Chemicals and their metabolites can be detected in a variety of biological substances such as blood, urine, exhaled air, hair, nails, feces, semen, breast milk, or saliva.  Blood and urine are the most commonly used in occupational safety and health.
Breast milk is a favored matrix (substance) to measure lipophilic (fat-loving) persistent, bioaccumulative, and toxic (PBT) compounds during lactation; this exposure route is dominant for breastfeeding children. A lipophilic compound might also be detected in blood, while a hydrophilic (water-loving) compound might be detected in urine.
Analytical methods used by the CDC include isotope dilution mass spectrometry, inductively coupled plasma mass spectrometry, or graphite furnace atomic absorption spectrometry.  Others include gas chromatography or high-performance liquid chromatography coupled with various detectors such as ultraviolet, electron capture, flame ionization, atomic emission, or mass spectrometric detectors.  Ligand-binding assays and immunoassays are also used.
As biomonitoring necessarily involves working with human subjects and specimens, biosafety procedures are necessary to prevent the transmission of pathogens.