Several cities have now banned which are a type of fat.

The correct answer is "trans fatty acids".

Trans fatty acids (colloquially as trans fats) are a type of unsaturated fatty acids wherein instead having its double bonds in the cis configuration giving the fatty acid "kinks" in the molecules making it more soluble; the double bonds in trans fats are in the trans configuration (hence the name) and this will not give the molecule kinks making it less soluble. Less soluble trans fats are notorious in depositing in peripheral arteries as well as in coronary arteries that increase the risk for developing atherosclerosis.