In one sentence, describe what the following article is about:

If you notice chest pains, nausea, dizziness, irregular heartbeat, or shortness of breath while exercising, stop immediately. The exercise may be straining your heart. Call your doctor or 911 if the symptoms do not disappear quickly. If you have a prescription for nitroglycerin, then carry it with you during exercise. You should also write down the symptoms you experienced, the time of day, when you last ate, how long they lasted, and how often these symptoms appear. Speak with your doctor about any other symptoms before continuing an exercise program. The doctor may want to perform another stress test before you resume exercising. Wear the correct clothing and shoes for the type of exercise you are doing. Stay hydrated during exercise and be sure that someone knows where you are going when you leave to workout outside. Always use good judgement and stay within your limits. It is much better to continue to work out each day at an intensity you feel is lighter than you can manage, rather than to be sidelined for weeks with an injury or be re-hospitalized with another cardiac event. In very cold or hot weather your body has to work harder to provide oxygen to your cells, including your heart. Do not exercise outdoors if the weather is colder than 35F (1.7C) or hotter than 85F (29.4C) with humidity greater than 80%.

Summary:
Look for signs that your heart is working too hard. Prevent injuries and accidents. Avoid exercising outdoors when temperatures are high or low.