Guns on display at a gun store in Miami.  (AP Photo/Alan Diaz)

Roughly 3 million Americans carry loaded handguns with them every day, primarily for protection, according to a new analysis of a national survey of gun owners published in the American Journal of Public Health.

The information comes from the National Firearms Survey, which the authors — a group of public health experts at the University of Washington, Harvard University and the University of Colorado — administered in 2015. The nationally representative survey was conducted online with 4,000 U.S. adults, including more than 1,500 who identified themselves as handgun owners.

The survey asked handgun owners how often they carried a loaded handgun on their person when away from home.

The peer-reviewed study concluded that roughly 9 million people carried loaded handguns at least one a month, including 3 million who carried them every day. People who carry handguns at least once a month were disproportionately likely to be conservative men between the ages of 18 and 29 residing in Southern states.

Four out of 5 of them said that personal protection was the primary reason they carried a loaded handgun. Nearly 6 percent reported being threatened by another person with a firearm at least once in the past five years. And 1 out of 5 reported carrying a concealed handgun without a permit, even in states where such a permit is required.

The researchers who conducted the study did so in part because good data on concealed-carry practices has been lacking.

“In light of the increasingly permissive concealed carry laws in the United States that we have observed over the past thirty years, it’s important to first, not only document the scope of this particular behavior that we did, but also take the next step and think about how this particular behavior may impact public health and public safety,” Ali Rowhani-Rahbar of the University of Washington, the lead author of the study, said in a statement.

Today, it’s easier than ever to carry a gun on you at all times. Many states have broadened their concealed-carry policies in recent years. Before 2003, for instance, Vermont was the only state where a person could legally carry a concealed handgun without a permit. Since then, 11 other states have passed laws eliminating permit requirements for concealed carry. Many other states have passed laws making it easier to obtain concealed-carry permits.

The result has been an explosion in the number of concealed-carry permit holders in the United States, from 2.7 million in 1999 to 14.5 million in 2016. That figure doesn’t account for individuals living in states without permitting requirements.

Gun rights advocates, such as the National Rifle Association, contend that permissive concealed-carry policies make society safer — “more guns, less crime,” as the adage goes. Early research into that question appeared to somewhat back that notion up, with studies in the late 1990s and early 2000s showing that liberal concealed-carry policies were either associated with lower rates of crime or had no effect on crime at all.

But more recent crime research has come to a very different conclusion. This year, for instance, a comprehensive analysis of decades of crime data found that states that made it easier to obtain concealed-carry permits saw a 10 percent to 15 percent increase in violent crime in the decade following the change.

A separate study published Thursday in the American Journal of Public Health, using more recent data, bolsters that finding. It specifically examined the difference between two types of concealed-carry laws — more permissive “shall-issue” laws, which require authorities to give permits to any individual meeting certain minimum requirements, such as age and residency; and more restrictive “may-issue” laws, which give authorities the discretion to decide whether to issue permits.

Examining crime data from 1991 to 2015, the study, conducted by a team of researchers from Boston University, Children’s Hospital Boston, and Duke University, found that “shall-issue concealed carry permitting laws were significantly associated with 6.5% higher total homicide rates, 8.6% higher firearm-related homicide rates, and 10.6% higher handgun-specific homicide rates compared with may-issue states.”

The study also offered an explanation for why earlier studies, using data primarily from the 1990s and earlier, showed different results. Demand for handgun permits was relatively modest in earlier decades. But during the concealed-carry boom of the 2000s, demand for handguns soared. Gun manufacturers' output increased dramatically.

“There has been a large increase, especially since 2005, in the share of firearms produced that are of higher caliber and therefore greater lethality,” according to a study published in the American Journal of Preventive Medicine this year. “In addition, the growing production of 0.380 pistols, which are generally compact, suggests a shift toward more-concealable weapons as well. Thus, firearm production has moved toward products designed to be more powerful and more concealable.”

In other words, the shift toward more lethal, more easily concealed firearms in recent years may be altering the relationship between concealed-carry laws and rates of crime even as states move to make concealed carry easier and more widespread.

Gun owners’ attitudes on these issues are far from monolithic. Only 33 percent of gun owners, and just over half of current NRA members, say they support permitless concealed carry as currently allowed in 12 states. And 44 percent of gun owners agree that the ease with which people can legally obtain firearms is a contributor to the rate of gun violence in America.

Regardless, the 3 million individuals who carry loaded guns with them every day are a testament to recent efforts to make concealed carry easier and more widespread. The public health implications of that shift are only just now beginning to be understood.