Holes in your teeth form when acid dissolves the hard mineral surface of your enamel faster than your saliva can repair it. This process, called tooth decay, is driven by bacteria that feed on sugars in your mouth and produce acid as a waste product. Nearly 21% of adults aged 20 to 64 have at least one untreated cavity, making it one of the most common health problems in the world.
What Actually Happens Inside Your Mouth
Your teeth are covered in enamel, the hardest substance in your body. Enamel is made of tightly packed mineral crystals, primarily calcium and phosphate. These crystals are stable under normal conditions, but they start dissolving when the environment around them becomes acidic enough, roughly below a pH of 5.5.
The acid doesn’t come from food directly. It comes from bacteria. Your mouth is home to hundreds of bacterial species, and one in particular is the primary driver of cavities: Streptococcus mutans. This bacterium feeds on carbohydrates, especially sugar, and produces acid as a byproduct. It also has a unique ability to build a sticky, protective film called a biofilm (what you know as plaque) that helps it cling to your teeth and shield itself from saliva.
S. mutans is especially dangerous because it thrives in acidic conditions that would slow down other bacteria. As it produces acid and the pH drops, it actually outcompetes its neighbors, creating a self-reinforcing cycle: more acid, more S. mutans, more acid. Sucrose (table sugar) is its preferred fuel because the bacterium uses it to build the sticky matrix that anchors plaque to your enamel in the first place.
How Acid Dissolves Enamel
When bacteria produce acid, the hydrogen ions in that acid react with the calcium and phosphate in your enamel, pulling those minerals out of the crystal structure and into the surrounding saliva. This is demineralization. At first, the damage is invisible and happens below the surface of the enamel, not on it. The outermost layer can look intact while the mineral structure underneath is weakening.
There’s also a second mechanism at work with certain acids, particularly those in fruit and acidic drinks. Some acid molecules can directly grab calcium ions and carry them away from the enamel surface in a soluble complex. This is why drinks like orange juice and soda can be hard on teeth even beyond the sugar they contain: the acid itself strips minerals through two separate pathways at once.
Your Mouth’s Built-In Repair System
Your body doesn’t just sit there while this happens. Saliva is a remarkably effective defense system. It contains dissolved calcium and phosphate ions that can redeposit onto weakened enamel, essentially patching the damage in a process called remineralization. Above a pH of about 5.5, saliva is actually supersaturated with the minerals needed to rebuild enamel, meaning the chemistry naturally favors repair over destruction.
Saliva also buffers acid, gradually neutralizing it and raising the pH back to safe levels after you eat or drink. Proteins in saliva form a thin protective coating on your teeth and help regulate mineral crystal growth. This is why dry mouth (from medications, medical conditions, or dehydration) significantly increases cavity risk. Without adequate saliva flow, your teeth lose their primary shield against acid.
The balance between demineralization and remineralization is the central story of tooth decay. A cavity forms only when acid attacks outpace your saliva’s ability to repair the damage over days, weeks, and months.
Why Sugar Frequency and Amount Both Matter
For decades, dentists emphasized how often you eat sugar rather than how much. The logic was straightforward: every time you eat something sugary, bacteria produce acid for roughly 20 to 30 minutes. More snacking sessions means more acid attacks per day, with less recovery time in between.
More recent research paints a slightly more nuanced picture. A large study of U.S. adults found that the total amount of added sugar consumed was clearly linked to more cavities and more tooth surfaces affected by decay. The relationship between frequency alone and cavity count was less consistent. In practical terms, this means both habits matter. Sipping a sugary drink all afternoon is a problem because it extends the acid attack, but consuming large quantities of sugar in fewer sittings isn’t harmless either. The safest approach is reducing both how much and how often.
How a Cavity Progresses Stage by Stage
Cavities don’t appear overnight. They develop through distinct stages, and the earlier you catch them, the better the outcome.
The first sign is a white spot on the enamel, an area where minerals have been lost but the surface hasn’t actually broken through yet. At this stage, there’s no hole and no pain. With good oral hygiene and fluoride exposure, these white spots can remineralize and the damage can actually reverse. This is the only stage where a cavity can heal on its own.
If demineralization continues, the enamel surface eventually breaks down and a physical hole forms. You still may not feel anything, because enamel has no nerve endings. This is why cavities are often discovered during dental exams rather than from symptoms.
Once the hole reaches the dentin, the softer layer beneath the enamel, things accelerate. Dentin is much less resistant to acid, so decay spreads faster. This is typically when you start noticing sensitivity to hot, cold, or sweet foods and drinks.
If left untreated, bacteria and acid eventually reach the pulp, the innermost part of the tooth containing nerves and blood vessels. The pulp swells in response to the infection, but because it’s enclosed inside a rigid tooth, there’s nowhere for the swelling to go. The nerve gets compressed, causing significant pain. At this point, the tooth may need a root canal or extraction.
Where Cavities Form on Your Teeth
Not all cavities are the same. They’re classified by location, and each type has a slightly different profile.
- Pit and fissure cavities form in the grooves on the chewing surfaces of your back teeth. These are the most common type in teenagers and tend to progress quickly because the grooves trap food and bacteria in areas a toothbrush can’t always reach.
- Smooth surface cavities develop on the flat sides of teeth, often in the tight spaces between them. These are more common in people in their 20s and grow slowly. They’re also the most responsive to improved brushing and flossing, and can sometimes be reversed before a hole forms.
- Root cavities occur on the exposed root surfaces of teeth, typically in older adults whose gums have receded. Roots aren’t covered by enamel, only a thinner layer called cementite, so they’re especially vulnerable to acid.
How Fluoride Changes the Chemistry
Fluoride works by swapping into the mineral crystal structure of your enamel, replacing some of the hydroxyl groups with fluoride ions. Because fluoride ions are physically smaller, the crystal becomes more tightly packed, with stronger attractive forces between its components. The result is a mineral that requires significantly more acid to dissolve.
The difference is dramatic in terms of acid resistance. When the pH drops from 7 to 5 (a common shift after eating), the conditions driving dissolution of normal enamel change by a factor of 10,000, pushing hard toward mineral loss. For fluoride-enriched enamel under the same pH drop, the shift is only about 3%, a trivial change. This is why fluoride toothpaste and fluoridated water have had such a measurable impact on cavity rates across populations.
Fluoride also promotes remineralization. When calcium and phosphate from saliva redeposit onto a tooth surface in the presence of fluoride, they form the more acid-resistant crystal structure rather than the original, weaker one. So fluoride doesn’t just protect existing enamel; it helps rebuild it stronger.
What You’d Feel at Each Stage
Early enamel cavities produce no symptoms at all. You won’t feel pain, sensitivity, or anything unusual. The first physical clue might be a visible white or brown spot on the tooth, but even that’s easy to miss without a mirror and good lighting.
Once decay reaches the dentin, sensitivity appears. You might notice a sharp twinge when drinking cold water, eating something sweet, or biting down. The tooth might also develop a visible dark spot or a rough area you can feel with your tongue.
Pain that lingers, throbs, or wakes you up at night signals that the decay has likely reached or is approaching the pulp. At this point, the cavity has been developing for a considerable amount of time. Smooth surface cavities in enamel can take months to years to progress, while pit and fissure cavities and dentin-level decay move faster. By the time a cavity hurts, it’s well past the stage where it could have been reversed.