Chemicals and chemical reactions are an integral part of everyday life. Photosynthesis occurs in plants, acids aid digestion, buffers balance the pH of blood, and, of course, industry uses many chemical reactions to produce modern products. However, more and more potentially toxic chemicals are becoming concentrated in our environment due to the continued and rapid industrialization of the world.
Products of industry, whether byproducts of manufacturing, such as heavy metals and polychlorinated biphenyls (PCBs), or finished products, such as antibiotics, pesticides, herbicides, batteries, fuels, and electronic equipment, are all potential toxins. Ground and surface waters used for drinking and fishing, soil used for agriculture purposes, and farmed animals raised with growth/production stimulators are all potential reservoirs of chemicals that can harm human health.
Public health agencies and clinicians should educate the public regarding ways to minimize heavy metal exposures. Known exposures should be discussed immediately with a local poison control authority.
Elemental mercury is a well-known toxin. Mercury poisoning may cause digestive, respiratory, renal, and neurologic disorders and can be lethal. (The phrase “mad as a hatter” refers to neurologic sequelae of mercury exposures in the felt hat industry of the 19th century.) Some evidence links mercury to cardiovascular disease. Mercury easily crosses the placenta in pregnant women and may cause birth defects, even in the absence of maternal symptoms. Mercury toxicity is usually diagnosed by a blood test. Chelation is available for acute mercury toxicity.
The most common exposure route is through ingestion of fish, especially those high on the food chain, such as shark, tuna, and swordfish, and certain fish taken from some freshwaters.
Very few vaccines contain thimerosal, a compound that includes mercury. Thimerosal-free vaccines have now become standard. However, the amount of mercury in vaccines is generally not considered to pose risks.
Alternatives to amalgam fillings such as resin composites are now the standard of care and do not involve the use of mercury. Dental procedures now use mercury only rarely. Older dental fillings are not believed to present a health hazard as they have very minimal release of mercury over the course of their use. Mercury is used in medical instruments, although less commonly than in the past.
Mercury is a component of several compounds and is a common environmental pollutant as a result of industrial processes. Family physicians may encounter patients who have inhaled elemental mercury from spills at home or in schools, although small exposures are unlikely to lead to toxicity.
Cadmium, a heavy metal common in the environment, can cause kidney, bone, and lung disease and is considered a “probable carcinogen” by the federal Environmental Protection Agency (EPA). Cigarettes are a common source of cadmium exposure. Incineration of household waste, particularly batteries, may release cadmium into the atmosphere, and industrial processes such as mining and land applications of sewage sludge can pollute water and air. Phosphate fertilizers commonly contain cadmium, and grain and vegetable crops easily absorb the metal through polluted irrigation waters.
Fish concentrate cadmium in their livers and kidneys, leading to toxicity in populations that commonly consume whole fish, especially fish taken from urban waters. Shellfish also concentrate cadmium.
Cadmium toxicity can be diagnosed through urine and blood tests, although blood generally shows evidence of acute exposures only. For this reason, prevention of excess exposure is of paramount importance. No well-studied and accepted chelating agent is available for cadmium in humans.
Lead is very common in landfills harboring old electronic devices (solder) and cathode ray tubes, in mine runoff areas, and in manufacturing facilities where lead is used in batteries, radiators, lead glazes, and other products. Groundwater and waterways may become contaminated from these sources, and many old houses still use lead pipes to connect to city water mains. In addition, older houses often have lead-based paint, which may flake and be consumed by small children.
Lead may also be present in toy jewelry, radiographs (if stored in lead-lined boxes), household crystal and glazed pottery used for serving foods and beverages, imported Mexican candy, and traditional medicines. For example, some Ayurvedic and Chinese medicines may contain lead, mercury, and arsenic. , Air emissions from combustion of leaded gasoline, may cause surface contamination of crops. Fortunately, leaded gasoline has been essentially eliminated from modern use.
Lead poisoning can lead to nervous system and kidney damage and can cause several nonspecific symptoms. It may also adversely affect fetal and childhood development and fertility in men. During pregnancy and lactation, women can mobilize lead stored in bone from past exposures.
Lead poisoning is diagnosed through blood lead concentrations, among other laboratory findings, and chelation agents are available for treatment of patients with high levels or acute symptoms.
High-calcium diets may protect against lead accumulation by reducing gastrointestinal absorption of this mineral. High blood levels of vitamin C are also independently associated with lower prevalence of elevated blood lead concentrations. However, neither calcium nor vitamin C has yet been found to reduce body lead burden in randomized controlled clinical trials.
Arsenic is present in some pesticides, treated wood, and mining runoff. Exposures may also come from the smelting process. Arsenic is also a component of chicken feed supplements used to treat parasites and often reaches drinking water, especially untreated well water. Exposure through water is of particular concern, because the arsenic compounds formed are readily bioavailable. Exposure may also result from occupational inhalation.
Long-term exposure is related to hyperkeratosis, neurologic and cardiovascular problems, diabetes mellitus, and liver disease, as well as increased risk of skin, lung, and other cancers. Arsenic poisoning is commonly diagnosed through urine tests, although hair and nail samples can also reveal exposure. Chelation therapy is available.
PCBs , are synthetic organic chemicals that were used in many products before 1977, when domestic PCB production was banned in the United States. Over 1.5 billion pounds were produced in the United States. PCBs now represent an environmental contaminant concentrated in fatty fish and other animal products (dairy products, eggs, and meats) and are also detectable in human tissues.
