LDL cholesterol is often called bad cholesterol because it is the main type that deposits inside artery walls. LDL particles carry cholesterol from the liver to tissues throughout the body. When there is too much LDL in the blood, or when LDL particles stay in circulation for too long, they begin to slip into the inner lining of arteries through tiny gaps in the vessel wall. Once inside the artery wall, LDL becomes chemically altered, especially in the presence of oxidative stress from smoking,