produces slow-healing, painful burns. Strong bases, such as the metal hydroxides and ammonia, make up another class of corrosive chemicals. Strong dehydrating agents, such as phosphorus pentoxide and calcium oxide, have a powerful affinity for water and can cause serious burns upon contact with the skin. Finally, strong oxidizing agents, such as concentrated solutions of hydrogen peroxide, can also have serious corrosive effects and should never come into contact with the skin or eyes.
A chemical allergy is an adverse reaction by the immune system to a chemical. Such allergic reactions result from previous sensitization to that chemical or a structurally similar chemical. Once sensitization occurs, allergic reactions can result from exposure to extremely low doses of the chemical. Allergic reactions can be immediate, occurring within a few minutes after exposure. Anaphylactic shock is a severe immediate allergic reaction that can result in death if not treated quickly. If this is likely to be a hazard for a planned experiment, advice on emergency response should be obtained. Allergic reactions can also be delayed, taking hours or even days to develop. The skin is usually the site of such delayed reactions, in which cases it becomes red, swollen, and itchy.
It is important to recognize that delayed chemical allergy can occur even some time after the chemical has been removed. Contact with poison ivy is a familiar example of an exposure that causes a delayed allergic reaction. Also, just as people vary widely in their susceptibility to sensitization by environmental allergens such as dust and pollen, individuals may also exhibit wide differences in their sensitivity to laboratory chemicals. Examples of substances that may cause allergic reactions in some individuals include diazomethane, dicyclohexylcarbodiimide, formaldehyde, various isocyanates, benzylic and allylic halides, and certain phenol derivatives.
Asphyxiants are substances that interfere with the transport of an adequate supply of oxygen to the vital organs of the body. The brain is the organ most easily affected by oxygen starvation, and exposure to asphyxiants can lead to rapid collapse and death. Simple asphyxiants are substances that displace oxygen from the air being breathed to such an extent that adverse effects result. Acetylene, carbon dioxide, argon, helium, ethane, nitrogen, and methane are common asphyxiants. It is thus important to recognize that even chemically inert and biologically benign substances can be extremely dangerous under certain circumstances. Certain other chemicals have the ability to combine with hemoglobin, thus reducing the capacity of the blood to transport oxygen. Carbon monoxide, hydrogen cyanide, and certain organic and inorganic cyanides are examples of such substances.
A carcinogen is a substance capable of causing cancer. Cancer, in the simplest sense, is the uncontrolled growth of cells, and it can occur in any organ. The mechanism by which cancer develops is not well understood, but the current thinking is that some chemicals interact directly with DNA, the genetic material in all cells, to result in permanent alterations. Other chemical carcinogens can modify DNA indirectly by changing the way the cells grow. Carcinogens are chronically toxic substances; that is, they cause damage after repeated or long-duration exposure, and their effects may become evident only after a long latency period. Carcinogens are particularly insidious toxins because they may have no immediate apparent harmful effects.
Reproductive toxins are substances that have adverse effects on various aspects of reproduction, including fertility, gestation, lactation, and general reproductive performance. Developmental toxins are substances that act during pregnancy to cause adverse effects on the embryo or fetus. These effects can include lethality (death of the fertilized egg, the embryo, or the fetus), malformations (this class of substances is also called teratogens), retarded growth, and postnatal functional deficiencies. When a pregnant woman is exposed to a chemical, generally the fetus is exposed as well because the placenta is an extremely poor barrier to chemicals. Reproductive toxins can affect both men and women. Male reproductive toxins can in some cases lead to sterility. Two well-known male reproductive toxins are ethylene dibromide and dibromochloropropane.
Neurotoxic chemicals can induce an adverse effect on the structure or function of the central and/or peripheral nervous system, which can be permanent or reversible. In some cases the detection of neurotoxic effects may require specialized laboratory techniques, but often they can be inferred from behavior such as slurred speech and staggered gait. Many neurotoxins