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 harmful 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, all have the potential to harm human health. Ground and surface waters used for drinking and fishing, soil used for agriculture purposes, and farmed animals raised with growth/production stimulators are also potential reservoirs of chemicals that, when consumed, can have short- and long-term health consequences.
The term toxicants can include both natural and synthetic substances and refers to toxic chemicals (individual chemical compounds), toxic substances (mixtures of chemicals), and toxins (harmful substances produced by living organisms, such as microorganisms).[1] Exposure to toxicants can be acute or chronic.
Public health agencies and clinicians should educate the public regarding ways to minimize these exposures. Known exposures should be discussed immediately with a local poison control authority. Primary care physicians may evaluate their patients if concerned about environmental exposures to toxicants. Toxicologists may also play a role in assessing adverse effects.
Elemental mercury has well-known toxic effects. 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. Because mercury accumulates in our tissues, including that of the heart, consumption of this neurotoxin increases the risk for high blood pressure, irregular and increased heart rate, and death from heart attack or stroke.[2] Mercury easily crosses the placenta 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 elemental mercury toxicity.
The link between mercury contamination and pancreatic problems has become more defined through recent studies. While a 2013 prospective observational study found that those with the highest levels of mercury exposure had a 65% increased risk for developing diabetes, compared with those with the lowest levels of mercury exposure, a 2022 systematic review and meta-analysis did not support this association.[3][4] However, mercury deposition in pancreatic islet cells was found to be significantly more common among individuals diagnosed with pancreatic adenocarcinoma compared to islet cells of individuals without pancreatic adenocarcinoma.[5]
The most common nonoccupational 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. A 2020 study found that as much as 84% of the world’s fish contains unsafe levels of mercury.[6]
Very few vaccines contain thimerosal, a compound that includes mercury. Thimerosal-free vaccines have now become standard.[7] 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. Mercury is used in medical instruments, although less commonly than in the past.
Mercury is a component of and is a common environmental pollutant as a result of industrial processes. Primary care physicians may encounter patients who have been exposed to elemental mercury from suboptimal care or destruction of compact fluorescent light bulbs, liquid crystal display (LCD) screens, broken mercury thermometers, and batteries 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.
Data from the National Health and Nutrition Examination Survey demonstrate that cereals and breads, leafy greens, and potatoes were the top food sources of cadmium in the US diet.[8] 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 have lead pipes connected to city water mains. In addition, older houses often have lead-based paint, which may flake off and be accidentally consumed.
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, spices, and traditional medicines. For example, some Ayurvedic and Chinese medicines may contain lead, mercury, and arsenic.[9][10] 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.[11] High blood levels of vitamin C are also independently associated with lower prevalence of elevated blood lead concentrations.[12] However, neither calcium nor vitamin C has yet been found to reduce body lead burden in randomized controlled clinical trials.
Arsenic exposure over the long term is related to hyperkeratosis, neurologic, cardiovascular, and renal disease, as well as increased risk of skin, lung, liver, and bladder cancers.[13] Arsenic poisoning is commonly diagnosed through urine tests, although hair and nail samples can also reveal exposure. Chelation therapy is available and recommended for acute toxicity.
Arsenic is present in some pesticides, treated wood, and mining runoff. Exposures may also come from the smelting process. Arsenic was historically a component of chicken feed supplements used to treat parasites.[14] Other arsenic-based antimicrobials were historically used in turkeys to prevent parasitic disease. As of 2016, there are no remaining arsenic based animal drugs on the market. When used in feed, arsenic can reach 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. Rice and fruit juices are sometimes contaminated with arsenic. Rice products from California, India, and Pakistan are typically lower in contamination.[15] Some brands are much lower in arsenic than others. Current levels can be found listed online. Contamination can be further reduced by boiling rice like pasta and draining the cooking water.[16][17]
PCBs are synthetic organic chemicals that were used in many products before 1977, when domestic PCB production was banned in the United States.[18][19]
Over 1.5 billion pounds of PCBs 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 have also been associated with cardiovascular risk factors.[20] PCBs can cross the placenta and may contribute to cognitive problems in children.[21] PCBs also enter breast milk, although the contribution of this route of exposure to health effects in infants is not well established, and breastfeeding is still encouraged.[22]
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.[23]
Pesticides (including herbicides) are agents that can both beneficially and adversely affect individual and public health.[24] 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 (e.g., carbaryl), organochlorines (e.g., lindane, DDT), organophosphates (e.g., chlorpyrifos, malathion), and pyrethroids (e.g., permethrin). Diethyltoluamide (DEET) is also commonly used to prevent mosquito and tick bites and can be harmful if not used as directed or if ingested. Handwashing 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 anti-lice shampoo.
