Let's assume that pretty much every smoker in the U.S. knows cigarettes are bad for them. And let's that assume that "bad for them" is understood as likely to kill them. Someday. But in the meantime, before that"someday" happens, millions of people continue to smoke, until for more than 400,000 Americans, someday becomes today. And that's just death - according to the CDC, about nine million Americans suffer some 13 million smoking-attributable health problems every year. No wonder the U.S. spends more than $75 billion in smoking-related health costs every year.

What we're left with, then, is a very clear picture that people continue to smoke even when they know it's bad for them - yet more evidence that, as I've said before, behavior change is hard.But what's especially surprising is when the harms caused by smoking hurts not just the smoker, but others. I'm not talking second-hand smoke. I'm talking real dangers. Dangers not evident someday, but very much in the here and now.

Today, the CDC offers some staggering evidence on that score: Two reports in the MMWR that demonstrate just how hard it is to quit smoking - even when it's somebody else's health at risk, not your own.

First, there's a study on smoking prevalence among women of child bearing age in the U.S. Aside from the familiar yet unsettling statistic that 22.4% of women 18 to 44 years old are smokers, there's this unnerving stat: among smokers aged  18 to 24 years old, a staggering 68% have tried to quit. But only 26% have been successful. Ouch. That means that even when these potential mothers are trying to stop smoking, they have only a 1/3 chance of success. That success rate gets better among older groups - by ages 36 to 44, about 46% of smokers manage to quit. But even so, that's a horrible success rate. And one can presume that the flip side - the majority of smokers can't quit - means that there are a whole lot of women smoking right through their pregnancy. When, one imagines, they're most motivated to quit, because it isn't their health on the line, it's their unborn child's as well.

If that's bad, this is worse: another study in the MMWR carries the gripping title, "Fatal Fires Associated with Smoking During Long-Term Oxygen Therapy." Yes, that's right: a study about how many oxygen tanks exploded because the people using them couldn't keep from lighting up. The study looks at fires in four states (Maine, Massachussetts, New Hampshire and Oklahoma), and finds some 38 fires from 2000 to 2007 can be attributed to exploding oxygen tanks due to cigarettes. Remarkably, only 37 people died in these fires, and only three of those weren't the smokers themselves. Nonetheless: can there be a more clear sign that cigarette addiction drives people to act in self-destructive ways - not only by continuing to smoke, but to do so at the risk of blowing themselves up? (I should note, these fires are a mere subset of  the number of fatal fires caused by cigarettes - which are the leading cause of residential fire deaths in the U.S., according to the CDC report).

Here's why I keep on about behavior change, and why smoking is a central illustration of the challenges involved: There's a lot of talk out there these days, including by me and here on this blog, about preventive medicine. The line goes that once we learn our risks - genetic, environmental, and otherwise - we'll be able to take action and change our lives, adjusting our behaviors to ward off those risks. It sounds good, and I continue to believe that preventive medicine will bring us to a more efficient, more effective, and more responsive healthcare system. We'll live smarter and longer and better.

But if we want to know the stakes involved in expecting millions of Americans to start living better, smokers offer perhaps the clearest example of how challenging this will be. After all, no behavior has better evidence for negative repercussions, no group is better informed about the health risks involved in their behavior, and no activity carries such a clear upside for the public health. 400,000 people die every year when they needn't have, if they'd just acted differently. And yet: the evidence shows that it isn't easy. People will still misbehave when they know it's against their own interests. People will misbehave when they're putting their children's health at risk, too.

And - holy cow - people will misbehave even when they're going to blow themselves up.

Photo courtesy Larry Taylor.