1) Deep vein thrombosis (immobility clots)
What’s going on: Sitting for 6–12 hours reduces venous return in the legs, thickens the blood through dehydration, and promotes clots that can migrate to the lungs (embolism).
Warning signs: pain or heaviness in the calves, unilateral swelling, local heat.
How to reduce your risk: hydrate, walk every 60–90 minutes, wear graduated compression stockings, choose an aisle seat and ask if you need prophylaxis if you take anticoagulants or have a history.
2) Cardiopulmonary stress from altitude, cabin pressure, and extreme weather
What’s going on: Pressurized cabins are equivalent to 1,800–2,400 m altitude; there is less oxygen per breath. Heat/humidity or extreme cold require quick adjustments to the heart and lungs.
Warning signs: Unusual shortness of breath, chest tightness, dizziness, palpitations, swollen ankles.
How to reduce your risk: Temperate climate destinations, shorter trips, portable oximeter if you have COPD/HF, and plan with your cardiologist or pulmonologist before long flights.
