Dioxins, a member of the “dirty dozen,” are of great concern because of their high toxicity. Dioxins survive in the body for a long time, because they are absorbed by fat tissues and have a chemical stability that aids them in their survival. Dioxins use the food chain as a platform for them to magnify. Several studies established that dioxins affect a number of body organs and systems (Marinkovi%u0107 et al. 2010, Sweeney and Mocarelli 2000, Institute of Medicine, 1994). They are responsible for conditions such as chloracne and patchy darkening of skin resulting from the short-term exposure to large doses of dioxins (Marinkovi%u0107 et al., 2010). The failure of the body systems result from the long-term exposure (Yu, 2001).long term exposure of animals to these compounds has resulted in several types of cancer (Institute of Medicine, 1994). 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) was named by the International Agency for Research on Cancer (IARC) and the World Health Organization as a cancer causing agent (Marinkovi%u0107 et al., 2010). However, 2,3,7,8-TCDD, regardless of its disease causing properties, does not affect genetic material (Institute of Medicine, 1994). However, one unique characteristic of dioxins is they have different effects on different species. The presence of dioxins in every component of the environment requires a better understanding of how they affect organisms.
This paper reviews physical and chemical characteristics of dioxins. The objectives of this paper are to discuss (1) how dioxins are formed, exposed, metabolized and excreted; (2) mechanism of dioxins’ gene regulation; and (3) toxic effects of dioxins on plants, animals, and human beings.
Characteristics of Dioxins
Dioxins represent a subset of halogenated aromatic hydrocarbons that have similar chemical configuration and a common poisonous system. There are 75 polychlorinated dibenzo dioxins (PCDDs) and 135polychlorinated dibenzo furans (PCDFs) in the dioxin family. However, only seven PCDDs and ten PCDFs can be determined in a laboratory, and among 210 dioxins, only 17 are toxic (Marinkovi%u0107 et al., 2010). 2,3,7,8-TCDD is the most poisonous dioxin, because it has the lowest known LD50 of only 0.6 μg/kg enough to kill a test animal (Hites, 2011).
Dioxins are colorless in their pure form, but they exist differently in mixed forms in the environment. Dioxins are compounds, whose ability to dissolve in organic solubles raises with the amount of chlorine available (Marinkovi%u0107 et al., 2010). Dioxins tend to magnify through the food chain as they strongly attach to any high organic content material.
Sources of Dioxins
Most dioxins result from industrial wastes (herbicide production, and paper mills) and burning processes (waste incineration). Power plants that use coal and oil produce dioxins in their emissions and so do compounds containing chlorine. Marinkovi%u0107 (2010) reported that these processes contribute 45% of dioxin emissions, while 40% is from uncontrolled combustion processes.
Exposure, Metabolism, and Excretion
Dioxins are available in the air through the herbicides used by man. Soil and water are also contaminated with dioxins which means that products such as fish and meat that a man eats are contaminated and act as sources of dioxins to humans. More than 50% of 2,3,7,8-TCDD is absorbed through the alimentary canal of the animals (Institute of Medicine, 1994).When absorbed into the body, the compound goes to other organs through the circulatory systems. Enzymes in the metabolize it to form derivates that are hydrophilic and easy to eliminate from the body. In rodents, 90% of the compound undergoes urinary and biliary excretion in polar biotransformed form (Institute of Medicine, 1994). The removal of this compound from the body relies on metabolism, but compounds that the body had not absorbed go through direct intestinal removal process. The excretion of 24.5% of this compound using bile in dogs took place within 110 hours (Institute of Medicine, 1994). When the compound was introduced with mixed function oxidase (MFO) inducers, elimination rate doubled (Institute of Medicine, 1994).Different animal species exhibit different rates and primary routes of excretion. The compound after a single prescribed amount undergoes a first order elimination process that exhibits very sluggish secretion characteristics. Different animals exhibit different half-life elimination periods (Yu, 2001). Most species remove TCDD from their body through feces.
Mechanism of Gene Regulation
TCDD does not exhibit any effects to DNA (Institute of Medicine, 1994) even though it has various effects emanating from the regulation of normal cells. Studies have focused on identifying and characterizing the interaction of TCDD with intracellular protein in liver cells known as Ah (Aryl Hydrocarbon) receptor. The availability of Ah causes the organism prone to problems that result from the presence of dioxins and PCDDs. Toxicities of PCDDs on invertebrates or more primordial fish, which do not have the receptor, are absent (Pickard and Clarke, 2008). The system of gene control of these compounds is simply as follows (Yu, 2001): (1) after entering the cell, 2,3,7,8-TCDD joins the Ah receiver to form a receiver composite, (2) The resulting compound is translocated to the nucleus, (3) the complex binds to dioxin-responsive elements (DREs) on DNA, (4) transcription of several genes is increased, and (5) translation of transcribed mRNA results to synthesis of cytochrome P450 monooxygenases.
Toxic Effects of Dioxins on Organisms
Effects on Plants
Plants experience complicated biochemical actions of phenoxyherbicides (PHs). Chemicals applied flow to the plant through the usual water absorption routes. Translocation of chemicals though vascular tissues in responsive plants disrupt development and essential food processes. Plants die if many enzymes are stimulated and inhibited leading to problems in the growth and metabolic processes (Yu, 2001).
