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為何食物 “超標” 但不會危害健康？
為何食物 “超標” 但不會危害健康？
曾耀源
(廠商會檢定中心)
當食物被驗出有害化學物質含量超出法定限值時，政府或會補充一句：「但按樣本驗出的化學物含量，在正常食用情況下，並不會對健康構成危害。」。
不少市民可能感到奇怪，既然食物已 “超標” (例如每公升食水含鉛量超出10微克；菜含有農藥並超出最高殘餘限值；魚被驗出零容許的孔雀石綠)，為何政府卻說對健康沒有影響？上月筆者擔任美食博覽食品安全演講嘉賓時，便有觀眾朋友提出這一個疑問。
簡單來說，這是因為決定食品是否 “合法”，我們是以 “限量標準值” 作判斷；但要評估問題食品的 “食用風險”，我們則要看有害物質的 “安全參考值”。
甚麼是限量標準值？
“限量標準值” 指法例規定不同食物中准許的化學物含量 (即法定限值)。以最近備受關注的食水為例，香港政府訂明飲用水准許的重金屬含量：
資料來源：政府新聞處, 2015
http://www.isd.gov.hk/drinkingwater/chi/faq.html
訂立一種有害化學物質的法定限值時需要考慮多個因素。例如，化學物天然存在於各食物的份量、化學物 (例如農藥) 在正確使用下無可避免殘留在食物的份量、檢測極限、國際標準或做法。此外，訂立限值時要作出 “前設” - 即使化學物超出預設限值數倍或以上，只要不過量進食，亦不會令人急性或慢性中毒 (即不超出安全參考值)。這才能有效保障市民健康。
法定限值的訂立可以方便監察及快速判斷食物是否 “可接受”。假如食物內有害物質超出法定限值，當然不能在市場上銷售。除非 “前設” 出錯 – 例如食品含化學物的份量遠超預計、過量進食問題食品等，否則在大多數的情況下，進食問題食品一般不會造成健康問題。
甚麼是安全參考值？
若要評估進食含有害物質的食品會否影響健康，我們要先計算攝入量（即有害物質在食物的份量乘以有關食物的進食量），然後與安全參考值比較。
安全參考值有很多種。大家可能都聽過“每日/每周/每月可容許攝入量”，它們被用作評估進食某化學物後帶來的慢性或長期健康影響。這些參考值指人一生中每日/每周/每月平均可攝入某一化學物而不致帶來可見健康風險的估計分量。由於容許攝入量是以人一生的攝入量為計算基礎，只要平均攝入量並非長期超出這一數值，偶然高於可容許攝入量也不會影響健康。
若要評估進食某化學物後會否帶來急性或即時不良反應，我們則會以 “急性參考劑量” 作為一段短時間 (例如一日) 內安全攝入該化學物的參考值。
安全參考值是由國際權威組織訂立，他們會以毒理學角度從動物實驗推算數值。例如，聯合國糧食及農業組織 / 世界衞生組織聯合食品添加劑專家委員會認為每日鉛攝入量只要低於每公斤體重1.2微克，便不會對成年人的健康構成危害。
結語
“限量標準值” 和 “安全參考值” 是兩種不同的概念，大家切莫混淆。一旦食物內有害物質超出法定限值，我們便知道該食物 “不合格”；但要進一步判斷進食問題食品後會否令人急性或慢性中毒，則要視乎攝入有害物質的總量是否超出“安全參考值”。
