Benign tumor A tumor that grows only at the site of origin; and does not invade adjacent tissues or metastasize.  It is generally treatable. Bias Systematic error that may be introduced in sampling by selecting or encouraging one outcome over another. Bioaccumulation The build-up of a substance in a biological organism such that the level in the organism is greater than in environmental source of the substance.  same as Bioconcentration. Bioactivation The metabolic process whereby a parent substance is chemically changed to a daughter substance with enhanced biological activity. Bioassay A laboratory study used to determine the ability of a substance to produce a particular biological effect. Bioavailability The physical and/or biological state of a substance rendering it capable of being absorbed into the body. Bioconcentration see Bioaccumulation Biodegradation Breakdown of a chemical into smaller less complex molecules by microorganisms in environmental media (e.g., soil, water, sediment). Biological Half-Life The time required to eliminate one-half the quantity of  a substance from the body. Biomarker Indicators of events occurring in biologic systems due to a xenobiotic.  The types of indicators are exposure, effects, or susceptibility. Biotransformation Conversion of a chemical from one form to another by a biological organism. Body Burden The concentration of a substance which has accumulated in the body. Bone Marrow The tissue within the internal open space of bones (e.g., shaft of long bones) in which the blood-forming elements exist. Bronchioles The very small branches of the tracheo-bronchial tree of the respiratory tract which terminate in the alveoli.