Effects on Animals
The poisonous content of dioxins is different among different species (Hites, 2011).
Carcinogenicity Carcinogenicity bioassays of TCDD have been conducted in rats, mice, and hamsters from one to two years (Institute of Medicine, 1994). Several body sites such as the liver, the thyroid, adrenal glands and others have reported an increase in tumor rates. In addition, TCDD has the ability of increasing tumor formation by other chemicals alone (Institute of Medicine, 1994).
Immunotoxicity. Studies indicate that the increasing TCDD dosage affected the ability to limit immune function positively. TCDD curtails the function of cells of the immune system, such as lymphocytes and the production of antibodies by B cells (Institute of Medicine, 1994). TCDD administration increased the number of tumors that formed when mice were injected with tumor cells (Institute of Medicine, 1994). There is a difference in the effects of TCDD on the immune system among species due to differences in sensitivity (Institute of Medicine, 1994).
Reproduction and developmental toxicity. Administering TCDD to test animals affects testicular function, decreases fertility and the rate of sperm production. Female animals report reduced fertility, decreased ability to remain pregnant throughout gestation, smaller size, increased fetal death, impaired ovary function, decrease of hormones such as estradiol and progesterone, and increase in rates of fetal abnormalities (Institute of Medicine, 1994). Several different toxicities such as neurotoxicity, gastrointestinal toxicity, and metabolic toxicity have been shown as the effects of TCDD (Institute of Medicine, 1994).
Effects of Dioxins on Human Exposure in History
Main exposure of humans to dioxins in history. The most famous dioxin accident was at the US Monsanto chemical company, which manufactured industrial chemicals, PCBs, pesticides, and herbicides (Marinkovi%u0107 et al., 2010). In 1970, dioxin contaminated oil used for spraying roads in Times Beach Missouri led to evacuation of the town (Hites, 2011). A 2,4,5- trichlorophenol reactor explosion in 1976 in a chemical plant located in Seveso, Italy, led to the exposure of many people to these toxic compounds (Marinkovi%u0107 et al., 2010). During the Vietnam War, the U.S. military sprayed 4.5×107L of Agent Orange, 150kg of 2,3,7,8-TCDD to the environment of southern Vietnam from 1965 to 1971 (Hites, 2011).
Effects of dioxins on human health. Exposure to high levels of dioxins damages liver function and also results to chloracne, which are the mainly recognized dermal consequences of contact to TCDD (Marinkovi%u0107 et al., 2010; Institute of Medicine, 1994). This condition is reported in at least 193 (0.6%) of Italian city residents, mainly young ones exposed after the accident. The former Ukrainian president Viktor Yushchenko was poisoned by dioxins, which resulted to chloracne (Marinkovi%u0107 et al., 2010). This poisoning case is the most famous high profile case involving dioxin poisoning.
Contact with the toxic compounds for a long time causes problems to the main body systems (Yu, 2001). Atherosclerosis, hypertension, diabetes, and nervous system damage are problems experienced if the compound remains in the body for a long time (Marinkovi%u0107 et al., 2010). Researchers concluded that contact with TCDD in both mother and father caused birth of mainly girl children.
TCDD is in the list of compounds known to cause cancer. Research conducted after the Seveso incident and in 12 countries where workers are exposed to herbicides show an increase in overall cancer and specific cancer cases (Marinkovi%u0107 et al., 2010). Research has established that there exists a relationship between the compound and cancer of the alimentary canal. This cancer is determined by the dose of TCDD absorbed into the body. In the case of the U.S war soldiers (1994) indicated a relationship between herbicide exposure and the occurrence of diseases associated with exposure to this toxic substance. The herbicide used may have had effect on the residents of sprayed areas. Many cases of hepatic cancers were reported in this area in the years the Vietnam war was going on (Yu, 2001). TCDD was detected in samples of acquatic animals in levels that confirmed that food chain was contaminated with the toxic substances. (Baughman and Meselson, 1973).After several decades, the concentrations of dioxins in affected areas remained high as compared to un affected areas (Nishijo et al., 2012). A recent study in dioxin-contaminated areas, found that there was a noticeable effect of exposure to dioxins shortly after birth, which mostly occurred in boys (Nishijo et al., 2012). In another study in Vietnam, results showed that the local residents were absorbing dioxins into their body that exceeded the threshold values acceptable, and this was a huge risk to their health (Tran et al., 2010). Areas marked as dioxin concentrated have been targeted for cleaning to make them secure for human occupation. The amount of soil in need of cleaning is approximated to be about 104 m2 (1 ha).
The effects of dioxins are well recorded from the numerous studies that have been carried out in this field. The findings have some differences, but all seem to agree on one thing that dioxins have health effects that can be devastating if consumed in large doses. Dioxins end up in the environment via a variety of routes available to them. These chemicals exist in different media in the environment, such as air, surface water, ground water, land, and sediments. Living creatures that exist in the world have been exposed to dioxins. This exposure is associated with negative health effects which include cancer. The levels of dioxins in the atmosphere increased in the period from 1935 to 1970, but since then the levels have reduced. The reduction is a result of the continued decrease in particle emissions from large combustion systems, the limitation of chemical waste burning, and the abandonment of the chlorinated phenol business by large sectors of the chemical industry. Good controls and hygienic practices are keys in reducing the exposure of humans to dioxins since it mostly occurs through food.