San Antonians know August isn’t just hot. It’s relentless.
The days routinely climb into the upper 90s and triple digits. More importantly, our nights often stay around 76°F, giving your body little chance to recover before the next day begins. That prolonged heat takes a toll on your heart, circulation, and overall health.
August is San Antonio’s worst month for it. In 2025, Metro Health recorded 121 heat-related illnesses in August alone, the highest of any month that year, when the heat index sat at 100 degrees. Those cases include heat exhaustion, heat stroke, and dehydration serious enough to send people to the emergency room.
At Medical Vein Clinic®, we’re also thinking about something many people don’t connect to summer: deep vein thrombosis (DVT), or blood clots.
What does the heat have to do with blood clots?
When you become dehydrated, your blood contains less water and becomes more concentrated. At the same time, people naturally move less during extreme heat. Whether you’re staying indoors, taking a long road trip, or simply avoiding outdoor activity, hours of sitting slow blood flow through the legs.
If you have varicose veins, chronic venous insufficiency, a history of blood clots, have recently had surgery, or have other circulation problems, the combination of heat, dehydration, and prolonged sitting can increase your risk of developing a life-threatening DVT. When blood clots move out of the legs and into the chest or brain, they can cause a heart attack, stroke or cut off oxygen in the lungs.
The good news is that prevention is often simple.
Stay ahead of dehydration
One of the biggest mistakes people make is waiting until they’re thirsty.
By then, dehydration has already begun.
Most adults need to drink water consistently throughout the day, but there isn’t a one-size-fits-all recommendation. Your fluid needs depend on your age, activity level, medications, overall health, and how much time you’re spending outdoors. During a South Texas summer, you’ll almost certainly need more than usual. Watch for early signs that your body needs fluids:
• Dark yellow urine
• Dry mouth
• Headache
• Fatigue
• Dizziness
• Muscle cramps
Don’t wait until you’re thirsty. By the time you notice thirst, your body is already starting to lose the fluids it needs to regulate temperature and keep your circulation working efficiently.
Don’t let the heat steal your movement
Regular movement is one of the simplest ways to support healthy circulation, but it’s easy to skip your daily walk when it’s 102 degrees outside.
Instead of avoiding exercise altogether:
• Walk early in the morning or after sunset.
• Get up every hour if you work at a desk.
• Stretch your calves during long drives.
• Stay active indoors when temperatures become dangerous.
Even a few minutes of movement several times a day is far better than sitting for hours.
Know when heat becomes dangerous
Respect the heat. Heat exhaustion usually starts with heavy sweating, dizziness, nausea, weakness, and muscle cramps. Get to a cool place and begin rehydrating right away. If symptoms progress to confusion, hot skin, or a temperature above 103°F, call 911. Heat stroke is a medical emergency and every minute matters.
Look out for one another
Older adults, outdoor workers, people without reliable air conditioning, and those with chronic medical conditions are at the greatest risk during prolonged heat waves. San Antonio opens cooling centers during heat advisories for anyone who needs a safe place to cool down.
Summer in South Texas isn’t going anywhere.Stay hydrated. Keep moving. Listen to your body.
Healthy veins depend on healthy habits, and even in August, every step you take is helping your circulation.
Frequently Asked Questions
Can hot weather actually cause blood clots?
Yes. Extreme heat raises your risk of deep vein thrombosis (DVT) through two mechanisms working together. Dehydration thickens and concentrates your blood, and people move less when it’s dangerously hot outside. Both slow blood flow through the legs, which is where most clots start. The risk climbs if you already have varicose veins, chronic venous insufficiency, a history of clots, or recent surgery.
Who is most at risk for a heat-related blood clot?
People with existing circulation problems face the highest risk. That includes anyone with varicose veins, chronic venous insufficiency, a prior DVT, recent surgery, or other vascular conditions. Older adults, outdoor workers, and people without reliable air conditioning are also more vulnerable during prolonged heat waves. If you fall into more than one of these groups, prevention matters even more during a South Texas summer.
How much water should I drink in San Antonio summer heat?
There’s no single number that fits everyone. Your fluid needs depend on your age, activity level, medications, overall health, and how much time you spend outdoors. During a South Texas August, you’ll almost certainly need more than usual. Drink consistently through the day rather than waiting until you feel thirsty. Thirst means dehydration has already started.
What are the early warning signs of dehydration?
Watch for dark yellow urine, dry mouth, headache, fatigue, dizziness, and muscle cramps. These show up before thirst becomes obvious, so treat them as your signal to rehydrate. By the time you feel thirsty, your body is already losing the fluids it needs to regulate temperature and keep circulation working.
How can I stay active when it’s over 100 degrees outside?
Move at the cooler ends of the day and break up long stretches of sitting. Walk early in the morning or after sunset, get up every hour if you work at a desk, stretch your calves during long drives, and stay active indoors when temperatures turn dangerous. A few minutes of movement several times a day protects your circulation far more than one long session or none at all.
What’s the difference between heat exhaustion and heat stroke?
Heat exhaustion usually starts with heavy sweating, dizziness, nausea, weakness, and muscle cramps. Move to a cool place and rehydrate right away. It becomes heat stroke, a medical emergency, when symptoms progress to confusion, hot skin, or a temperature above 103°F. Call 911 immediately if that happens. Every minute counts.
When should a leg symptom send me to a vein specialist?
Persistent leg swelling, aching, heaviness, cramping, or visible varicose veins are worth evaluating, especially heading into a hot summer when clot risk rises. Sudden swelling, pain, warmth, or redness in one leg can signal a DVT and needs prompt medical attention. Medical Vein Clinic evaluates both venous and arterial circulation at two San Antonio locations, Stone Oak and Alamo Ranch. Call 210-622-8000 or book a consultation.
