A tooth cavity is a permanently damaged area in the hard surface of a tooth that develops into a tiny hole. It forms when bacteria in your mouth produce acid that eats away at tooth enamel over time. Cavities are one of the most common health problems worldwide: about 21% of U.S. adults between 20 and 64 have at least one untreated cavity right now, and roughly half of all children aged 6 to 9 already have some history of tooth decay.
How a Cavity Forms
Your mouth is home to hundreds of species of bacteria. Some of them, particularly one called Streptococcus mutans, feed on sugars and carbohydrates left on your teeth after you eat. As these bacteria digest those sugars, they produce acid as a byproduct. The bacteria also create a sticky film called plaque that clings to tooth surfaces and holds that acid right against your enamel.
Enamel is the hardest tissue in your body, but it’s made mostly of minerals, and acid dissolves minerals. Every time you eat something sugary or starchy, the bacteria ramp up acid production, and your enamel loses a small amount of its mineral content. This process is called demineralization. Your saliva naturally works to reverse this by depositing minerals back onto the tooth surface (remineralization), but if acid attacks happen too often or last too long, the balance tips. The enamel weakens, breaks down, and eventually a hole forms. That hole is a cavity.
The Five Stages of Tooth Decay
Cavities don’t appear overnight. Decay progresses through distinct stages, and catching it early makes a real difference in what treatment looks like.
White spots (demineralization). The earliest sign is a chalky white spot on the tooth where minerals have started to leach out. At this stage, there’s no actual hole yet, and the damage can still be reversed with fluoride and good oral care.
Enamel decay. If mineral loss continues, the white spot may darken to brown, and the enamel breaks down enough to form a small hole. This is the point where it officially becomes a cavity and needs to be treated by a dentist.
Dentin decay. Beneath the enamel sits dentin, a softer layer that contains tiny tubes leading toward the tooth’s nerve. Once decay reaches dentin, it spreads faster because dentin is less resistant to acid. This is often when you start feeling sensitivity to hot, cold, or sweet foods and drinks.
Pulp damage. The innermost part of the tooth, the pulp, contains nerves and blood vessels. When decay reaches the pulp, it causes significant pain and typically requires a root canal to remove the damaged tissue.
Abscess. Left untreated even further, bacteria can infect the pulp and create a pocket of pus called an abscess. This can cause intense pain, facial swelling, and fever. In severe cases, the tooth may need to be pulled entirely.
What a Cavity Feels Like
Early cavities often have no symptoms at all. You can have one developing for months without knowing it, which is why regular dental checkups matter. As decay progresses deeper into the tooth, symptoms tend to appear in a predictable pattern:
- Sensitivity: A mild zing when you eat or drink something sweet, hot, or cold
- Visible changes: Brown, black, or white stains on the tooth surface, or a visible pit or hole
- Toothache: Spontaneous pain that ranges from dull and persistent to sharp
- Pain when biting: Discomfort when you chew or press down on the tooth
If pain becomes throbbing or lingers after exposure to hot or cold, that often signals the nerve inside the tooth is damaged, meaning the decay has reached a more advanced stage.
How Dentists Find and Treat Cavities
During a routine checkup, your dentist looks for visual signs like staining, soft spots, or sticky areas on the tooth surface. X-rays reveal decay hidden between teeth or beneath existing fillings that isn’t visible to the eye.
Treatment depends entirely on how far the decay has progressed. A white-spot lesion caught early can sometimes be reversed with concentrated fluoride treatments, no drilling required. Once an actual hole has formed, the decayed portion of the tooth needs to be removed and replaced with a filling. If the cavity is too large for a standard filling but hasn’t destroyed the tooth, your dentist may use an inlay, onlay, or crown to restore the tooth’s shape and strength. When decay reaches the pulp, a root canal removes the infected tissue, and the tooth is sealed and capped. Extraction is the last resort, reserved for teeth that can’t be saved.
What Makes Some People More Prone to Cavities
Frequent snacking and sugary drinks are the most obvious risk factors because they give mouth bacteria a steady supply of fuel. But several other factors play a significant role.
Dry mouth is one of the biggest. Saliva does far more than keep your mouth moist. It rinses away food particles, neutralizes acid, and carries minerals that rebuild enamel. When saliva flow drops, whether from medications, medical conditions, or simply breathing through your mouth at night, your teeth lose that natural defense. Without adequate saliva flow, tooth decay and oral infections develop more readily.
The location of the tooth matters too. Back teeth (molars) have grooves and pits that trap food and are harder to clean thoroughly, making them prime targets for decay. Teeth that are crowded or overlapping create tight spaces where plaque builds up and floss can’t easily reach. Receding gums expose the tooth root, which lacks the protective enamel layer and decays more quickly.
How to Prevent Cavities
Fluoride is the single most effective tool for cavity prevention. When fluoride is present in your saliva, it gets incorporated into tooth enamel during the natural remineralization cycle. The resulting mineral structure is harder and more acid-resistant than what your body builds on its own. Even at very low concentrations, fluoride is effective. You get it from fluoridated tap water, fluoride toothpaste, and professional fluoride treatments at the dentist.
Dental sealants offer another strong layer of protection, especially for children. Sealants are thin coatings painted onto the chewing surfaces of back teeth to seal off the grooves where food and bacteria collect. Studies involving nearly 2,000 participants found that sealants reduced the risk of developing new cavities by 73% compared to fluoride varnish alone.
Beyond those two tools, prevention comes down to limiting how often your teeth are exposed to acid. Brushing twice a day with fluoride toothpaste removes plaque before it does damage. Flossing clears the tight spaces between teeth that a brush can’t reach. Drinking water after meals helps rinse away sugars. And spacing out snacks rather than grazing throughout the day gives your saliva time to neutralize acid and repair enamel between meals.
Cavities in Children
Tooth decay is the most common chronic disease in children. About 11% of kids aged 2 to 5 have untreated decay in their baby teeth, and that number climbs to nearly 18% among children aged 6 to 8. By the time kids are 6 to 9 years old, roughly half have had at least one decayed, filled, or missing tooth.
Baby teeth are especially vulnerable because their enamel is thinner. And while baby teeth eventually fall out, cavities in them still matter. Decay can cause pain, difficulty eating, and infections. It can also damage the permanent teeth developing underneath. Starting dental visits by age one, applying sealants once the permanent molars come in (usually around age 6), and supervising brushing until a child has the dexterity to do it well on their own all reduce cavity risk significantly.