Dioxins

Dioxins are a family of highly toxic and carcinogenic compounds that contaminate pesticides and other chemicals. They persist in the environment and accumulate in the food chain where they can cause reproductive and developmental disorders, hormone interference, immune system damage, and cancer.

There are more than 400 different dioxins, although only about 30 are considered significantly toxic. The term dioxin refers specifically to 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD)—the most toxic and a known human carcinogen. TCDD is one of 30 urban air pollutants that pose the greatest health risks. Dioxins are grouped into three classes:

These three classes often occur together. PCDDs and PCDFs are byproducts of natural processes or human activities. PCBs are manufactured chemicals that are no longer made in the United States. PCDFs are major contaminants of PCBs.

Dioxins are toxic to the immune system and are endocrine disrupters that cause changes in thyroid and steroid hormones and reproductive functioning. Children, especially breastfed infants, are most at risk from developmental and neurologic effects. Dioxins have also been associated with diabetes.

Dioxins have always been present in the environment from natural processes and, more recently, from industrial processes, but two events—the Vietnam War and Love Canal—brought them to public awareness. U.S. troops sprayed Agent Orange, a defoliating herbicide, over 15% of southern Vietnam, and two-thirds of the Agent Orange was contaminated with TCDD. For years, U.S. officials denied responsibility for the environmental damage and physical and mental disorders that afflicted Vietnamese civilians and American veterans. However, the many thousands of sick Vietnam vets—and, in some cases, their offspring—forced the Department of Veterans Affairs (VA) to recognize that certain cancers and other health conditions were associated with exposure to Agent Orange or other herbicides. In the late 1970s, investigative reporting and door-to-door health surveys found hundreds of people in a working-class neighborhood of Niagara Falls, New York, suffering from a range of inexplicable illnesses. Their homes had been built adjacent to Love Canal, a waste dump for some 21,000 tons of toxic chemicals, including dioxins.

There have been other dioxin-related tragedies:

Almost all living organisms, including plants, as well as air and water, contain dioxins at very low levels. Meat, dairy, fish, and shellfish and some soils and sediments have the highest levels. Uncontrolled or incomplete burning by solid- and hospital-waste incinerators releases the most dioxins into the environment. PCB-based waste industrial oils, which often have high levels of PCDFs, are stored worldwide; both long-term storage and improper disposal can result in dioxin release. However, most human exposure occurs through food, primarily via animal feed produced with clay, fat, or citrus pulp pellets. In addition to atmospheric emissions, water discharges, and soil deposits, sources of dioxin exposure include:

The major risk factors for dioxin exposure are eating large amounts of fish or shellfish or working in the pulp and paper industry, incinerator plants, or hazardous waste sites.

Dioxins occur worldwide, but most reports of contamination come from industrialized countries with regulations and food-safety monitoring. Dioxin contamination is increasing in some developing countries due to uncontrolled burning and dismantling and recycling of electronic products.

Developing fetuses and newborns are most sensitive to dioxins. The World Health Organization (WHO) has carried out global surveys of dioxins in human milk since 1987. PCDD and PCDF levels are highest in India and some European and African countries, whereas PCB levels are highest in Europe. Although breast milk levels generally appear to be decreasing, they remain above what is considered safe. Nevertheless, the health benefits of breastfeeding far outweigh potential dangers, and in utero exposure is considered of greater concern.

Dioxins can enter the body via the following ways:

When dioxin enters the body, it binds to aryl hydrocarbon receptors (AhRs) inside cells. This binding can change the expression of certain genes and disrupt normal cellular functions, which are ultimately responsible for the adverse effects of dioxins.

Universal background dioxin exposure is not believed to affect human health. However, short-term, high-level dioxin exposure can cause chloracne, a severe skin disease on the face and upper body that is a classic sign of dioxin poisoning. Dioxin exposure can also cause rashes, skin discoloration, and mild liver damage.

