tobacco), or a variant of the external substance processed by the body (a metabolite) that usually can be quantified. history the widespread use of the term "biomarker" dates back to as early as 1980. the manner in which the environment was monitored and studied near the end of the 1980s was still mainly reliant on the study of chemical substances that were considered dangerous or toxic when found in moderate concentrations in water, sediments, and aquatic organisms. the methods used to identify these chemical compounds were chromatography, spectrophotometry, electrochemistry, and radiochemistry. although these methods were successful in elucidating the chemical makeup and concentrations present in the environment of the contaminants and the compounds in question, the tests did not provide data that was informative on the impact of a certain pollutant or chemical on a living organism or ecosystem. it was proposed that characterizing biomarkers could create a warning system to check in on the well being of a population or an ecosystem before a pollutant or compound could wreak havoc on the system. now, due to the development of biomarker studies, biomarkers can be used and applied in the fields of human medicine and in the detection of diseases. definition the term "biological marker" was introduced in 1950s. in 1987, biological markers were defined as “indicators signaling events in biological systems or samples” that could be classified into three categories: exposure, effect and susceptibility markers. in 1990, mccarthy and shugart defined biomarkers as, “measurements at the molecular, biochemical, or cellular level in either wild populations from contaminated habitats or in organisms experimentally exposed to pollutants that indicate that the organism has been exposed to toxic chemicals, and the magnitude of the organism's response”. in 1994, depledge defined a biomarker as, “a biochemical, cellular, physiological or behavioral change which can be measured in body tissues or fluids or at the level of the whole organism that reveals the exposure at/or the effects of one or more chemical pollutants.” in 1996, van gestel and van brummelen attempted to redefine biomarkers to unambiguously differentiate a biomarker from a bioindicator. according to van gestel and van brummelen, a biomarker by definition should be used only to describe sublethal biochemical changes resulting from individual exposure to xenobiotics. in 1998, the national institutes of health biomarkers definitions working group defined a biomarker as "a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention." in 2000, de lafontaine defined the term biomarker as a “biochemical and/or physiological change(s) in organisms exposed to contaminants, and thus represent initial responses to environmental perturbation and contamination". active biomonitoring de kock and kramer developed the concept of active biomonitoring in 1994. active biomonitoring is a comparison of the chemical/biological properties of a sample that has been relocated to a new environment that contains different conditions than its original environment.