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How Come Food Exceeding Legal Limits Do Not Cause Adverse Health Effects?

How Come Food Exceeding Legal Limits Do Not Cause Adverse Health Effects?

YY TSANG

(CMA Testing and Certification Laboratories)



When food is detected with a harmful chemical substance exceeding the legal limit, our government may add a statement “...However, based on the level of the chemical detected in the sample, adverse health effects will not be caused under normal consumption”.


Many citizens may wonder why the government says so. How come food with chemicals exceeding the legal limits (e.g. drinking water with lead exceeding 10 mg/L; vegetable with pesticide exceeding the maximum residue level; fish detected with prohibited malachite green) will not affect our health? When I gave a food safety talk at Food Expo last month, an audience asked me about this.


Simply speaking, this is because we use “legal limit” to judge whether a food is “lawful” or not; while we use “safety reference value” to evaluate the risk of consuming a problematic food.



What are Legal Limits?

Legal limits, or end-product standards, refer to the amounts of chemicals allowed in different foods stipulated under the law. Using drinking water as an example, our government has specified the maximum levels of different heavy metals allowed in drinking water:

Source: Information Services Department, 2015

http://www.isd.gov.hk/drinkingwater/eng/faq.html



In order to establish a legal limit of a harmful chemical substance in food, several factors shall be considered. For example, amount of the chemical naturally present in different food, residues of the chemical (e.g. pesticides) left in food even after a proper usage, detection limits, international standards or practices. Moreover, an “assumption” shall be made – even if the actual amount of the chemical exceeds the specified legal limit several times or more, no acute or chronic toxicity will be observed provided that food consumption is normal (i.e. not exceeding the safety reference value). This is essential to effectively protect our health.


Establishment of a legal limit facilitates monitoring and evaluating whether food is “acceptable”. If food contains excessive harmful substances, of course it cannot be sold. Unless the “assumption” made is wrong (e.g. amount of the harmful substance in food is much higher than expected, over-consumption of problematic food, etc.), otherwise in most cases ingestion of problematic food will not result in health consequences.



What are Safety Reference Values?

In order to know whether consumption of a problematic food containing harmful substances will lead to adverse health impacts, we need to calculate the total intake (i.e. amount of the harmful substance in food x consumption amount) first, and then compare it with its safety reference value.


Several safety reference values can be used. For assessment of chronic or long term health impacts due to consumption of a certain chemical, we can use reference values such as tolerable daily/weekly/monthly intake that you have heard about. They refer to the amount of a chemical that can be ingested by a person over a period of time without appreciable health risk. Since they are established based on lifetime intake, an occasional exposure above these values would not affect health provided that the average intake does not continuously exceed the chronic reference value.


On the other hand, if we want to evaluate acute or immediate toxic effects due to consumption of a problematic food, “acute reference dose”, a reference for safe ingestion of a chemical over a short period of time (e.g. 1 day) shall be used.


Safety reference values are established by prestigious international organizations, using toxicological principles to predict the values from animal test results. For example, Joint FAO/WHO Expert Committee on Food Additives (JECFA) opined that health impact for adult is considered negligible when the exposure level is lower than 1.2 mg/kg body weight/day.



Epilogue

Legal limits and safety reference values are two different concepts, and should not be mixed up. If the level of a harmful substance in food exceeds the legal limit, the food is concluded as “unacceptable”. However, in order to further tell whether consumption of the problematic food will lead to acute or chronic toxic effects, total intake of the harmful substance must be calculated and compared with its safety reference value.



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