Advisory Council on Food and Environmental Hygiene

Study on Dietary Exposure of Secondary School Students to Dioxins and Heavy Metals


This paper presents the findings of the study conducted by the Food and Environmental Hygiene Department (FEHD) on dietary exposure of secondary school students to dioxins and heavy metals.


2. Dioxins and heavy metals are harmful environmental contaminants which can cause adverse effects on human health. Dietary intake is one of the major routes of human exposure to these contaminants. In the Policy Address 2000, FEHD pledged to conduct a study on dietary exposure of local secondary school students to dioxins and heavy metals in order to identify the major dietary sources of these contaminants and to evaluate the risk posed to the students. The study has been completed. The executive summaries of the study reports on dioxins and heavy metals are enclosed at Annex I and Annex II respectively. Hard and soft copies of the full reports of the study have been deposited with the Secretariat for perusal by Members.


3. Dioxins are a group of polychlorinated aromatic compounds arising either naturally or as by-products of industrial activities e.g. metal smelting, bleaching of paper pulp, etc. They are toxic and stable. Once produced, they tend to persist in the environment and concentrate in the food chain. Dioxins are most commonly found in food items that are rich in animal fat such as meat, fish and dairy products. While acute oral toxicity is rare, studies have shown that chronic exposure to dioxins may be cancer causing to humans.


4. Heavy metals are environmental contaminants that are present naturally in the Earth's crust. They may also be discharged to the environment through industrial uses. The possible health effects of each kind of heavy metals vary depending on the unique features of individual metal and the route of exposure. Acute toxicity resulting from ingesting food contaminated with heavy metals is uncommon, but chronic exposure to these metals may result in undesirable toxic effects. In our study, three types of heavy metals, namely arsenic, cadmium and mercury, are covered. They are chosen for study among the various types of heavy metals mainly because of their relatively pronounced toxicities. Inorganic arsenic, the toxic form of arsenic, is a human carcinogen whereas cadmium can affect renal function. Mercury is a toxic chemical, particularly in its organic form, which affects particularly the nervous system.


5. The level of dietary intake of the selected contaminants (i.e. dioxins, arsenic, cadmium and mercury) by secondary school students, which are the target population of our study, is evaluated by assessing the average level of these contaminants contained in the target food groups and the amounts of those foods consumed by the students. We completed a food consumption survey in late 2000 to obtain the food consumption data on a list of commonly consumed food items by secondary school students. Based on the data collected during the survey, the dietary patterns of 903 secondary school students from 27 secondary schools are analyzed in our study. As regards the estimation of the contaminant levels in the food items, we have made use of the data collected under our food surveillance programme between 1999 and 2001. The dietary exposure data thus computed are then compared with international reference on safety intake levels, and the adverse effects likely to occur in the target population are estimated.



6. The World Health Organisation (WHO) recommended a tolerable daily intake (TDI) of 1-4 TEQ pg/kg of body weight (bw) for dioxins in 1998. TDI is the amount of a toxic substance, expressed on a body weight basis, which an individual may ingest daily over a lifetime without appreciable risk to health. It stresses on lifetime exposure. Occasional short-term excursions above the TDI would have no health consequences provided that the averaged intake over long period is not exceeded.

Heavy Metals

7. The Joint Food and Agriculture Organization (FAO) / WHO Expert Committee on Food Additives (JECFA) recommended the safe exposure levels in terms of Provisional Tolerable Weekly Intakes (PTWIs) for inorganic arsenic, cadmium and mercury at 15, 7 and 5mg/kg bw/week respectively. PTWI is an estimate of the amount of a chemical that can be ingested per week over a lifetime without appreciable risk.



8. The study reveals that the dietary exposure of an average secondary school student to dioxins is 0.85 pg TEQ/kg bw/day. This level of intake is well below the TDI as recommended by WHO in 1998, thus implying that the average secondary school student in Hong Kong would not experience major toxicological effects of dioxins. Meat, poultry and their products are identified as the major dietary sources of dioxins. Dioxin concentration in milk is not high but the relatively large consumption amount renders it an important dietary source of dioxins. Conversely, although the dioxin concentration in eggs is high, its low consumption level makes it a less significant contributor to total dioxin exposure.

