Biomarkers are convenient tools to characterize 1) specific human exposure to air pollution, 2) responses of the human organism to exposure, and 3) individual variations of both exposure and response. This paper presents two effect monitoring tests as recent examples to study human responses to exposure and the application of the tests in environmental .

epidemiology.

The common feature of these test methods is the use of the stable isotope tracer technique as a basic working principle. One of the methods, the in vivo [^N]methacetin liver function test, has already been approved for use on humans to characterize SO] load effects on the detoxifi...