Your blood pressure drops in hot weather because your body opens up blood vessels near the skin’s surface to release heat, a process called vasodilation. This redirects a large volume of blood away from your core and toward your skin, reducing the resistance your blood encounters as it flows through your circulatory system. Combined with fluid loss from sweating, this can lower your blood pressure enough to cause dizziness, fatigue, or even fainting.
How Heat Triggers a Blood Pressure Drop
When your body temperature rises, your nervous system launches a cooling response. Temperature-sensitive receptors in your skin detect the heat and trigger the release of chemical signals that relax the smooth muscle lining your blood vessel walls. This relaxation widens the vessels, especially those close to the skin, allowing more blood to flow to the surface where heat can escape into the surrounding air. Roughly 80 to 95 percent of the increase in skin blood flow during heat exposure comes from this active widening process.
The scale of this shift is substantial. Your body can redirect 7 to 8 liters per minute of additional blood flow to the skin during significant heat stress. To make that possible, your heart pumps harder (cardiac output can rise to around 12.5 liters per minute) and your body pulls blood away from other areas, reducing blood flow to your digestive organs by about 40 percent and to your kidneys by 15 to 30 percent. Under normal circumstances, these compensatory shifts keep your blood pressure relatively stable. But when the heat is intense, prolonged, or your body can’t keep up, the balance tips and blood pressure falls.
Sweating and Fluid Loss Make It Worse
Vasodilation alone isn’t usually enough to cause problems. The second hit comes from sweating. As you lose fluid through sweat, your total blood volume shrinks. Less blood returning to the heart means each heartbeat pumps out a smaller volume, which is measured as stroke volume. In studies comparing heated individuals who were dehydrated versus those who received fluid replacement, the dehydrated group had a stroke volume about 17 milliliters lower per beat and a cardiac output roughly 1.7 liters per minute less. Plasma volume dropped by about 6.6 percent in the dehydrated group, compared to essentially no change in those who received fluids.
That combination of widened blood vessels and reduced blood volume is what makes hot weather particularly effective at lowering blood pressure. Your heart is trying to pump more blood to the skin for cooling, but there’s simply less blood to pump. The result is a drop in pressure that you feel as lightheadedness, especially when you stand up quickly.
What It Feels Like
Heat-related blood pressure drops don’t always announce themselves dramatically. The early signs overlap with general heat exhaustion: heavy sweating, fatigue, a weak or rapid pulse, nausea, headache, and muscle cramps. The more specific clue that your blood pressure is involved is dizziness or faintness when you stand up from sitting or lying down. You might also notice cool, clammy skin with goose bumps despite being in the heat, which signals your body is struggling to regulate circulation.
Standing up adds an extra challenge because gravity pulls blood into your legs, temporarily reducing what’s available for your brain. In a study of older women, systolic blood pressure dropped an average of 17.4 mmHg upon standing in 86°F heat, compared to 14.2 mmHg in a cooler 68°F room. About 35 percent of participants experienced drops exceeding 20 mmHg in the warmer condition. A drop of that size is enough to cause noticeable dizziness or a brief blackout.
Why Older Adults Are More Vulnerable
Aging changes the body’s ability to manage this heat response in several ways. Older adults produce a blunted increase in cardiac output during heat exposure, meaning the heart doesn’t ramp up as effectively to compensate for widened blood vessels. They also have less effective constriction of blood vessels in the gut and kidneys, so the body can’t redirect blood to the skin as efficiently.
Interestingly, the nervous system signals telling blood vessels to constrict aren’t weaker in older adults. The problem appears to be that the blood vessels themselves respond less to those signals, likely due to age-related changes in vessel wall function and reduced availability of nitric oxide, a key molecule that helps vessels dilate and constrict on demand. The cooling response in the skin is also impaired, which means older adults get less benefit from the blood pressure trade-off: pressure drops, but they don’t cool down as effectively either.
Blood Pressure Medications and Heat
If you take medication for high blood pressure, hot weather can amplify the blood pressure-lowering effect beyond what’s intended. Several common drug classes carry specific risks in the heat.
- Diuretics (like hydrochlorothiazide or furosemide) increase urine output, which accelerates fluid loss on top of sweating. This can cause electrolyte imbalances, dehydration, and a greater risk of fainting.
- Beta blockers (like metoprolol or atenolol) can reduce your body’s ability to widen skin blood vessels and may decrease sweating, impairing your cooling response while also lowering blood pressure further.
- ACE inhibitors and ARBs (like lisinopril or losartan) lower blood pressure through their normal mechanism but can also reduce your sensation of thirst, making it easier to become dehydrated without realizing it.
- Calcium channel blockers (like amlodipine) add to blood pressure reduction and can contribute to electrolyte imbalances.
Combinations of these medications compound the risk. Taking an ACE inhibitor or ARB together with a diuretic, for example, significantly increases the chance of a dangerous blood pressure drop during heat exposure. If you’re on any of these medications and notice increased dizziness during hot weather, that’s worth a conversation with whoever prescribes them. Dose adjustments during summer months are not uncommon.
How to Prevent Heat-Related Blood Pressure Drops
Staying hydrated is the single most effective thing you can do. Water is the best default choice, but if you’re sweating heavily for several hours, a drink with electrolytes helps replace what sweat carries away. Be cautious with high-sugar, high-caffeine, or high-sodium beverages, which can worsen dehydration. If you’re on a sodium-restricted diet for heart-related reasons, check the sodium content of sports drinks before relying on them.
Timing matters. Schedule outdoor activity for the coolest parts of the day, typically early morning or evening. Even spending a few hours in an air-conditioned space during peak heat significantly reduces your cardiovascular strain. The American Heart Association identifies access to air conditioning as one of the most effective interventions for reducing heat-related health risks. If you don’t have air conditioning, cooling your skin with a damp cloth, cool shower, or spray bottle helps. Fans work when indoor temperatures are below 90°F, but above that threshold they can actually increase body temperature by blowing hot air across your skin.
Clothing choices make a measurable difference. Light-colored, loose-fitting clothes that cover your arms and legs reduce direct sun exposure while allowing air circulation. A brimmed hat that shades your face, ears, and neck keeps your head cooler and reduces the overall heat load on your body.
Stand up slowly, particularly if you’ve been sitting or lying down for a while. This gives your cardiovascular system a few extra seconds to adjust before gravity pulls blood into your legs. If you feel lightheaded, sit or lie down immediately and get to a cooler environment. Eating regularly throughout the day also helps maintain blood volume and electrolyte balance, since food provides both fluid and sodium that support circulation.