Biomonitoring is a scientific technique for assessing human exposures to natural and synthetic chemicals, based on sampling and analysis of an individual's tissues and fluids. While blood, urine, breast milk and expelled air are most commonly measured, hair, nails, fat, bone and other tissues may also be sampled. This technique takes advantage of the knowledge that chemicals that have entered the human body leave markers reflecting this exposure. The marker may be the chemical itself. It may also be a breakdown product of the chemical or some change in the body that is a result of the action of the chemical on the individual. For example, alterations in the levels of certain enzymes or other proteins may serve as markers; so too might modifications of normal body processes, such as the transfer of oxygen to tissues.