Here are a few facts about high blood pressure, or hypertension.

• More than 70 million Americans have it, according to Blue Cross Blue Shield.
• But one-third are not aware they do, the insurer says.
• Half of those who have been diagnosed with hypertension do not have it under control.
• Hypertension is a leading cause of stroke, heart attack, kidney disease and other issues that can lead to death.

Why is high blood pressure so easy to miss?

“Hypertension is often referred to as the silent killer because you can have high blood pressure for a long time and have no symptoms, even while the high blood pressures are doing damage to your body,” says Dr. Leigh Simmons, general internist at Massachusetts General Hospital and medical director of the Mass General Health Decisions Sciences Center.

“Hypertension can cause heart disease, including heart failure and coronary artery disease, stroke, vision loss, kidney disease and aneurysmal disease,” Simmons adds.

So how do you find out if you have high blood pressure? Here’s what you need to know.

Blood pressure is the pressure within the arteries. It is expressed in two numbers: The systolic and the diastolic blood pressure. The numbers represent measures of the pumping action of the heart, says Dr. Leslie Thomas, a nephrologist at Mayo Clinic.

The higher number, called systolic blood pressure, is the pressure from the contraction of the heart’s left ventricle. The lower number, diastolic blood pressure, measures the relaxation of the heart’s left ventricle.

The numbers are expressed in millimeters of mercury, or mm Hg, a reference to the device used to measure blood pressure, which is called a sphygmomanometer. You may recognize it as the cuff that your doctor places around your arm to measure your arterial blood pressure.

Normal blood pressure is a systolic pressure of less than 120 mm Hg and a diastolic pressure less than 80 mm Hg, says Dr. Raoul Hermanns, a physician in the Netherlands. It is expressed like this: 120/80 or “one twenty over eighty.” Elevated blood pressure — not quite high blood pressure — is systolic pressure of 120 to 129 and diastolic pressure less than 80. High blood pressure or hypertension is systolic pressure equal to or greater than 130 or diastolic pressure equal to or greater than 80.6254a4d1642c605c54bf1cab17d50f1e

Blood pressure higher than 180/120 mm Hg is considered a hypertensive emergency; if you have this measurement, you should call for medical help immediately.

“Hypertension is a very common condition affecting up to 40 percent of adults,” Thomas says. “It is one of the most common conditions for which medications are prescribed.” 

Most people with it have primary hypertension, Thomas adds. “How primary hypertension develops is not entirely understood,” he says. “However, it is felt to result from many inherited and environmental factors that interact in complex ways within the body.”

“Risks for the development of primary hypertension include family history, advancing age, obesity, high sodium diet, alcohol consumption and physical inactivity,” he adds.

The Mayo Clinic advises checking your blood pressure at least every two years, starting at age 18, and more frequently if you have risk factors such as smoking, inactivity, unhealthy diet, obesity or other conditions.

People who are 40 or older should check their blood pressure at least annually, the clinic adds. If you have hypertension, your doctor may advise even more frequent checks.

“Children age 3 and older may have blood pressure measured as a part of their yearly checkups,” the Mayo Clinic says.

You can get a free blood pressure screening at health fairs or community clinics, at some pharmacies or other places with public blood pressure machines. Consult your doctor or pharmacist.

In rare cases, hypertension may manifest in symptoms, according to the American Heart Association.

They may include the following:

• Blood spots in the eyes, called subconjunctival hemorrhage. This is more common if you have diabetes.
• Face flushing.
• Dizziness. Sudden dizziness or inability to walk could be a warning sign of stroke. 

“Most people with hypertension don’t have any symptoms of the condition,” Simmons says. “However, some symptoms of dangerously elevated blood pressures can include blurred vision, headache, chest pain and shortness of breath. If you have any of these symptoms, that warrants an immediate evaluation by your clinicians or in the emergency room.”

Don’t try to diagnose yourself, the American Heart Association advises. Only your doctor can determine if you have high blood pressure.

Once you know your blood pressure numbers, you can make lifestyle changes to mitigate the issue and avoid serious illness.

“Lifestyle is the biggest and the first most important step in treating high blood pressure,” says Leo M. Baily, an osteopathic doctor at the Brentwood Medical Center in Newport News, Virginia. 

“These days our diets are laden with sodium,” Bialy adds. “Our lifestyles are overrun with sedentary behaviors, and the first thing we need to do is increase our activity, decrease the amount of salt [and] fatty foods we get in our diets. And if those things don’t get us to our goal, then obviously we can start introducing medications to get us closer to that goal.”

There are several medications to control your high blood pressure. “Most people are turned off by the idea of being on medications,” Bailey says. “They think, ‘I’m on medications for life.’ But as long as you maintain follow-up with your primary care doctor or whoever’s managing your blood pressure, whether it’s your cardiologist or your kidney doctor, you can expect to have a longer, healthier life with reduced risks of having a stroke, having a heart attack, going on dialysis and basically having more longevity.”