Scientists have long understood that our bodies absorb substances, whether natural or man-made, from our environment. Today's technology allows us to be able to detect and measure extraordinarily low levels of many substances in human samples.
Substances may enter our bodies through eating, breathing, drinking and direct contact. After the substance enters the body and is distributed and metabolised, the body usually gets rid of it ('excretion'). The chemicals we are exposed to in our daily lives, including in cosmetics and foods, are so well-studied and measured that their combined effects are largely predictable and the cosmetic industry's assertions of the safety of its products are based on robust scientific data that adheres to strict safety guidelines.
In some cases, substances remain in the body in trace amounts. However, just because something can be detected in the urine or blood, doesn't mean it is going to cause us any harm. It is possible that all chemicals in our environment may be found in our blood, urine or tissues at some point in time.
In a similar fashion, analytical chemistry can now detect very small traces of many substances in products too. The safety assessment carried out for each cosmetic takes into account any traces to ensure they are not at a level that could possibly cause any harm