Xenobiotics are substances that are foreign to the human organism.  

Examples are drugs, food additives, tobacco, chemicals in toiletries and cosmetics as well as general environmental toxins.  These can have an additional impact on wellbeing and longevity.  Some such as smoking and environmental toxins are clearly harmful.  Others, such as nutritional supplements can be positive, negative or neutral.  

Hopefully we can simplify here how to make positive choices when shopping for skincare products, deciding on multivitamins and the evidence base for specific nutritional supplements (some are better than others).