Then does that mean an ASD is harmless ?

Well, not quite. As I said before, frequent chest colds occur. If very severe, these may prevent normal growth and development of a child. Also, the lungs receive a large blood volume, and this is harmful after some years. The blood vessels in the lung become hard and thick, and pressure inside them increases. This condition is called PULMONARY HYPERTENSION - "pulmonary" for lung, "hyper" for high, "tension" for blood-pressure - which is a very serious complication. Also, after several years, the ventricle may "fail" - HEART FAILURE. Another serious complication is ARRHYTHMIA - which means an irregularity in the rhythm of the heart. Since it handles a large volume of blood, the right atrium enlarges. This causes a disturbance in the heart's electrical activity, causing it to beat faster - a disease called ATRIAL FIBRILLATION. All of these problems are common in LARGE ASD's.

But what about the little ones - are they safe ?

They are less dangerous than the large ones. But there is one other problem which may occur in both large and small ASD's - PARADOXICAL EMBOLISM.
What is paradoxical embolism ? Blood flow in the veins is normally slow and sluggish, and some small clots may form. CLOTS are small pieces of hardened blood. In a normal person, these clots may pass from the veins, through the heart and into the lungs. Here, they are "filtered" and prevented from entering the ARTERIES along with the purified blood. When there is an ASD, however, the clot, on entering the right atrium, may pass across the ASD into the left atrium. Along with the "pure" blood, it can then pass into the arteries, and from here to the brain. In the brain, it may block a blood vessel, preventing blood flow to a part of the brain. This causes a STROKE. A stroke is an injury to the brain. It may cause weakness or paralysis of an arm or leg, or inability to speak or unconsciousness. It can be a very serious problem, sometimes. It's because of this risk of stroke that doctors advise that even small ASD's be closed, by surgery or other methods.