Chemicals

Unwanted chemicals may be ingested in contaminated food or water, inhaled into the lungs, or absorbed through the skin. Exposure may occur in the workplace or at home. Food and water may have been contaminated by industrial wastes; for example, freshwater fish may absorb mercury in lakes and rivers. It now appears that farmed salmon may contain toxic chemicals contained in feed. Some water processing plants test for over 300 chemicals, including PCBs, DDT, and dioxin, as well as lead and mercury. It is not unusual for chemical wastes to remain in the original dangerous form; alternatively, they may undergo transformation into more toxic materials or break down into harmless substances. For example, although pesticides may remain in the environment for a long time, some, such as DDT, do not break down into harmless chemicals, and therefore high levels gradually accumulate in the environment. Many ecosystems are disturbed by the use of pesticides, including those of microorganisms, some of which may become pathogenic, or disease causing. The increased availability of organically grown fruits and vegetables has provided an opportunity for consumers to reduce their exposure to chemicals ingested in food.

Tissue damage may result from a large dose in a single incident, or, more often, damage results from repeated exposure to small amounts of the unwanted material. The chemical may cause damage at the site of entry, or it may enter the blood and circulate to other sites in the body. Frequently this process occurs without the knowledge of the individual. Normally the liver is responsible for detoxification, or inactivation and removal of foreign chemicals from the body. In many cases, however, these chemicals bypass the liver and are stored in certain tissues, gradually accumulating to dangerous levels over years of exposure. Usually there are no obvious signs of this accumulation. For example, hexachlorophene was widely used in hospitals as well as in homes as an antiseptic in soaps and powders until it was discovered that it was absorbed through the skin, particularly broken skin. Heavy use eventually caused brain damage. Now the use of hexachlorophene is restricted. However, new types of antibacterial soaps are being used extensively, and it may be some years before the effects of these new chemicals are documented.

Currently there is increasing concern about children’s exposure to plastics in the environment. Chemicals such as phthalates are used to soften plastics and prevent shattering. Toys, bottle nipples, and soothers made of phthalates have been withdrawn from the market in the United States and Canada, but products produced overseas may still contain the banned product. Bisphenol A (BPA) is a hardening agent used in plastic baby bottles and water bottles. This chemical has been recently classified as a toxin by the Canadian government. The concern about such plastics relates to their ability to mimic hormones such as estrogen and act physiologically within the tissues. It is thought that such endocrine disrupters can lead to infertility and promote the growth of endocrine-sensitive cancers.

There is also increasing concern about chemicals and regulation of the menstrual cycle and reproduction in women. Recent surveys link exposure to solvents to increased risks in women for altered menstrual cycles, spontaneous abortions (miscarriages), and stillbirths.

Chemicals may affect the body in different ways. Chemical substances often injure cells directly by damaging the cell membrane and causing swelling and eventual rupture of the cell. This results in inflammation and necrosis in the tissue. Some chemicals alter the metabolic pathways in the cell, leading to degenerative changes. Many chemicals are carcinogenic; that is, they cause mutations of the cell and lead to the onset of cancers such as leukemia. A few examples of dangerous chemicals are described in the following section.