What Are Dioxins and Dioxin-like Polychlorinated Biphenyls
- Dioxins and dioxin-like polychlorinated biphenyls (DL-PCBs) are a group of environmental pollutants with similar structures and toxic effects.
- There are 210 dioxins, but only 17 are of the most concern for human health. As regards the 209 polychlorinated biphenyls (PCBs), only 12 PCBs have a structure and toxic effects similar to dioxins; hence, they are termed DL-PCBs. Among dioxins and DL-PCBs, the most toxic one is 2,3,7,8-TCDD.
- Dioxins are produced as unwanted by-products of combustion and some industrial processes (e.g. bleaching paper pulp using chlorine). Natural disasters such as forest fires and volcanic eruptions can also release dioxins. In contrast, PCBs were manufactured in the past for a variety of industrial uses such as electrical insulators, lubricants, plasticisers and specialised hydraulic fluids.
How Do Dioxins and DL-PCBs Enter Food Chains and Contaminate Foods
- Dioxins and DL-PCBs are not soluble in water and highly resistant to chemical and biological degradation. Once released, they contaminate plant and soil surfaces as well as aquatic sediments. Land animals ingest contaminated vegetation and soil and accumulate dioxins and DL-PCBs in their bodies. In water bodies, small living organisms eat contaminated particles and sediments and store the contaminants in their bodies. Small fish eat a number of small organisms and the contaminants pass to the small fish. Large fish eat a number of small fish and accumulate the contaminants in their bodies, leading to bioaccumulation and biomagnification (i.e. increasing concentration) of the contaminants along the food chains (Figure 1).
Figure 1. Diagram to illustrate how dioxins and DL-PCBs enter and pass along the food chain.
How are Humans Exposed to Dioxins and DL-PCBs
- Generally, levels of dioxins and DL-PCBs in air and water (such as drinking water) are very low. Over 90% of the human intake of dioxins and DL-PCBs is through food, mainly from animal origin.
- Since dioxins and DL-PCBs are fat-soluble, they accumulate in fatty tissues of animals. Hence, fatty foods such as meat, poultry, seafood and dairy products are the major dietary sources of dioxins and DL-PCBs in the general population. The levels in foods produced in this way are considered background levels of dioxins and DL-PCBs in foods. According to World Health Organization, dioxins and DL-PCBs are found throughout the world in the environment and all people have background exposure which is not expected to affect human health.
Contamination above background levels can occur if food animals are fed products which are adulterated, either by natural or unnatural means, with high concentrations of dioxins and DL-PCBs. For example,
- In December 2008, Irish authorities found the presence of dioxins and DL-PCBs at levels up to 200 pg WHO-TEQ/g fat (i.e. about 130 times of EU limit in 2008) in pork. The use of contaminated animal feed was identified to be the source.
- In 1999, high levels of dioxins were found in pork, poultry and eggs from Belgium. The cause was traced to animal feed contaminated with illegally disposed PCB-based waste industrial oil.
- The actual intake of dioxins and DL-PCBs for any one person will depend on a number of factors, including the amount and duration of exposure and the concentration of the contaminants in food and the environment (e.g. soil, water and air). In situations where local sources (e.g. certain types of industrial plants, poorly-designed waste incinerators, etc.) that introduce dioxins and DL-PCBs into the food or environment may significantly increase one's exposure to the contaminants.
Public Health Significance of Dioxins
- Accidental exposure (i.e. industrial accidents) to large amount of dioxins could lead to the development of chloracne, a skin condition, excessive body hair and other skin lesions such as skin rashes and skin discolouration.
- Long-term exposure to dioxins and DL-PCBs can cause developmental problems in children, lead to reproductive problems in adults, damage the immune system and interfere with the levels and activities of hormones. Associations with diabetes, thyroid dysfunction and heart diseases in humans have been reported in some studies.
- The International Agency for Research on Cancer (IARC) considered that DL-PCBs and some dioxins (e.g. 2,3,7,8-TCDD) are carcinogenic to humans (Group 1). There is no evidence to suggest that dioxins and DL-PCBs cause direct genetic damage.
Assessment of Dioxins and DL-PCBS
- Some carcinogens induce cancers via a mechanism not involving direct damage to DNA (i.e. non-genotoxic carcinogens); they induce tumours as a result of another toxic effect which has a threshold (i.e. a level below which no effect occurs). Dioxins and DL-PCBs are non-genotoxic carcinogens and belong to this category of substances. For these substances, exposure below the threshold poses no health risk, both cancer and other toxic effects
- The persistency (i.e. resistance to degradation) of dioxins and DL-PCBs means that exposure to a small dose of these substances on a given day may have little or no impact on human health; however, long periods of exposure to small doses can have a cumulative adverse effect on human health. Hence, to assess the long-term cumulative risk to health, total intake of these substances should be assessed over months rather than days. In 2001, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) decided to express the tolerable intake for dioxins and DL-PCBs on a monthly basis in the form of a provisional tolerable monthly intake (PTMI) of 70 pg WHO-TEQ/ kg bw per month.
- It should be noted that PTMI is not a limit of toxicity. Short-term intakes slightly above the PTMI would not necessarily result in adverse health effects; however, long-term intakes above the PTMI would erode the safety factor built into the calculations of the PTMI. It is not possible, given current knowledge, to define the magnitude and duration of excess intake that would be associated with adverse health effects.
Dietary Exposure of the Hong Kong Population
- Dietary exposure of dioxins and DL-PCBs is estimated by assessing the levels of the contaminants in food as well as the food consumption pattern of the population. The estimated dioxins and DL-PCBs intake is then compared with the PTMI established by JECFA to assess the associated health risk.
- According to the results of the First Hong Kong Total Diet Study, the dietary exposures to dioxins and DL-PCBs for both the average and high consumers of the local population were below the PTMI set by JECFA; meaning that the local population is unlikely to experience major undesirable health effects of dioxins and DL-PCBs.
Reduction of Dioxins and DL-PCBs Exposure
Dioxins and DL-PCBs are found throughout the world in the environment. Nonetheless, according to WHO, remedial actions have led to reductions in exposure in the developed world, with a fall to around 10% of levels seen in the 1970s. Countries with rapidly expanding development are experiencing increasing exposure, particularly to dioxins, but levels are still below those developed countries in the 1970s. Examples of remedial actions include:
- PCB manufacture is prohibited under the Stockholm Convention on Persistent Organic Pollutants, although their release into the environment still occurs from the disposal of large-scale electrical equipment and waste.
- Low levels of dioxins emission is achieved through improved technology of waste incineration.
- As dioxins and DL-PCBs accumulate in fats of animals, trimming fat from meat and consuming low fat dairy products may decrease the exposure.
- Also, a balanced diet (including adequate amounts of fruits, vegetables and cereals) will help to avoid excessive exposure from a single source.
Risk Assessment Section
Centre for Food Safety