Excerpted from The Norm Chronicles: Stories and Numbers About Danger and Death by Michael Blastland and David Spiegelhalter. Out now from Basic Books.

The hazards of life can be instant, like violence or accidents, the kind that hit us over the head with a swift goodnight. But lifestyle is a more sinister threat, another type of mortal hazard with slower effects that go stealthily into the blood one cancerous bacon sandwich or poisonous drink at a time, potential killers by degrees that might catch up with us later in life, as something surely will.

The first mortal hazard—the quick one—is known as acute risk; the second is chronic. Murder with a chainsaw is an acute risk, obesity a chronic one that takes time to do its worst. Of course, the same hazard might be both: too much booze can do you in quickly when you fall under a bus, or slowly stew your liver. But in general it helps to separate them.

To quantify the daily impact of chronic risks, such as obesity, we use a little device we have called the MicroLife, or ML. Here’s how it works.

Imagine the duration of your adult life divided into 1 million equal parts. A MicroLife is one of these parts and lasts 30 minutes. It is based on the idea that as young adults we typically have about 1 million half-hours left to live, on average.

Sounds unimpressive. But we like the MicroLife. It is a revelatory little thing. It brings life down to a micro-level that’s easy to think about and compare: half-hour chunks, of which we have 48 a day. Think of it as your stock of life to use up any way you choose, 1 million micro-bits of a whole adult life, each worth half an hour, yours to spend. Watching the World Series? Bang, 6 MicroLives gone, just like that, never to have again.

So the simple passing of time uses up MicroLives. Every day we get up, move around, stuff tasty things into our bodies, discharge smelly things out of our bodies, and go to bed—perhaps with the thought, if we’re gloomy, that there go another 48 MicroLives from our allotted span.

But extra MicroLives can also be used up by taking chronic risks. So although time passes to its own beat, our bodies can age faster or slower according to how we treat them. If we jump around more, and stuff less or better, how much can we slow the steady tick-tock toward disease, decrepitude, and death? And if we indulge and allow ourselves to be couch potatoes, how fast might our own clocks run?

In other words, MicroLives can measure how fast you are using up your stock of life, faster or slower depending on the chronic risks to which you’re exposed. If your lifestyle is chronically unhealthy, you’ll probably burn up your allotted MicroLives that much quicker, and die sooner, on average.

For example, lung cancer or heart disease often follows a lifetime of smoking, and subsequently reduces life expectancy—again not for everyone, but overall. Some people seem indestructible, smoke like a chimney, and drink like a fish, and never look the worse for it. But, on average, even if chronic risks don’t kill you straight away, they tend to kill you sooner than if you had avoided them. Again, if we count the bodies, we can estimate how many years are lost overall, whether to obesity, smoking, or sausages, and we can convert this loss of life into the number of MicroLives burned up by unhealthy living. Thus, exposure to a chronic risk equal to 1 ML shortens life, on average, by just one of the million half-hours that people have left as they enter adulthood.

It turns out that one cigarette reduces life-expectancy by about 15 minutes, on average, and so two cigarettes cost half an hour, or 1 ML. Four cigarettes are equal to 2 MLs.

The first two pints of strongish beer also equal about 1 ML. Each extra inch on your waistline costs you around 1 ML every day, 7 a week, about 30 a month, and so on. According to recent research, so does watching two hours of TV. An extra burger a day is also about 1 ML. We’ll reveal the calculations behind these risks in a moment.

We could simply add up all these MicroLives, half-hour by half-hour, to see roughly how much time, on average, you lose in total from whatever your lifespan might have been. But the end of life is often far away, like the end of the story, and a lost half-hour deferred until you are in your dotage hardly seems to count. As a doctor said in a British newspaper: “I would rather have the occasional bacon sarnie than be 110 and dribbling into my All-Bran.” But by thinking of exposure to chronic risks like an acceleration of the speed at which you use up your daily allotment of MicroLives, we can do something more vivid and immediate. We can show how much your body ages each day according to the chronic or lifestyle risks that you take.

Ordinarily, remember, we use up 48 MLs a day. But remember, too, that smoking 4 cigarettes burns an extra 2 MLs. So if you smoke 4 cigarettes in a day, you’ve used not 48 but 50 MLs that day. In other words, after a 24-hour, four-fag (to use the British slang) day, we could say that you are 25 hours older.

And that’s not a bad representation of what can happen biologically. Bodies do often age faster when we do bad stuff to them. Twenty cigarettes daily means you burn an extra 10 MLs a day, on average, or become 29 hours older with every 24 that pass, or move toward death five hours faster, every day.

By bringing chronic risk down to the small scale of what happens today, rather than thinking of it only as a life-size problem deferred, the MicroLife makes chronic risk a good deal more real and immediate.

But should it? You might argue with this. You might prefer to put off facing up to your lifestyle risks. You might argue that you shouldn’t be confronted with the payback until it actually occurs, late in life. On the other hand, it could be argued that the damage is done now, at the point of consumption, so we should measure it now.

Consider the 1 ML cost of an extra burger every day, mentioned earlier. This was reported in the British newspaper the Daily Express in a story about the dangers of red meat, based on a study from Harvard University. The Express said: “If people cut down the amount of red meat they ate—say from steaks and burgers—to less than half a serving a day, 10 percent of all deaths could be avoided.”

