A cavity is a hole in your tooth created by acid. Bacteria living on your teeth feed on sugars from your food, produce acid as a waste product, and that acid slowly dissolves the hard mineral structure of your tooth. Left alone, the hole gets deeper, eventually reaching the nerve and causing pain. About 22% of adults aged 20 to 34 have at least one untreated cavity, making it one of the most common chronic conditions in the world.
How a Cavity Actually Forms
Your teeth are coated in enamel, the hardest substance in your body. Enamel is made of tightly packed mineral crystals, primarily a compound of calcium and phosphate. These crystals are strong, but they have a weakness: acid dissolves them.
Here’s the chain of events. Your mouth is home to hundreds of bacterial species, and they naturally form a sticky film on your teeth called plaque. When you eat something containing sugar or starch, bacteria in that plaque consume those carbohydrates and release acid. This acid lowers the pH on your tooth’s surface. When the pH drops below roughly 5.5, the environment around your tooth becomes chemically unbalanced. Phosphate starts getting pulled out of your enamel to restore equilibrium in the surrounding fluid, and the mineral crystals dissolve. This process is called demineralization.
That threshold isn’t identical for everyone. If your saliva naturally contains less calcium and phosphate, your enamel may start dissolving at a pH as high as 6.5. People with mineral-rich saliva may not see damage until pH drops to 5.5 or lower. This partly explains why some people seem more cavity-prone than others despite similar diets.
The key bacterium driving this process is Streptococcus mutans. It thrives on sugar, tolerates acidic environments, and produces a sticky scaffold that helps plaque cling stubbornly to teeth. With frequent sugar exposure, the bacterial community in your plaque gradually shifts toward more acid-producing species, accelerating the damage.
What Happens at Each Stage
Cavities don’t appear overnight. They progress through distinct stages, and catching them early makes a real difference.
White spot lesions. The earliest sign is a chalky white patch on the tooth surface. At this point, minerals have been lost from beneath the enamel’s surface, but no actual hole has formed. This stage is fully reversible. Fluoride treatments, improved brushing, and reducing sugar intake can drive calcium and phosphate back into the weakened enamel, restoring its strength. Fluoride is central to this repair process, but it works best when calcium and phosphate are also available, either from your saliva or from mineral-containing dental products.
Enamel decay. Once enough mineral has been lost, the surface breaks down and a physical hole forms in the enamel. You typically won’t feel anything at this stage because enamel has no nerves. This is the point where a dentist would recommend a filling.
Dentin decay. Beneath enamel sits dentin, a softer, yellowish layer that makes up most of the tooth. Dentin contains thousands of microscopic tubes that connect to the tooth’s nerve. Once decay reaches dentin, it spreads faster because the material is less minerite-dense. This is often when you start noticing sensitivity to hot, cold, or sweet foods.
Pulp involvement. The innermost part of your tooth, the pulp, contains nerves and blood vessels. When decay reaches this layer, you’ll likely experience significant, persistent pain. Treatment at this stage typically requires a root canal, where the damaged inner tissue is removed, the space is cleaned and sealed, and a crown is placed over the tooth.
Abscess. If bacteria reach the pulp and infection spreads beyond the tooth root, a pocket of pus called an abscess can form. This causes intense throbbing pain that may radiate to the jaw or ear, and it can become a serious health concern. A root canal may still save the tooth, but in severe cases, extraction is necessary.
Why Cavities Hurt
Tooth pain from a cavity follows a specific mechanism. Dentin contains thousands of tiny fluid-filled tubes running from the outer surface toward the nerve. When decay exposes these tubes, temperature changes, sugar, or even air movement can cause the fluid inside them to shift rapidly. That sudden outward flow of fluid triggers the nerve endings at the base of the tubes, producing a sharp, shooting pain. This is why a cavity that felt fine for months can suddenly become painful: the decay finally broke through to dentin and opened those tubes.
How Cavities Are Found
Not all cavities are visible to the naked eye. Dentists use several tools to catch decay early. X-rays remain the standard for spotting cavities between teeth, where they most often hide. Newer technologies include laser fluorescence devices that detect bacteria in decayed tissue by measuring the fluorescence they emit, and near-infrared light cameras that illuminate the tooth from behind to reveal internal damage without radiation. Visual examination also plays a role; dentists use standardized scoring systems to classify discoloration, surface roughness, and other visible signs of early breakdown.
Each method has strengths. Laser fluorescence is particularly good at catching early enamel lesions, while X-rays excel at confirming deeper decay that has reached dentin. Dentists often combine methods to avoid missing anything.
How Fillings Work
When a cavity has progressed past the reversible white-spot stage, the standard treatment is a filling. Your dentist removes the decayed portion of the tooth and fills the space with a restorative material. Two main types are used today.
Composite fillings are made from a blend of plastic and ceramic particles. They’re tooth-colored, so they blend in visually, and they bond directly to the tooth structure. This bonding means less of your healthy tooth needs to be removed during placement. They typically last 7 to 10 years.
Amalgam fillings are a mix of metals, silver in color, and more noticeable when you open your mouth. They don’t bond to the tooth, so slightly more tooth structure is removed to hold them in place. Their advantage is durability: amalgam fillings last 10 to 15 years or longer with proper care. They’re also faster to place. Amalgam has declined in popularity due to cosmetic preferences, but both materials remain effective.
Why Some People Get More Cavities
Diet is the most obvious factor. Frequent snacking on sugary or starchy foods gives mouth bacteria a constant fuel supply, keeping acid levels elevated for longer periods. But biology matters too. The mineral content of your saliva, which varies from person to person, determines how quickly your teeth can repair minor acid damage between meals. People who produce less saliva overall, whether from medications, medical conditions, or simply genetics, lose that protective buffering effect and face higher risk.
The bacterial makeup of your mouth also plays a role. Some people harbor higher populations of acid-producing bacteria. Frequent sugar consumption doesn’t just feed existing bacteria; it shifts your entire oral microbiome toward species that thrive in acidic conditions, creating a self-reinforcing cycle. Children aged 6 to 8 have an 18% rate of untreated decay in baby teeth, while 11% of children aged 2 to 5 already show untreated cavities, illustrating how early these patterns begin.
Reversing Early Damage
The single most important thing to understand about cavities is that the earliest stage can be reversed. Remineralization is the process of depositing calcium and phosphate back into weakened enamel, and it happens naturally every time your saliva washes over your teeth after an acid attack. Fluoride supercharges this process by helping minerals integrate into the crystal structure more effectively, creating a surface that’s actually more acid-resistant than the original enamel.
For this repair cycle to win out over damage, three conditions need to be met: fluoride exposure (through toothpaste, water, or professional treatments), adequate calcium and phosphate (from saliva or dental products), and enough time between acid attacks for repair to occur. If you’re snacking on sugar every hour, your teeth never get that recovery window, and demineralization outpaces repair. Spacing out meals and limiting sugary snacks between them gives your saliva time to do its job.