How do doctors or investigators tell whether a chemical actually got into someone's body? Biomonitoring is the measurement of chemicals, their breakdown products, or other biological markers in human tissues or fluids such as blood, urine, breath, hair, or breast milk. It is used to show exposure from the inside out, rather than only measuring what was present in the air, water, soil, or workplace. Depending on the substance, biomonitoring may help identify recent exposure, long-term accumulation, or a body's response to a toxic agent.

In practical terms, biomonitoring can strengthen or weaken a toxic exposure claim because it may connect an alleged source of harm to a person's actual body burden. That can matter when proving causation, especially in cases involving solvents, heavy metals, pesticides, or industrial chemicals. It can also help separate background exposure from unusually high exposure tied to a job site, apartment building, or contamination event.

For an injury claim, biomonitoring is usually one piece of the larger record, alongside medical findings, workplace records, expert opinions, and environmental testing. A positive test does not automatically prove illness, and a negative test does not always rule exposure out if the chemical leaves the body quickly. In New York, fault can still be divided under pure comparative negligence in CPLR §1411 (1975), but biomonitoring often becomes central to proving exposure and damages in the first place.