After some harmful substances enter the human body, they will be metabolized and discharged, which has little effect on the human body. Such as rotten fish and shrimp, are naturally discharged through diarrhea.
After some harmful substances enter the human body, they are scattered to all parts, and only when they reach important organs can they endanger human health. For example, entering the muscle has relatively little influence. What enters the liver can have a serious impact.
However, harmful substances are not missiles. They specifically attack important organs and need to be accumulated continuously. Symptoms (qualitative change) will only occur when the vital organs reach a certain amount (threshold).
Therefore, this sentence makes scientific sense.
Similarly, A/B, chemical substances are divided into chemical purity, analytical purity and chromatographic purity according to the content of impurities (which can be understood as harmful substances). It is also divided into chemical grade and food grade.
Food-grade additives are less harmful to human body, as long as the intake is controlled.