40 tonnes of poisonous methyl isocyanate vapor, an intermediate chemical in the manufacturing of an agricultural insecticide. This emission caused the death of almost 3,000 people and more than 20,000 others were seriously injured.

As an example of toxicity caused to other animals, consider the effects of the use of carbofuran, an insecticide used in agriculture in North America. Carbofuran exerts its toxic effect by poisoning a specific enzyme, known as acetylcholine esterase, which is essential for maintaining the functioning of the nervous system. This enzyme is critical to the healthy functioning of insects, but it also occurs in vertebrates such as birds and mammals. As a result, the normal use of carbofuran in agriculture results in toxic exposures to numerous birds, mammals, and other animals that are not the intended targets of the insecticide application. Many of these non-target animals are killed by their exposure to carbofuran, a chemical that is well-known as causing substantial ecological damages during the course of its normal, legal usage in agriculture.