Bundling up before you head outside may do more than just protect you from shivering. According to the National Registry of Myocardial Infarction, 53 percent more heart attacks occur in winter than in summer (peaking in January), and the reason may be linked to simply breathing in very cold air. A recent study from Penn State suggests that people who engage in strenuous activities (like shoveling snow) in below-freezing temps may experience impaired blood flow in their coronary arteries—where heart-attack-causing blockages can occur. Researchers believe the blood vessels constrict in response to cold air, decreasing the volume of oxygen-rich blood that's pumped to your heart. In healthy people, the body will quickly compensate, but for those with heart disease, the added constriction could prove dangerous and trigger chest pain or, worse, a heart attack. Whether you have heart disease or not, before you head outside you should wrap a scarf around your nose and mouth to warm the air you're breathing, and leave any outdoor activities that raise your heart rate for warmer winter days.