of coronary artery disease as well. These factors make it hard to judge how much not brushing one's teeth elevates the risk of coronary heart disease, but there is a proven correlation between poor oral health and coronary heart disease. Another negative side effect of not brushing one's teeth is halitosis or bad breath. According to the American Dental Association, not brushing properly allows remnants of food to collect on the teeth, gum line, and the surface of the tongue. Tooth plaque leads to gingivitis and periodontitis bacteria build-up, which produces bad-smelling odors. Having bad breath is very common, and most people experience it, but not brushing one's teeth particularly increases the risk. The ADA states that properly brushing teeth to remove bacteria that contribute to oral odors will improve oral hygiene and keep breath smelling as fresh as possible. Tooth-brushing guidelines Frequency A 2008 review cites studies from 1969–1973 that gum and tooth health were maintained if brushing removed dental plaque more often than every 48 hours, and gum inflammation happened if brushing happened at intervals longer than 48 hours. The 2008 review noted that tooth brushing can remove plaque up to one millimeter below the gum line, and that each person has a habitual brushing method, so more frequent brushing does not cover additional parts of the teeth or mouth. Dentists consider the extra abrasion of dentin from brushing multiple times per day to be insignificant, since modern toothpastes have relative dentin abrasivity below 250. A 1997 study simulated six months of brushing with manual or sonic toothbrushes. Neither caused observable wear of tooth surface or fillings. Both caused a small loss of cement at the edge of gold inlays. A 2017 study put composite fillings on both sides of extracted teeth. The study simulated seven years of brushing one side of each tooth with a sonic toothbrush, then measured microleakage around the fillings. No leakage was found at any bond between filling and enamel. At bonds between filling and cementum, there was sometimes leakage, and it happened to the same extent on the brushed sides of teeth and the unbrushed sides. When asked to brush "to the best of their abilities," young adults brushed longer, but did not cover any additional parts of their mouths. They brushed especially long on the grinding surfaces of back teeth (occlusal), which are the prime location for cavities in young children,