Cotinine levels in blood are often used to measure exposure to tobacco smoke.

VOA: special.2009.01.13

"We found that people with high exposure to second hand smoke had increased levels of depression and anxiety."

VOA: standard.2010.06.15

So in other words, if you live with a smoker, you really are at very high risk of demonstrating high levels of second-hand smoke exposure."

VOA: standard.2010.06.15