Summarize this article:

You should not apply electricity to areas that are close to your heart or that are likely to be particularly sensitive. Keep them away from your: Temples Mouth Eyes/Ears Front or side of your neck near the major arteries Spinal column (can be crossing from opposite sides of the spine though) Left side of your chest, i.e.: near your heart Front of your chest with one on your back Varicose veins Broken skin or a new scar that is still healing Areas that are numb Some health conditions make using TENS risky. If you have a pacemaker or another electrical device in your body, the electrical impulses could interfere with these devices or their signals, or cause a failure. If you have epilepsy, you may be more sensitive and it is better not to use TENS. If you have a disorder which affects the rhythm of your heart/heartbeat, your heart might be particularly sensitive to electrical impulses, and malfunction. If you are allergic to the electrode pads, you may be able to get hypoallergenic electrode pads. If you are pregnant or might be pregnant, do not use TENS without it being prescribed. The risks of using TENS during pregnancy aren't known, so do not use it without consulting your doctor. Some women find it helpful for pain relief during labor pains, but be sure to ask your doctor whether it is safe for you and your baby before using it. If you are unsure whether TENS is safe for you, consult your doctor. These activities make using TENS more risky. If you are in the bath, shower, or swimming pool, the water will change how and where the electricity is conducted. Do not use TENS while you are sleeping. If you are driving a motor vehicle the sensations caused by TENS may be a distraction. If you are operating machinery, do not use TENS to avoid unexpected confrontations. The electrical impulses that TENS devices produce should not cause problems for airlines, but ask them before you use it during flight.

Summary:
Do not place the electrodes on areas of the body which could harm you. Do not use TENS at all anywhere on your body if you have a condition that makes it unsafe for you. Do not use TENS while you are engaged in certain activities.