When Clarissa Dalloway thinks that it’s “very, very dangerous to live even one day,” what, exactly, does she have in mind? She’s probably contemplating something abstract—the passage of time, the obscurity of fate. She isn’t worried about stumbling over her own feet and careening into London traffic. Then again, she hasn’t read “Careful: A User’s Guide to Our Injury-Prone Minds,” a terrifying primer on the absurd and humiliating dangers of daily life by the psychologist and safety expert Steve Casner. Every year, Casner writes, people “trip while walking down the sidewalk too close to the curb, fall into the street, and get hit by cars, trucks, and buses.” Reading “Careful” makes you want to stay in—a mistake, Casner writes, since fifty per cent of all fatal accidents happen in “that house of horrors we call home.” Puttering around the house is so dangerous that even people with hazardous jobs, such as electric-power-line installers, are more likely to do themselves in at home than at work.

Casner is no ordinary worrywart. He is a jet and helicopter pilot with degrees in computer science and “intelligent systems”; he works as a research psychologist at NASA, where he studies safety as part of the “human factors” division. He usually publishes in journals such as Aviation, Space, and Environmental Medicine (one of his recent papers investigated the worthy question of airline pilots’ “knowledge and beliefs about over-the-counter medications”). In “Careful,” Casner treats quotidian life like a NASA space mission. When his young daughter requests a bunk bed for her room, a “quick fact check” tells him that “about thirty-six thousand kids per year are taken to an emergency room following a bunk bed injury.” Casner calculates that, if six per cent of the U.S. population is around his daughter’s age, then “one out of every five hundred kids in the country gets wheeled into the E.R. each year solely because of bunk beds.” He continues, “How many kids even have bunk beds? What if it’s one in five? That would mean one percent of all kids with bunk beds are heading to the E.R.” Bunk-bed request: denied.

Some people think that we live in a safety-obsessed age. They argue that a certain amount of danger is not just acceptable but desirable—that people in past eras were tougher, more independent, more courageous. Casner recalls his own childhood, in the unsafe nineteen-sixties. As a kid, he rode unsecured in the back of his grandfather’s pickup truck, leaning over the side to get closer to the road. When the dry cleaning came home, he ran around with the plastic bag over his head; he made gunpowder with his chemistry set; he biked around town, helmetless, with a friend’s iguana on his shoulders. He had bunk beds. He interviews Ryan, an acquaintance who, as a young man, positioned the family trampoline between the roof of their house and its swimming pool. During drunken high-school parties, Ryan and his brother Jesse led their friends in leaping from the roof to the trampoline to the water. “Today, Ryan is an investment advisor with three kids,” Casner writes, and Casner himself enjoys skateboarding around San Francisco. It’s hard to balance fun and risk, he admits, because they “are so often one and the same thing.”

Even so, Casner argues, we’re in the midst of a safety crisis. In 1918, one in twenty people died in an accident of some kind; by 1992, that number had been reduced to one in forty, through regulations, innovations, and public-awareness campaigns. But then the decline in the accidental-death rate stopped—and, since 2000, it has actually risen. In 2015, after almost a century of steady decline in car fatalities, driver, pedestrian, and cyclist deaths shot up eight, ten, and twelve per cent, respectively. Other kinds of accidents are more common, too, and we are now, Casner says, about as safe as we were thirty years ago. Casner has some theories about why this is happening. One is “risk homeostasis”—our tendency, once we’re safer, to take more risks. (Bicyclists who wear helmets, for example, tend to ride closer to cars than those who don’t.) New inventions play a role—smartphones that distract us, medications that confuse us; so does the new popularity of adventure sports, such as rock climbing. There’s the ascendant culture of D.I.Y.: “People are once again building their own furniture, blowing glass, upgrading their homes, and chopping and chainsawing their own firewood,” Casner writes. Many injuries happen when people are trying to “cook, make, decorate, or fix something.” Another significant factor is that people are living longer, into frail, accident-prone old age.

“We have come to the end of a really good run,” Casner concludes. “We have wrung all the big gains we’re going to get from putting rubber corners on stuff and saying, ‘Hey, don’t do that.’ ” From here on out, if we want to be safer, we will have to do it ourselves, by making better decisions. We’ll have to become the kinds of people who, before trampoline-bouncing into the pool, think twice (or, more likely, for the first time).

Accidents come in many forms. Casner devotes several pages to the proper technique for slicing a bagel; annually, three hundred and thirty-three thousand Americans cut themselves so badly with kitchen knives that they have to go to the E.R. Twenty-one thousand people hurt themselves with food processors; twenty-eight thousand injure themselves with hammers; forty thousand are wounded, somehow, by their washing machines. In 2010, fifty-one thousand car crashes and four hundred and forty deaths resulted from objects, such as mattresses, falling off automobile roof racks and into traffic. A hundred and twenty-five thousand people die annually because of “medication nonadherence,” and, according to the American Cancer Society, untold numbers perish because they think their cancer symptoms will “resolve themselves with time” and so never visit a doctor—an accidental death in slow motion. Needless to say, Casner’s chapter on accidents involving infants and small children is more horrifying than all these statistics combined.

Casner finds the word “accident” misleading; he distinguishes between “mistakes” and “errors.” A mistake is “the flawless execution of a mostly dumb idea”—it’s what happens when you should have known better. Many of the hundred and forty thousand people who fall off ladders every year do so because they stand on the rung that says “Not a step.” That’s a mistake. But errors are inevitable: even a competent and well-trained pilot will, eventually, glance at a lever in the “On” position and think that it is actually “Off.” The psychologist James Reason has found that people are aware of their own errors only eighty-five per cent of the time. Even workers in nuclear power plants make “errors in reading gauges, interpreting indicator lights, and selecting the wrong button to push,” Casner writes. The core problem is that minds wander. A French psychologist surveyed E.R. patients who had been in car accidents; he found that half of them were lost in thought at the moment of the crash.

“The U.S. airline crash rate over the past ten years is approaching 0 percent,” Casner writes, in large part because pilots, in addition to training themselves not to make mistakes, also employ various systems designed to combat error. Every commercial flight has two pilots, two air-traffic controllers, and even two flight computers. The pilots rely on checklists to make sure no steps are skipped; they use “callouts”—“Gear down, flaps fifteen”—to insure that everyone is paying attention. Pilots never multitask: if a pilot finds that she has to look at a map, she tells her co-pilot, “It’s your airplane,” and waits for an affirmative response—“I’ve got the jet”—before shifting focus.