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Food Safety Focus (18th Issue, January 2008) – Incident in Focus

Heavy Metals in Dried Oysters

Reported by Ms. Joan YAU, Scientific Officer,
Risk Assessment Section, Centre for Food Safety


In December 2007, a local media conducted a survey on heavy metals in oysters and dried oysters. It was found that the cadmium levels of two oyster samples from Lau Fau Shan were 2.68 ppm and 4.19 ppm while the cadmium levels of four dried oyster samples ranged from 2.37 to 4.4 ppm. The chromium levels of the two dried oyster samples were 1.03 and 1.15 ppm. According to the Food Adulteration (Metallic Contamination) Regulations, Cap. 132V, the maximum permitted concentrations for cadmium and chromium in oysters are 2 ppm and 1 ppm respectively. Questions have been raised whether the levels of cadmium and chromium in the dried oyster samples exceeded the legal limits.

Illustration: Oysters: fresh (left); after sun drying (right).

What are the International Practices in Interpreting Maximum Permitted Concentrations for Heavy Metals in Dried Food?

Heavy metals and a number of environmental contaminants are ubiquitous in the nature. While their total amounts in food may not change upon processing, their levels may be either concentrated or diluted. In light of this fact, the Codex Alimentarius Commission (Codex) has recommended in its General Standard for Contaminants and Toxins in Foods that "in general however, maximum levels should preferably be set for primary agricultural products and may be applied to processed, derived and multi-ingredient foods by using appropriate factors" to take into account the effect of processing. This approach has been adopted by countries including Australia and member countries in the European Union, and is considered reasonable and scientifically sound from both risk assessment and risk management perspectives.

In other words, when interpreting the laboratory result, an appropriate factor has to be applied to the test result wherever indicated so as to obtain the "original" level of the contaminant present in food before processing that the relevant legal limits are set against.

How Can We Derive the Appropriate Factors for Comparison with the Legal Limits?

The "original" level of a heavy metal for a processed food which has been dried, dehydrated or concentrated prior to drying, dehydration or concentration can be determined using the following formula:

Food Item
Water Content in Unprocessed State
Water Content in Processed State
轉換系數 *
Conversion Factor*
克/每 100克
g/100g edible portion
克/每 100 克
g/100g edible portion (%)
鮑魚 Abalone 68.6b – 77.5a 18.3a 0.28 – 0.38
淡菜 Mussel 79.9a 15.6a 0.24
鱆魚 Octopus 79.2a – 84.9b 24.8a 0.20 – 0.28
蠔 Oyster 82.0a – 87.1a 13.1b 0.15 – 0.21
元貝 Scallop 84.2a 27.4a 0.22
海參 Sea cucumber 77.1a,b 18.9a 0.28
蝦 Shrimp 73.6a – 80.6a 13.8b – 25.0b 0.23 – 0.35
魷魚 Squid 80.4a – 82.2b 21.8a 0.23 – 0.25
香菇 Shitake mushroom 91.6b – 91.7a 12.3a – 13.2b 0.09 – 0.10
黑木耳 Black wood ear 89.8b 11.4b – 15.5a 0.12

The "conversion factor" can be derived from:

  1. laboratory test result of water content of a food sample before and after drying, dehydration or concentration; and / or
  2. generally accepted data (e.g. food composition database) regarding the water content of the processed food and its unprocessed counterparts.

Examples of conversion factors for selected dried groceries are illustrated below:


  1. China Food Composition 2002
  2. ASEAN Food Composition Table

It is important to note that water content of a food sample before and after dehydration may vary with a number of factors including species, seasons, geographical locations, processing requirements, etc. Therefore, direct laboratory analysis of the water content of food sample before and after processing would provide a better estimate of the "conversion factor". If the "conversion factor" is derived from generally accepted data, the trade should ensure that the data sources are accurate and reputable.

The Centre for Food Safety (CFS) has followed the above Codex's recommendation in assessing whether the level of heavy metal in a food item has exceeded the legal limit. By applying the "conversion factors", the reported cadmium and chromium levels in the four dried oyster samples are therefore within the legal limits in Hong Kong .

Can the Above Approach Ensure Safe Food Supply in Hong Kong ?

Legal limits stipulated in law are often confused with the respective safety reference values such as the Provisional Tolerable Weekly Intake (PTWI) established by the Joint Food and Agriculture Organization (FAO) / World Health Organization (WHO) Expert Committee on Food Additives (JECFA). The law states the amounts of chemicals allowed in different foods for monitoring and maintaining food standards. Consuming food products with chemicals exceeding the legal standards does not automatically imply that the consumer's health is at risk. The CFS will take into account the level of the contaminant in the food and the average quantity of the food consumed when assessing the overall safety of the sample and the associated health effects of the metals detected.

Advice to the Trade

The trade should ensure the foods they sell or import are fit for human consumption and comply with the legal standards.