Teeth rot when bacteria in your mouth feed on sugars and produce acid that dissolves the mineral structure of your enamel. This process, called demineralization, begins whenever the pH on a tooth’s surface drops below about 5.5. Your mouth naturally repairs minor acid damage throughout the day, but when acid attacks happen too often or last too long, the repair process can’t keep up, and decay sets in.
What Tooth Enamel Is Made Of
Enamel is the hardest substance in your body, composed of about 95% mineral by weight, with only 1% organic material and 4% water. That mineral is a crystalline form of calcium and phosphate called hydroxyapatite. This dense crystal structure is what allows your teeth to withstand bite forces up to 770 newtons and endure the constant temperature and pH swings inside your mouth.
But hydroxyapatite has a vulnerability: hydrogen ions from acids pull calcium and phosphate out of the crystal. When acid contacts enamel, it strips those ions away and replaces them with water. If this happens in small doses, your saliva can deposit calcium and phosphate back into the enamel and reverse the damage. If it happens repeatedly, the crystal structure weakens, and a cavity begins to form.
How Bacteria Turn Sugar Into Acid
Your mouth contains hundreds of bacterial species, but one group is especially effective at causing decay. These bacteria thrive on fermentable carbohydrates, breaking down sugars through a process that produces lactic acid as a byproduct. They live in dental plaque, a sticky film of microbes and proteins that clings to tooth surfaces and traps acid right against the enamel.
Not all sugars are equally dangerous. Sucrose, ordinary table sugar, is the most cavity-promoting sugar in the human diet. It does double duty: bacteria metabolize it into acid, and it also stimulates the formation of more plaque, giving those bacteria a better environment to grow in. Glucose and fructose can also fuel decay, but they’re less effective than sucrose. Interestingly, some snack foods low in sugar but high in starch cause a more severe and prolonged acid spike in plaque than foods high in sugar alone, likely because starchy residues cling to teeth longer and break down slowly into simpler sugars.
The Five Stages of Tooth Decay
Decay doesn’t happen overnight. In most cases, cavities develop over months to years, depending on your diet, oral hygiene, and saliva quality. The progression follows a predictable path from the outer surface inward.
White Spot Lesions
The earliest sign of trouble is a chalky white spot on the enamel where minerals have started to leach out. At this stage, no hole has formed yet. The damage is still reversible if conditions in the mouth change, for example through better brushing, reduced sugar intake, or fluoride exposure that helps minerals redeposit into the weakened area.
Enamel Breakdown
If acid attacks continue, that white spot darkens to brown and the enamel surface actually breaks. Small holes, the classic “cavities,” appear. Once the enamel is physically breached, the damage can no longer reverse on its own.
Dentin Decay
Beneath the enamel sits dentin, a softer tissue riddled with tiny tubes that connect to the tooth’s nerve. Once decay reaches dentin, it accelerates because the softer material dissolves more easily. This is also when you may start feeling sensitivity to hot, cold, or sweet foods, since those tiny tubes transmit sensation directly toward the nerve.
Pulp Infection
The innermost part of the tooth, the pulp, contains nerves and blood vessels that keep the tooth alive. When bacteria reach the pulp, the tissue becomes inflamed and swells. Because the pulp is enclosed in a rigid shell of dentin and enamel, there’s nowhere for the swelling to go. The result is intense, throbbing pain. The immune system sends white blood cells to fight the bacteria, but their defensive chemicals can actually cause additional tissue damage in such a confined space. If oxygen levels inside the pulp drop low enough, the inflammatory response escalates further, and the damage can become irreversible.
Abscess
If the infection spreads beyond the tooth root into the surrounding bone and gum tissue, a pocket of pus called an abscess forms. This can cause facial swelling, fever, and serious pain. At this point, the tooth typically cannot be saved without significant intervention.
How Saliva Protects Your Teeth
Saliva is your body’s primary defense against tooth rot. It works through three mechanisms. First, it physically washes food particles and bacteria off tooth surfaces. Second, it contains three separate buffering systems (the most important being a bicarbonate system) that neutralize acid and bring the mouth’s pH back to safe levels after eating. Third, at normal pH, saliva is supersaturated with calcium and phosphate ions, meaning it actively pushes those minerals back into enamel that has been slightly weakened by acid.
Fluoride amplifies this repair process. When fluoride ions from toothpaste or drinking water are present alongside calcium and phosphate in saliva, they incorporate into the repaired enamel and make it more resistant to future acid attacks. This is why fluoride is so effective at preventing cavities: it doesn’t just coat the teeth, it changes the chemistry of the repair process itself.
Why Some People Decay Faster Than Others
The single biggest accelerator of tooth decay outside of diet is dry mouth. Saliva is so critical to protecting enamel that anything reducing saliva flow dramatically increases cavity risk. Medication is the most common cause of dry mouth, and several hundred drugs can trigger it, including blood pressure medications, antidepressants, painkillers, anti-anxiety drugs, diuretics, and antihistamines. When salivary proteins and electrolytes are missing, the acid-buffering system fails and cavity-causing bacteria multiply more freely.
Diabetes also raises decay risk, partly through dry mouth and partly because higher glucose levels in saliva give bacteria more fuel. People who breathe through their mouth at night, whether from congestion, sleep apnea, or habit, dry out their oral tissues for hours at a stretch, giving acid an uninterrupted window to work on enamel.
Frequent snacking matters more than total sugar intake. Every time you eat something containing fermentable carbohydrates, your mouth’s pH drops for roughly 20 to 30 minutes before saliva can neutralize the acid. Three meals a day means three acid exposures. Grazing on crackers or sipping a sugary drink over several hours can mean dozens of overlapping acid attacks, with your saliva never fully recovering between them. The duration and frequency of sugar contact with your teeth is often more damaging than the sheer amount you consume.
Why Decay Tends to Hit Certain Teeth
Rot doesn’t strike teeth evenly. The most vulnerable spots are the ones saliva can’t easily reach and your toothbrush tends to miss: the pits and grooves on the chewing surfaces of molars, the tight spaces between teeth, and the area along the gumline. Plaque accumulates in these sheltered zones, and the acid it produces stays concentrated against the enamel instead of being diluted and washed away. This is why flossing prevents cavities in places brushing alone cannot, and why molars are far more likely to decay than smooth-surfaced front teeth.
Receding gums expose the tooth root, which is covered in cementum rather than enamel. Cementum is softer and less mineral-dense, so it dissolves at a higher pH than enamel does. This is why older adults who have some gum recession often develop root cavities even if they avoided decay for most of their lives.