Blood Pressure Rises in the Cold and Drops in the Heat

When it comes to blood pressure, we are to some degree creatures of our environment. Hot weather, cold weather and other types of weather changes can affect your blood pressure.

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During the winter months, blood pressure tends to rise, notes the Mayo Clinic. Why? Because colder weather causes a narrowing of the blood vessels. That, in turn, can drive your blood pressure numbers up, as more pressure is required to ensure that blood continues flowing through narrower veins and arterial passages.

Research published in the journal Hypertension in July 2013 concluded that, among middle-aged men and women, every 18-degree Fahrenheit (10-degree Celsius) drop in temperature was associated with a 1.85 increase in systolic blood pressure (the top number) and a 1.18 rise in diastolic blood pressure (the bottom number).

The study team offered several explanations as to why, including the effect cold weather appears to have on the sympathetic nervous system, which controls the body's "fight or flight response" to a stressful or dangerous situation. Decreased sweating was another cited reason.

And speaking broadly, the weather can also have an indirect effect on blood pressure levels. For example, when the cold of winter hits, people generally tend to become less active, Mayo notes. That decrease in activity levels is often accompanied by a related increase in weight. The resulting off-season "winter bod" may ultimately drive up high blood pressure risk, particularly among those who already have a history of high blood pressure or a long-standing struggle with excess weight.

In contrast, the reverse may also occur. Beyond a relative widening of blood vessels, hot weather can also set in motion a series of reactions that can drive blood pressure down.

For example, according to Harvard Health Publishing, sweating resulting from sustained exposure to hot weather can also pull out sodium and potassium, while simultaneously driving up stress hormones to try to counteract it.

Healthy people may not always feel particularly affected by such shifts and may not feel the effects of dehydration. But the elderly and those who have a history of heart disease may experience enough of a resulting blood pressure drop to feel dizzy. And this could in turn up the risk for falling, Harvard Health points out.

"If you get dehydrated, it can certainly become a blood pressure issue," agrees Willie E. Lawrence, Jr,, MD, chief of cardiology with Midwest Health's Heart & Vascular Specialists in Kansas City, Missouri.

Read more: About High Blood Pressure and Low Pulse

Dr. Lawrence did stress that not everyone will feel blood pressure-driven faint in hot weather because the body has a "a pretty tremendously active regulator mechanism" that controls blood pressure and serves as a safety mechanism. "For instance, when you go from lying down to standing, your blood pressure would otherwise drop, but your body regulates for that and keeps things stable," he says.

"However, if you get dehydrated in hot weather and you don't have enough fluid in your body, then hypotension, or low blood pressure, can certainly become a problem," he notes. "Because when you sweat a lot you lose not only water, you also lose electrolytes, including sodium. And your regulatory system cannot compensate. It can begin to fail. And then you might faint."

The American Heart Association (AHA) points out that hot tubs and saunas pose another heat-related risk. AHA researchers warn that people who know they have, or are at risk for, high blood pressure should be very careful about using saunas and hot tubs in spas or gyms. Moving quickly between cold water and hot water can also drive up blood pressure.

Blood pressure also can be affected at times by other types of weather changes that are not related to temperature, notes the Mayo Clinic.

For example, sudden shifts in humidity or atmospheric pressure can cause your blood pressure to rise, for many of the same reasons it would go up in reaction to the cold. So, too, can changes in wind conditions or cloud cover, particularly among seniors.