Heavy Metals

9. The estimated dietary exposure of an average secondary school student to inorganic arsenic, cadmium and mercury are 2.52, 2.49 and 2.98 mg/kg bw/week respectively. Major toxicological effects arising from dietary exposure of the secondary school students to these heavy metals are not anticipated as the estimated dietary exposure levels are below the PTWI established by JECFA. Cereal and cereal products are identified as the main source of mercury in the diet. For cadmium and inorganic arsenic, food items falling under the food group "seafood other than fish", in particular shellfish, are the main dietary sources.


10. Similar to dietary exposure studies conducted by other countries which all have their inherent limitations such as time and resources constraints, our study, as the first of its kind ever conducted in Hong Kong, also has a number of limitations. As we have made use of existing data collected from the food consumption survey and the food surveillance programme to compute the dietary exposure, the following limitations have to be noted:

  1. The use of food surveillance data in estimating the contaminant concentration levels of food items might produce biased results on the high side. It is because our food surveillance programme adopts a risk-based approach. As a result, food samples chosen for testing would tend to contain higher level of contaminants.

  2. The dietary consumption pattern of secondary school students is obtained through a food consumption survey using self-administered food frequency questionnaires. Although the questionnaire is very comprehensive, some food items may be missed out. This may lead to underestimation of the dietary exposure.

  3. The limits of detection (LOD) adopted in the testing of food samples under the food surveillance programme are for enforcement purpose and are therefore usually set at relatively higher levels. This poses a problem when we use the results of the food surveillance programme in estimating the actual concentration of the contaminants in our study as the concentration might lie anywhere between 0 and LOD. Such problem is most obvious for those food groups with the majority of food samples below the LOD.

11. While it would be ideal to collect very comprehensive data to enhance the accuracy of dietary exposure estimates, we are mindful of the long time needed (easily taking several years) and costs involved. A simplified, yet scientifically adequate approach using existing data like our study, is able to provide information on our dietary exposures within a short period of time, providing early alert if actions need to be taken promptly. Despite the limitations of the study, these have not affected the overall findings and conclusions which compare favourably with those of other countries. It is prudent to conclude that an average secondary school student in Hong Kong would not experience major toxicological effects of dioxins or heavy metals studied.


12. The findings of the study have provided us with useful and reliable initial information on the major dietary sources of dioxins and heavy metals, and their risks posed to the population arising from dietary exposure. To monitor the trend of the exposures and produce more accurate estimates, we will explore the possibility of conducting a population-based food consumption survey so that population-wide dietary exposure studies can be conducted in the future. In addition, we plan to use our newly established Food Research Laboratory in support of any further dietary exposure studies. In particular, we will employ analytical methods to detect contaminants in food samples at much lower concentrations. This will enable us to obtain a more accurate assessment on dietary exposure to food contaminants which will in turn contribute to the overall planning of our food control strategy. We will also continue to monitor closely the international development on regulatory standards of dioxins and heavy metals, and to ensure that the foods available in Hong Kong, especially those food items that may contain higher level of contaminants, are fit for human consumption.


13. A balanced diet is essential to avoid excessive exposure to contaminants that are concentrated within a small range of food items. Since dioxins are mainly present in the fatty parts of foods, to lower their intake consumers are advised to consume low-fat products, to trim fat from meat and meat products, to reduce the amount of animal fat used in food preparation and to use cooking methods that can reduce fat. Vulnerable groups such as children and pregnant women should be particularly careful in the selection of foods. They are advised not to consume excessive amount of predatory fish such as sharks and tuna which may contain higher concentration of mercury. Consumers are also advised not to overindulge in shellfish as they tend to contain higher concentration of arsenic and cadmium.


14. We will later publicize the findings of the study through various channels to advise the public of the risk factors concerned and the ways to reduce the possible risk. We will also report the findings of the study to the LegCo Panel on Food Safety and Environmental Hygiene. The reports of the study will be uploaded onto the website of FEHD ( and will be available at the Communication Resource Unit, major public libraries as well as the Health Education Exhibition and Resources Centre of FEHD.

Food And Environmental Hygiene Department
October 2002

1 The toxicity of dioxins, expressed in toxic equivalence (TEQ), is measured relative to the toxicity level of the most toxic dioxin, known as TCDD.

Annex 1: Executive Summary - Dietary Exposure to Dioxins of Secondary School Students

Annex 2: Executive Summary - Dietary Exposure To Heavy Metals of Secondary School Students