For markers of intermediate health outcomes, the most important property is the positive predictive value for clinical outcomes of interest. Unless the marker has a known relationship with disease, a marker response conveys no information about disease risk.
Most markers are nonspecific in that various exposures cause the same marker response.
Although nonspecificity can be an asset in studies of complex mixtures, it leads to problems with confounding and dilution of exposure-response associations in the presence of other exposures. Noninvasive markers, such as those measured in urine, blood, or nasal lavage fluid, are generally more useful for field studies than are invasive markers. However, invasive markers, such as those measured in bronchoalveolar lavage fluid or lung specimens from autopsies, provide the most direct evidence of pulmonary damage from exposure to air pollutants. This manuscript was prepared as part of the Environmental Epidemiology Planning Project of the Health Effects Institute, September 1990 - September 1992.