It is estimated that 50% of lifetime pesticide exposure occurs in the first 5 years of life.[25] 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.[26]
Poison control should be contacted immediately if pesticide ingestion is suspected. 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.
Pharmaceuticals, such as the anthelmintic morantel, widely used in animal agriculture, may pass easily into soil and water supplies once released into the environment.[27] Public health authorities are actively combatting multidrug-resistant pathogens that are commonly found in pigs, cows, and poultry, despite efforts to limit antibiotic use in agriculture.[28]
Antibiotic resistance in animals is known to cause antibiotic resistant infections in humans, whether infections are transmitted from animals to farm or slaughterhouse workers, or to people through the consumption of meat infected with a resistant strain of bacteria.[29] According to the Centers for Disease Control and Prevention, 2.8 million antibiotic-resistant infections occur in the United States each year, and more than 35,000 people die as a result.[30]
The World Health Organization holds that antibiotic resistance is one of the biggest threats to health and global food security, and notes changes that the agricultural sector can make to mitigate this threat, including discontinuing the use of antibiotics to promote growth or prevent disease in healthy animals.[31] Global and regional authorities have crafted detailed plans to regulate such use of antibiotics, however, the overuse and misuse of antibiotics in livestock continue to be prevalent issues.[32]
Heterocyclic amines (HCAs) are carcinogenic compounds that form during the cooking of meat, poultry, and fish.[33][34] HCAs are capable of inducing genetic damage after ingestion.[35] The amount of HCAs and their mutagenic activity increases with temperature and certain methods of food preparation.[36] 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 are associated with HCAs, including those arising in the colon and rectum, stomach, breast, lung, and prostate.
The intake of cruciferous vegetables, such as broccoli and Brussels sprouts, has been found to favorably increase HCA metabolism in humans by induction of hepatic detoxification enzymes.[37][38][39]
Nitrate poisoning is a risk for millions of Americans who obtain drinking water from untreated wells. Infants are particularly vulnerable to nitrate toxicity. Nitrate toxicity can lead to methemoglobinemia (blue baby syndrome).[40]
Nitrates and nitrites are used as preservatives in hot dogs, other processed meats like bacon, pickled meats and vegetables, some cheeses, and other foods. They are metabolized in the body to form N-nitroso compounds, which are associated with certain cancers such as gastric and esophageal. N-nitroso compounds also form during smoking of foods, such as fish.
Nitrates are a natural component of many vegetables. However, despite the presence of nitrates in 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 and other phytonutrients 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.[41][42][43]
Microplastics occur commonly in foods and have been found in human kidney, liver, and brain tissues.[44] More research is needed on their health effects. Fish and shellfish are known sources, due to seawater and bioaccumulation through trophic levels, as is bottled water and tea.[45][46] Many other foods have also been found to contain significant concentrations of microplastics, with some studies finding that more highly processed foods contain more than less processed.[47][48] While not avoidable, microplastic intake may be reduced through reduced use of plastic food containers and avoidance of highly processed and animal foods, notably fish and shellfish.
See Basic Diet Orders chapter.
Many harmful chemicals are concentrated in fatty animal tissues (including fish, liver, and kidneys) or produced during cooking of animal products. To reduce exposure to these chemicals, it is best to eliminate consumption of animal products as described in the basic diet orders.
Certain fish species are common sources of toxic exposures. 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 breastfeeding 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. For reference, The Environmental Working Group has a regularly updated list of produce items, “The Dirty Dozen,” with the highest concentration of pesticides.[49]
When organic fruits and vegetables are not available, conventionally grown varieties should not be avoided. The benefits of fruits and vegetables outweighs the relatively minor risk of pesticides.[50]
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.
Parents should follow regular schedules with their physician for childhood developmental visits and discuss any concerns with their physician.