Think you're safe from lung cancer because you don't smoke? Here's a fun fact: Secondhand smoke is estimated to account for at least 35,000 deaths from heart disease and 3,000 deaths from lung cancer in nonsmokers nationwide each year. And a recently-published study suggests that New Yorkers are even more at risk because our dense urban environment results in a greater exposure to secondhand smoke.

The study, conducted in 2004—one year after the city's smoke-free air law took effect—used blood tests to determine that 57% of adult non-smoking New Yorkers (2.5 million) have elevated cotinine levels, compared to 45% of non-smoking adults nationwide. Agh, what's cotinine!? Well, the DOH says it's a "harmless" by-product of nicotine breakdown, which, in a perfect world, wouldn't be in your system if you don't smoke cigarettes. At the time of the study, NYC had fewer smokers per capita than most other American cities; only 23.3 percent of adults were smokers, compared with a national average of 29.7 percent.

Health commissioner Dr. Thomas Frieden tells the Times, "This is not what we expected. It is a shocking number." Harvard tobacco expert Dr. Jonathan Winickoff thinks it has to do with apartment dwellers who are exposed to smoke drifting from one unit to another within a building: "Smoke doesn’t know to stop at a doorway. It fills the full capacity of every indoor location in which the cigarette is smoked." Asian adults in NYC were the most likely to have elevated cotinine levels (69%); studies show that Asian Americans, particularly Chinese Americans, are more likely to smoke at home. And lower-income New Yorkers were also more likely to be exposed than those with higher incomes (63% versus 54%).

The city's Department of Health and Mental Hygiene has a lot more on alarming info on secondhand smoke, and some useful tips [PDF] on how to politely but firmly turn your home into a smoke-free zone. These include posting a "Smoke-Free Home" sign on your door and letting people know you’re "rejecting the smoke, not the smoker."