The number of Americans who have heart-worn highway markers has increased by 7 million since 2014, according to the National Heart, Lung, and Blood Institute.

Heart-wearing markers were added to highway pegs as part of the Interstate Highway System, which began in 1955 and was expanded in 1970.

The institute found that about a third of the population of the United States has one or more markers, and the percentage is up from about half in 2014.

The rate of heart-worn markers has risen in each state since 2014.

The number of heartworns has remained stable in most states over the past two decades, the institute said.

In addition to the four major states, there are smaller cities and towns along the highways.

Heartworns are still more common in some states, including Pennsylvania, Michigan, Wisconsin, Ohio, Pennsylvania, North Dakota, and Idaho, the study found.

Heartworn markers are generally visible only when passing through a highway, but some are visible on the highway’s shoulder.

“Heartworns represent the largest share of heart disease markers, but they have not shown a clear increase in prevalence since the mid-1990s,” the institute found.

“Heartwound prevalence has been declining in all states and the number of individuals with a heartwound has decreased by about 50 percent since 2000.”

According to the institute, about 9.1 million Americans have at least one marker.

This represents approximately 1.3 percent of the nation’s population.