Oh to be one of the 10 percent for whom death could be avoided! But this is not what the study said. Its main conclusion was that an extra portion of red meat a day—this being a lump of meat around the size of a pack of cards or slightly smaller than a standard quarter-pound burger—is associated with a “hazard ratio” of 1.13—that is, a 13 percent increased risk of death. Put aside any doubts about the validity of this number for a moment and take it at face value. What does it mean? When our risk of death is already 100 percent, surely a risk of 113 percent is an exaggeration?

Let’s consider two friends—whom we’ll call Kelvin and Norm—who are both age 40, and just for the moment let’s make the unrealistic assumption that they are pretty much alike in most lifestyle respects that matter, apart from the amount of meat they eat.

Carnivorous Kelv eats a quarter-pound burger for lunch from Monday to Friday, while Normal Norm does not eat meat for weekday lunches, but otherwise has a similar diet to Kelvin’s.

Each person faces an annual risk of death, the technical name for which is their “hazard,” or, somewhat archaically but poetically, their “force of mortality.” A “hazard ratio” of 1.13 means that, for two people like Kelvin and Norm, similar apart from the extra meat, the one with the risk factor—Kelvin—has a 13 percent increased annual risk of death—not an overall risk, obviously—during a follow-up period of about 20 years.

This does not imply that his life will be 13 percent shorter. To work out what it really means, we have to go to the life tables provided by the U.S. Centers for Disease Control and Prevention. These tell us the risk that an average man—Norm, say—will die at each year of age. In 2008 this risk, or hazard, was at its lowest for those age 10, at less than one in 10,000; it then rises to one in 1,000 at age 19, then to one in 100 at age 59, until, at age 85, one in 10 will die before their 86^th birthday. Very roughly, the annual chance of death increases tenfold about every 27 years, which works out at doubling every nine years, or about a 9 percent extra risk of dying before the next 12 months are out, for every year that we are older. The tables also tell us life expectancy at any given age, assuming the current hazards, and, having survived to age 40, Norm is expected to live 38 more years, until he is 78.

From this we can work out Carnivorous Kelv’s prospects by multiplying all Norm’s hazards by 1.13. After a little work in a spreadsheet, we find that Kelvin can expect to live 37 more years, on average, a year less than Norm. So Kelvin’s lunch—if he eats the same lunch all his life and if we believe this hazard ratio—is associated with the loss of one year in expected age at death, from 78 to 77.

Is that a lot? Kingsley Amis, the English novelist and poet, said, “No pleasure is worth giving up for the sake of two more years in a geriatric home at Weston-super-Mare.” (You do not need to have an intimate knowledge of Weston-super-Mare to get the flavor of this comment.) It is for readers to decide. But we cannot say that precisely this amount of time will be lost. We cannot even be very confident that Kelvin will die first. In fact, there is only a 53 percent chance that Kelvin will die before Norm, rather than 50:50 if they eat the same lunch. Not a big effect.

But it sounds rather more important if we say that this lost year (about one-fortieth of the remaining life) translates very roughly to one week a year, or roughly half an hour a day: That’s one extra MicroLife burned up for each daily burger. So, unless you’re a very slow eater, you expect to lose more life than the time it takes to eat your burger.

But we can’t even say the meat is directly causing the loss in life expectancy, in the sense that, if Kelvin changed his lunch habits and stopped shoveling down the burgers, his life expectancy would definitely increase. Maybe there’s some other factor that both encourages Kelvin to eat more meat and leads to a shorter life.

Income could be such a factor—poorer people in the United States tend to eat more burgers and also live shorter lives, even allowing for measurable risk factors. But the Harvard study does not adjust for income, arguing that the people in the study—health professionals and nurses—are broadly doing the same job. We think that many of these studies about diet should be taken with a pinch of salt (although perhaps not too much, since it may increase your risk of heart disease).

So what about the booze? The precise effect of alcohol on all-cause mortality is controversial, since, although it can cause accidents (particularly for drivers and young binge drinkers), give you liver disease, and increase the risk of some cancers, it can also protect your heart. So the “dose-response” curve for annual risk is J-shaped in middle age, meaning that the risk falls slightly with the curve of the J as you consume a small amount of alcohol, then rises as you consume more. Very roughly, it looks like the first drink each day adds a MicroLife, but extra drinks take it away again, and more. So the first is medicine, the second poison, the third poison, and so on. It does not go medicine/poison/medicine/poison. That would be ridiculous, and anyway would mean that one would need to have an odd number of drinks.

All this is depressing, but what about the benefits of a good diet and hearty exercise—eating muesli and running? A recent review estimated a risk that translates to about 1 ML lost for every day that you are 5 kilograms (about 11 pounds) overweight. Seriously obese people can lose 10 years off their life expectancy, similar to smokers.

Both the U.S. and the U.K. guidelines recommend we all get 30 minutes of moderate or vigorous activity five days each week—a total of two and a half hours per week, or 22 minutes a day.

A huge review of 22 studies involving nearly 1 million people concluded that two and a half hours a week of “non-vigorous” activity was associated with a hazard ratio of 0.81 compared with being a complete couch potato—producing a 19 percent reduction in annual risk of death. This works out as about an hour per day, or 2 MLs, added to life expectancy, for an average of 22-minutes-a-day activity—quite a good return for the investment of getting off the sofa.

Excerpted from The Norm Chronicles: Stories and Numbers About Danger and Death by Michael Blastland and David Spiegelhalter. Out now from Basic Books.