Evidence strongly suggests that PCBs are human endocrine disruptors and adversely affect the immune, reproductive, nervous, and endocrine systems of animals and humans. PCBs can cross the placenta and may contribute to cognitive problems in children. PCBs also enter breast milk, although the contribution of this route of exposure to health effects in infants is not well established, and breast-feeding is still encouraged.
Dioxins are usually byproducts of industrial processes, including incineration, although they also result from volcanic eruptions. Like PCBs, they are found mostly in animal products near the top of the food chain and ultimately can affect animal and human immune, reproductive, nervous, and endocrine systems. Although less than 10% of dioxins are considered significantly toxic, their long half-life—about 7 years in the human body—makes them an important public health concern. Destroying dioxins requires incineration at temperatures above 850°C to 1000°C.
Pesticides  (including herbicides) are agents that can both beneficially and adversely affect public health. They are beneficial in that they can restrict the spread of disease. On the other hand, over 4 billion pounds of pesticides are applied annually in the United States, so the prevention of unnecessary and accidental exposure through direct contact, or through water and food sources, is essential.
Several common classes of pesticides have the potential for adverse effects on the central nervous system: carbamates (carbaryl), organochlorines (lindane, DDT), organophosphates (malathion), and pyrethroids (permethrin). DEET is also commonly used to prevent mosquito and tick bites and can be toxic if not used as directed or if ingested; hand-washing after application is essential.
Lindane and permethrin are available by prescription for the treatment of scabies. Permethrin is safer for infants, children, and during pregnancy and is also available as an antilice shampoo.
It is estimated that 50% of lifetime pesticide exposure occurs in the first 5 years of life. Developing fetuses and children are at high risk of pesticide toxicity due to their rapid growth and developmental vulnerability. Some pesticides have the potential to disrupt endocrine pathways involving estrogen, androgen, and thyroid receptors. In addition, young children are particularly vulnerable, because they spend more time outdoors, often put their hands in their mouths, and ingest a much greater amount of food per unit body weight. For some pesticide residues, breast milk may contain several times the concentrations found in maternal blood samples.
Acute pesticide ingestion can be treated with gastric lavage, charcoal, pralidoxime (for organophosphates), and atropine. Diazepam may also help prevent seizures in acute organophosphate poisoning.
Untreated pharmaceuticals, such as the antihelminthic morantel, which is used widely in animal agriculture, may pass easily into soil and water supplies once released into the environment. Public health authorities are concerned that subtherapeutic antibiotic use on farms may spawn antibiotic resistance. In Oklahoma, turkey, cattle, and chicken farms and retail meats showed multi-antibiotic-resistant Klebsiella pneumonia bacteria, which could transfer the gene for resistance to E coli.
In developed countries, studies have suggested an association between subtherapeutic antimicrobial use in farmed animals and the development of antibiotic resistance in humans. Some experts believe antibiotic resistance is more commonly due to antibiotic use in human patients or to contact with hospital environments. Others have suggested that the development of resistance in humans may be due, in some cases, to antibiotic-resistant microflora of farm animals contaminating products entering the human food supply and passing resistance genes on to human microflora. Subtherapeutic antimicrobial use to increase yields for plants, such as lettuce and aquaculture, spreads similar risk of antimicrobial resistance in humans.
Heterocyclic amines (HCAs) are carcinogenic compounds that form during cooking of all meats, including fish. HCAs are capable of inducing genetic damage after ingestion. In general, grilling causes the greatest amount of HCA formation, followed by pan-frying. Chicken products contribute the greatest quantity of HCAs in North American diets, compared with other meats, in part due to the quantity of chicken products consumed. Several cancers along the GI tract are associated with HCAs, including those arising in the colon and rectum, stomach, breast, lung, and prostate. Modification of cooking methods (such as microwaving) and reducing meat consumption are effective preventive steps.
Nitrates and nitrites are used as preservatives in hot dogs, pickled meats and vegetables, some cheeses, and other foods. They are metabolized in the body to form N-nitroso compounds, which are associated with gastric and esophageal cancer. N-nitroso compounds also form during smoking of foods, such as fish. Nitrates are also a natural component of many vegetables, and produce commonly contains nitrate residues from fertilizers. However, despite the presence of nitrates in and on many fruits and vegetables, protection from gastric cancer is afforded by the consumption of these foods, presumably because of the inhibitory effect of vitamin C on the formation of N-nitroso compounds.
Other foods and nutrients also inhibit N-nitroso compound formation. These include polyphenolic compounds in fruits and vegetables; garlic and other allium species; and vitamin E and selenium.
Many toxic chemicals are concentrated in fatty animal tissues (including fish liver and kidneys) or produced during cooking. To reduce exposure to these toxic chemicals, it is best to reduce consumption (and trim visible fat) as described in the basic diet orders.
Certain fish species are common sources of toxic exposures. While some evidence shows fish to be more healthful than other meats for various health outcomes (such as cardiovascular events), the toxic load of some fish species raises important concerns. This is of particular relevance for women prior to and during their childbearing years and for pregnant and lactating women. Despite the presence of some toxic chemicals in breast milk, the benefits of breast-feeding outweigh the presumed risks to the baby.
Organic produce is increasingly available. Nonorganically produced fruits and vegetables can be washed thoroughly with warm water and a soft brush to reduce pesticide residues. Certain fruits and vegetables, such as apples, berries, tomatoes, and grapes, tend to carry larger pesticide residues.
Household pesticides, if used at all, should be carefully stored. When they are used, family members and domestic animals should be protected from exposure for the period of time specified in the product instructions.