free radicals

It is said that while 97–98% of oxygen converts nutrients generated within the body into energy and performs beneficial functions for the human body, 2–3% of oxygen becomes reactive oxygen species that adversely affect cellular metabolism. Excessively generated reactive oxygen species are said to destroy the skin's antioxidant defense network and break down collagen and elasticity—key components of skin elasticity—thereby reducing skin firmness and becoming the main culprit behind skin aging.