dr. whyatt: you can get information about exposure by asking somebody. if i asked you how many cigarettes did you smoke in the last week you can probably tell me. if i asked you how much chlorpyrifos were you exposed to last week, you probably don't have any idea. probably don't even know what it is. but if i measure it in your blood -- so i take a blood sample and i measure that in your blood i know how much exposure you had. that measurement in the blood is a biomarker or a biologic marker. what it means is that, rather than asking you about your exposures i am measuring the exposure or the effect of the exposure in your body. and what we are doing is we are measuring the insecticide in the mother's blood at birth and in the baby's blood at birth. narrator: dr. deliang tang is in charge of the lab at columbia university where the blood samples are tested. these tests pick up even the smallest levels of exposure in the blood. each spike on this graph indicates a separate compound and the amount found in the sample. man: we're measuring a very, very trac