My Husband Has Had A Doppler Done On His Carotid

My husband has had a doppler done on his carotid arteries and today he had a TEE done. Both tests showed significant plaque. 50% blockage in Right Carotid Artery and 50%+ build up in Aorta. His serum cholesterol has always been low. He smokes, but doesn't think that less than a pack a day causes damage. Could you point me to research that proves smoking IS or at least LIKELY is the cause for this? He's only 61 and the doctor who did the test told me this was the worst he'd seen. Not even 90 year old men have this much buildup.