Dioxins can have widespread effects at every stage of development, especially fetal development. Long-term effects can include:

Long-term dioxin effects depend on various factors, including the following:

U.S. Vietnam War veterans are eligible for compensation if they develop any of the following conditions associated with Agent Orange:

Tests for dioxins are performed in only a few laboratories and are very expensive, although as of 2018, screening methods for biological samples, as well as for food and animal feed, were becoming increasingly available. For example, an assay that causes AhRs to luminesce in the presence of dioxin can be used to test human tissues, animal feed, and food.

There is no treatment for dioxin exposure. Treatment for dioxin toxicity is supportive and symptomatic.

Prior to the establishment of regulations and safeguards, dioxin releases were a major problem. In 1979, the Environmental Protection Agency (EPA) banned the manufacture of PCB-containing products, and additional controls on the incidental production and release of dioxins were introduced in subsequent years. In 1999, the United States, Canada, and Mexico agreed to cooperate on the further reduction of dioxin releases. The EPA manages the Superfund program, including National Priority List sites that commonly contain dioxins. In 2009, the EPA was told to speed up its reassessment of human health risks from dioxin exposure and remediation goals for dioxin-contaminated soil. Because many different compounds can bind to and activate AhRs, government-supported scientists are studying the health effects of combinations of dioxins and other chemicals. The EPA has established maximum allowable dioxin levels for drinking water, and the Food and Drug Administration monitors for dioxins in certain foods.

A civil suit by U.S. veterans against Agent Orange manufacturers Monsanto and Dow Chemical was settled in 1984. In 1991, Congress passed the Agent Orange Act that compensated veterans with certain health problems associated with “presumptive exposure to dioxin,” as well as their children born with the spinal defect spina bifida. In 2007, Congress began appropriating increased funding for remediation of dioxin hot spots at three former U.S. airbases in Vietnam, as well as assistance for severely disabled Vietnamese and their families in the most contaminated provinces. Cleanup of the Phu Cat airfield and its surroundings was completed in 2011, and cleanup of the even more contaminated Da Nang airport was completed in 2017. In 2010, the U.S. Air Force chopped up and melted down a fleet of C-123 aircraft that had conducted aerial spraying of Agent Orange, after C-123 crews continued to develop health problems. As of 2018, veterans—and some of their offspring—were still battling the VA over diagnosis, treatment, and compensation for dioxin-associated effects.

In 1978, President Jimmy Carter declared a state of emergency at Love Canal, and the government relocated 239 families. However, another 700 families were refused assistance even though it was shown that toxins were leaching into their homes. In 1981, Carter was forced to declare a second state of emergency to relocate the remaining families. The Times Beach, Missouri, disaster sparked passage of Superfund legislation to help clean up abandoned hazardous waste sites.

Countries are expected to implement the Food and Agriculture Organization of the United Nations (FAO)/WHO Code of Practice for reducing dioxin contamination of food and animal feed and monitoring contamination in foods and human milk. Indeed, many countries do monitor their food supplies, and these actions have enabled early detection of contamination. In 2001, FAO/WHO determined a provisional tolerable monthly intake of 70 picograms (10^-12 g)/kg (2.2 lb.) of body weight as the amount of dioxins that could be ingested over a lifetime without detectable health effects. In 2015, WHO published its first estimate of the global foodborne burden on fertility and thyroid function, which indicated that dioxin was a significant contributor in some parts of the world. Thus, WHO declared reducing dioxin exposure as an important public health goal for disease reduction.

U.S. dioxin production was reduced by 90% between 1990 and 2018. Although dioxins are environmentally persistent, they do break down somewhat over time, and as of 2018, areas of Vietnam that had been aerial sprayed with Agent Orange were trending toward recovery.

Reducing or preventing dioxin exposure depends primarily on strict government controls of industrial processes that produce and release the chemicals. Dioxin-contaminated materials, including PCB-based waste oils, should be incinerated at temperatures above 1,562°F (850°C). Large amounts of material require 1,832°F (1,000°C) or higher. Since most human exposure is food-based, good practices and monitoring are required at all steps of the food chain, especially animal feed.

Although opportunities to reduce individual dioxin exposures are limited, it is important to minimize body burden, especially in girls and young women who must protect their future children. Methods include:

See also Agency for Toxic Substances and Disease Registry ; Agricultural chemicals ; Love Canal ; Rachel Carson Council .