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Food Safety Focus (71st Issue, June 2012) – Incident in Focus

Understanding Pesticide Residues in Crops

Reported by Dr. Ken CHONG, Scientific Officer,
Risk Assessment Section,
Centre for Food Safety

In April and May, a green group announced a series of reports alleging that multiple pesticide residues, including some that were prohibited in the Mainland, were detected in vegetables, loose tea leaves and tea bags collected in the Mainland and Hong Kong, and considered pesticide abuse was a serious problem. What are the public health implications if pesticide residues are found in crops? In this issue, let's take a closer look into this subject.

Pesticide "RESIDUES" and Their Safety Assessment

To-date, the use of pesticides for effective pest management remains the prevalent practice of modern agriculture. The application of pesticides in a way strictly follows Good Agricultural Practice (GAP) can ensure the levels of pesticide residues in food as low as practicable. In general, Maximum Residue Limits (MRLs) stipulated in law are the maximum concentrations of pesticide residues to be legally permitted in or on food and agricultural commodities. They are set for individual pesticide-food pairs, established by field trials using GAP. Internationally, the Codex Committee on Pesticide Residues, under the Codex Alimentarius Commission, is charged to set standards including MRLs for pesticides in foods based on the recommendation of the Joint Food and Agriculture Organization (FAO) / World Health Organization (WHO) Meeting on Pesticide Residues.

Proper use of pesticides
Proper use of pesticides in accordance with Good Agricultural Practice (GAP) can reduce the levels of pesticide residues in food to minimal levels.

Regarding the safety aspect of pesticides, we will make reference to the safety reference values established based on scientific studies. The safety reference values are set based primarily on the toxic nature of the pesticide chemicals in animal models. The safety reference value of a pesticide is the amount of pesticide intake (per body weight) below which adverse health effects are unlikely to occur. The safety reference values usually have large safety margins and they also include benchmarks for prolonged exposure. For example, Acceptable Daily Intake (ADI) is a chronic safety reference value. The life-long consumption of food with pesticide residue below its ADI would be unlikely to cause adverse health effects. The WHO has set safety reference values for pesticides commonly used worldwide.

Pesticide residue level in a food item found exceeding its corresponding MRL does not automatically mean that the respective safety reference value would be exceeded. The dietary health risk of a pesticide residue is assessed by comparing the overall intake of the residue in food to the respective safety reference value. MRLs are very conservative estimates of pesticide residue levels in food based on GAP and the actual residual levels in food are often substantially lower than the MRLs. Occasional exposure to pesticide residue levels above their MRLs will not cause immediate health risk under normal situations. Simply speaking, food detected with pesticide residues below MRLs is safe for consumption.

Multiple Pesticide Residues

The presence of multiple pesticide residues in food may arise from many causes. For example, alternating use of pesticides of different chemical groups is often recommended to prevent the build-up of pesticide resistance. Different pesticides may also be applied at different growing stages of a crop for controlling different target pests. Both are normal agricultural practices likely to result in the detection of multiple pesticide residues in the final produce. From the risk assessment perspective, multiple pesticide residues should not raise health concern if their respective exposures are below their respective safety reference values.

As for the claim on "non-permitted pesticides", it should be noted that some pesticides are prohibited from use in certain crops but not in all kinds of crops, i.e. the prohibition applies to specific "pesticide-food" pairs. The pesticide residues detected in the samples concerned may not be a result of blatant violation of prohibition. Further investigation would be required to reveal the potential sources of contamination.

Follow-up Actions

With regards to the reports of the green group, the CFS had evaluated their testing results and took samples of vegetables from relevant vendors for testing of pesticide residues. All results were satisfactory. The CFS will liaise with the Mainland authorities for follow-up actions.

The CFS has in place vigorous surveillance programmes for food commodities to safeguard public health. In recent years, the results of the CFS's pesticide residues surveillance programme have been highly satisfactory. From January 2009 to March 2012, more than 57 000 samples were tested and only four unsatisfactory results were recorded. Nevertheless, there is absolutely no room for complacency. The Government has further stepped up its efforts to regulate pesticide residues in food. The Legislative Council has recently passed a pesticide residue in food regulation which provides comprehensive control of pesticide residues in food in Hong Kong. The regulation will come into operation on 1 August 2014 after a grace period of some two years.

Key Points to Note:

  1. The international communities allow the presence of pesticide residues in food at levels that are unlikely to pose risks to human health.
  2. The application of pesticides in a way strictly follows GAP can reduce the levels of pesticide residues in food to minimal levels.
  3. The Government introduced a regulation specifying MRLs for effective control of pesticide residues in food to further enhance the protection of public health.

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