Toxic Pollutants- Dioxins 

Presented ByAbu Khairul Bashar

Roll No: 584Session: 2011-2012

Introduction

Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollutants, the components of pollution, can be either foreign substances or naturally occurring contaminants. Pollution is classed as point source or nonpoint source. There are many types of ocean pollutants that endanger ocean life. Some of them are more obvious than others, but all contribute to an unhealthy ocean and many times, the death of its creatures.

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Toxic materials have the side effects on living organisms. Dioxins are environmental pollutants. Dioxin is a general name for a large group of chemical compounds with similar structure. These compounds are made up of carbon, oxygen, hydrogen and chlorine atoms. The most toxic dioxin has four chlorine atoms in positions 2, 3, 7 and 8. This dioxin (2, 3, 7, 8-tetrachlorodibenzo-p-dioxin) is often referred to as TCDD or “dioxin”. TCDD is the most studied and the most toxic of all dioxins.

Toxicological Properties

There is a large number of literature on dioxins. The available data show that it is difficult to discuss the environmental characteristics and risks of dioxins in general terms, basically because of 4 essential complicating factors.

Dioxins are in fact a large, heterogeneous group of chemicals. The differences between congeners, with respect to environmental characteristics such as ecotoxicity, bioaccumulation and environmental fate and therefore the associated risks, can be very large.

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Most of the dioxin congeners are very hydrophobic. The hydrophobic nature is a complicating factor in studying the environmental characteristics. Regular ecotoxicity tests for instance, are designed for water-soluble compounds. By using additional solvents, concentrations in the test medium can lead to ecological levels. This has resulted in a number of data on dioxins, which are not suitable for risk assessment.

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In fact a similar problem occurs in determining the toxicity to humans, where high doses used in animal testing make it extremely difficult to estimate the risks at very low concentrations as found in the environment .

A large part of the body of evidence was generated for the 2,3,7,8 TCDD congener which is regarded as the most toxic one, but which is not the environmentally most relevant one.

Sources of Dioxin Contamination

Dioxins are mainly by products of industrial processes.

But can also result from natural processes, such as volcanic eruptions and forest fires.

Dioxins are unwanted by products of a wide range of manufacturing processes including smelting, chlorine bleaching of paper pulp and the manufacturing of some pesticides and herbicides .

 

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In terms of dioxin release into the environment, uncontrolled waste incinerators (solid waste and hospital waste) are often the worst culprits, due to incomplete burning.

The highest levels of these compounds are found in some soils, sediments and food, especially dairy products, meat, fish and shellfish.

Very low levels are found in plants, water and air.

Figure: Dioxin and Furan Emissions Trends 1990 – 2010, Excluding Natural Sources

Effects of Dioxins on Human Health

Short-term exposure of humans to high levels of dioxins may result in skin lesions and altered liver function.

Long-term exposure is linked to impairment of the immune system, the developing nervous system, the endocrine system and reproductive functions.

Chronic exposure of animals to dioxins has resulted in several types of cancer.

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TCDD does not affect genetic material and there is a level of exposure below which cancer risk would be negligible.

Due to the omnipresence of dioxins, all people have background exposure and a certain level of dioxins in the body, leading to the body burden.

Current normal background exposure is not expected to affect human health on average.

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The newborn, with rapidly developing organ systems, may be more vulnerable to certain effects.

Some individuals or groups of individuals may be exposed to higher levels of dioxins because of their diets or their occupations .

Human concentrations are much less, 10-100 pg/g.

Figure: Contamination in different parameter

Environmental Persistence and Bioaccumulation

All groups of dioxin-like compounds are persistent in the environment. Ultraviolet light is able to slowly break down these compounds. Lipophilicity and very poor water solubility make these compounds move from water environment to living organisms having lipid cell structures. This is called bioaccumulation. Increase in chlorination increases both stability and lipophilicity .

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The compounds with the very highest chlorine numbers are so poorly soluble that this hinders their bioaccumulation. Bioaccumulation is followed by biomagnification. Lipid soluble compounds are first accumulated to microscopic organisms such as phytoplankton . Phytoplankton is consumed by animal plankton, this by invertebrates such as insects, these by small fish, and further by large fish and seals. At every stage, the concentration is higher, because the persistent chemicals are not "burned off" when the higher organism uses the fat of the prey organism to produce energy. Due to bioaccumulation and biomagnification, the species at the top of the trophic pyramid are most vulnerable to dioxin-like compounds.

Figure: Dioxin is released into the environment and builds up in sediment

Prevention and Control of Dioxin Exposure

Proper incineration of toxic material is the best available method of preventing and controlling exposure to dioxins. It can also destroy PCB-based waste oils.

Prevention or reduction of human exposure is best done via source-directed measures.

Strict control of industrial processes to reduce formation of dioxins as much as possible.

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More than 90% of human exposure to dioxins is through the food supply, mainly meat and dairy products, fish and shellfish.

Good controls and practices during primary production, processing, distribution and sale are all essential to the production of safe food. Food contamination monitoring systems must be in place to ensure that tolerance levels are not exceeded.

It is the role of national governments to monitor the safety of food supply and to take action to protect public health.

Figure: Danger Dioxin Waste in waste box.

WHO Activities Related to Dioxins

Reducing dioxin exposure is an important public health goal for disease reduction, also with respect to sustainable development. In order to give guidance on acceptable levels of exposure.

WHO has held a series of expert meetings to determine a tolerable intake of dioxins to which a human can be exposed throughout life without harm.

In 2001, WHO Expert concluded that a tolerable intake could be established for dioxins on the basis of the assumption that there is a threshold for all effects, including cancer.

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WHO is now working with the United Nations Environmental Programme (UNEP) on the implementation of the ‘Stockholm Convention’, an international agreement to reduce emissions of certain persistent organic pollutants (POPs), including dioxins.

The experts established a tolerable monthly intake of 70 picogram/kg per month. This level is the amount of dioxins that can be ingested over lifetime without detectable health effects.

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Since 1976, WHO has been responsible for the Global Environment Monitoring System’s Food Contamination Monitoring and Assessment Programme, provides information on levels and trends of contaminants in food through its network of participating laboratories in over 70 countries around the world. Dioxins are included in this monitoring programme.

Since 1987, WHO has conducted periodic studies on levels of dioxins in human milk, mainly in European countries. These studies provide an assessment of human exposure to dioxins from all sources.

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During the last 15 years, WHO, through the International Programme on Chemical Safety (IPCS), has established toxic equivalency factors (TEFs) for dioxins and related compounds through expert consultations. WHO-TEF values have been established which apply to humans, mammals, birds and fish.

The last such consultation was held in 2005 to update human TEFs. These international TEFs have been developed for application in risk assessment and management, and have been adopted formally by a number of countries including Canada, Japan, the United States and the European Union.

 

Conclusion

Dioxins occur as a complex mixture in the environment and in food. In order to assess the potential risk of the whole mixture, the concept of toxic equivalence has been applied to this group of contaminants. TCDD, the most toxic member of the family, is used as reference compound and all other dioxins are assigned a toxic potency relative to TCDD, based on experimental studies.

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