Table 2. Biomarkers as an objective measurement of exposure to the tobacco smoke, characteristics, and advantage/disadvantage of each biomarkers.

Biomarker	Exposure status	Advantages	Disadvantages
Urine	Recent exposure	Noninvasiveness Higher sensitivity Advantage in investigating the recent exposure to tobacco smoking on the human health	Need for creatinine clearance adjustment for hydration
Serum	Recent exposure	No need of adjustment Advantage in investigating the recent exposure to tobacco smoking on the human health	Invasiveness Fast clearance rate in late pregnancy Lower sensitivity
Saliva	Recent exposure	Noninvasiveness Convenience Multiple measurement with short-term interval	Various influencing factors; oral pH, diet, dehydration, drug, age, race, gender Lower sensitivity
Dried blood spot	Recent exposure	Easy to ship and store Advantage in a population-based study	Need for further research
Hair	1 cm of proximal hair reflects the last month's ETS exposure	Noninvasiveness Convenience Advantage in investigating long-term, cumulative hazard of tobacco smoking on the human health	Various influencing factors; hair dyeing, age, gender, or race
Toenails	1 mm of toenails reflects the last month's ETS exposure	Convenience Advantage in investigating long-term, cumulative hazard of tobacco smoking on the human health	Need for further research

ETS, environmental tobacco smoke.