Table 1.

Comparison of Bottom-up versus Top-down Exposomics12,17.

	Bottom-up Exposomics	Top-down Exposomics
Measurements	“External” environment	“Internal” environment
Analysis performed	Direct measurement of external exposures to determine associations between exposure(s) and health outcome(s)	Direct biological measurement of exogenous and endogenous analytes to determine intermediate biomarkers associated with health outcome(s)
Follow-up studies	Targeted toxicological and metabolism studies	Targeted search for associations between biomarkers and retrospective external exposures
Major advantages	Enables direct link between exposure and health outcome for faster implementation of abatement strategies	Includes: exogenous measurements; endogenous measurements that reflect biological response of genetic susceptibility or of transient exposure from chemicals with short biological half-lives; and includes measurement of reactive compounds