A cavity is a hole in your tooth caused by decay. It forms when bacteria in your mouth feed on sugars and produce acid that slowly dissolves the hard outer layer of your teeth. Cavities are one of the most common chronic health conditions: nearly 21% of U.S. adults between 20 and 64 have at least one untreated cavity, and about 10% of adolescents do as well.
How a Cavity Forms
Your mouth is home to hundreds of species of bacteria, and some of them thrive on the sugars and starches you eat. One key player, a bacterium called Streptococcus mutans, breaks down sugar almost exclusively through a process that produces lactic acid as a byproduct. That acid is strong enough to lower the pH inside your mouth below 5.5, which is the critical threshold where tooth enamel starts to dissolve.
Enamel is the hardest substance in your body, but it’s made of mineral crystals that are vulnerable to acid. When the pH drops below 5.5, those crystals begin to break apart in a process called demineralization. Your saliva naturally works to neutralize the acid and redeposit minerals back onto your teeth, but if acid attacks happen too frequently (from snacking, sipping sugary drinks throughout the day, or poor brushing habits), your saliva can’t keep up. The mineral loss outpaces the repair, and decay sets in.
Stages of Tooth Decay
Cavities don’t appear overnight. They progress through distinct stages, and catching them early can make a significant difference in treatment.
White Spot Lesion
The earliest sign of trouble is a white spot on the tooth surface. These chalky, opaque patches appear because mineral loss has made the enamel more porous, changing the way light reflects off it. They’re often only visible after the tooth surface has been dried, which is why you might not notice them at home. At this stage, the enamel surface is still intact, and the damage is reversible. Fluoride treatments, improved brushing, and other remineralization strategies can halt and even reverse the process without any drilling.
Enamel Decay
If the acid attacks continue, the enamel surface becomes rough and eventually develops tiny cracks and pits. Once the surface breaks through, you have an actual cavity. At this point, the damage can’t repair itself and needs a filling.
Dentin Decay
Beneath the enamel sits dentin, a softer, yellowish layer that makes up most of the tooth’s structure. Dentin is more vulnerable to acid than enamel (it starts dissolving at a pH of about 6.0, a less acidic threshold). Once bacteria reach the dentin, decay accelerates. Your tooth responds by producing a protective layer of new dentin to shield the innermost pulp, but this defense has limits. You may start feeling sensitivity to hot, cold, or sweet foods at this stage.
Pulp Involvement
The pulp is the living core of your tooth, containing nerves and blood vessels. When decay reaches it, the result is pulpitis, an inflammation that can cause throbbing or sharp pain lasting 30 seconds or more after exposure to heat, cold, or sweets. If the nerve dies entirely, you may actually lose sensitivity to temperature and sweets, though you’ll likely still feel pain when pressure is applied to the tooth. At this point, a root canal is typically needed: the dead or infected pulp is removed, the inside of the tooth is cleaned and sealed, and a crown is placed over it a few weeks later.
Who Gets Cavities
Nearly everyone is at risk, but some groups are hit harder. According to the CDC’s 2024 Oral Health Surveillance Report, about 11% of children aged 2 to 5 already have untreated decay in their baby teeth. That number jumps to nearly 18% for children aged 6 to 8. Among adults 65 and older, about 13% have untreated cavities.
Risk factors include frequent snacking or sipping on sugary or acidic drinks, dry mouth (since saliva is your primary defense against acid), receding gums that expose tooth roots, and not brushing or flossing consistently. Cavities are also more common in people without regular access to dental care.
How Dentists Find Cavities
Not all cavities are visible to the naked eye, especially those that form between teeth. Traditional X-rays (bitewing radiographs) have been the standard detection tool for decades, but they have limitations. Their sensitivity is low in early stages, and they can’t distinguish between a cavity that has broken through the surface and one that hasn’t yet. Studies show digital X-rays detect cavities between teeth with roughly 55 to 63% sensitivity, meaning they miss a significant portion of early lesions.
Newer tools like laser fluorescence devices perform better for certain types of cavities. These pen-like instruments measure how much fluorescent light a tooth emits, since decayed tooth structure fluoresces differently than healthy structure. In clinical studies, laser fluorescence devices detected cavities between teeth with 100% sensitivity and 85% specificity, significantly outperforming digital X-rays. Your dentist may use a combination of visual exams, X-rays, and these newer tools to catch decay as early as possible.
How Fluoride Protects Your Teeth
Fluoride works by swapping into the mineral structure of your enamel, replacing part of the original crystal and creating a version that is more compact and more stable. This modified mineral is harder for acid to dissolve, which means your teeth can withstand more acid attacks before demineralization begins. Fluoride also helps drive the remineralization process, encouraging minerals from your saliva to redeposit onto weakened areas of enamel.
This is why fluoride shows up in so many forms: toothpaste, municipal water supplies, professional varnishes applied at the dentist. For early white spot lesions that haven’t broken through the surface, fluoride application is one of the most effective ways to reverse the damage entirely.
What Treatment Looks Like
Treatment depends entirely on how far the decay has progressed. For white spot lesions, no drilling is needed. Your dentist may recommend prescription-strength fluoride toothpaste, fluoride varnish applied in the office, or products containing calcium and phosphate compounds that help rebuild enamel. Another option is resin infiltration, where a liquid resin is applied to the porous enamel to seal it and stop the decay from progressing.
Once a cavity has formed an actual hole, a filling is the standard treatment. The decayed portion of the tooth is removed and replaced with a filling material, commonly tooth-colored composite resin or, less frequently today, silver amalgam. For larger areas of decay that compromise the tooth’s structure, a crown may be needed to cap and protect what remains.
If decay has reached the pulp, a root canal removes the infected tissue and saves the tooth. The alternative is extraction, which creates its own set of problems: neighboring teeth can shift, bite alignment changes, and bone loss can occur in the empty socket over time.
Reducing Your Risk
The biology of cavities points to a few practical strategies. Since acid production ramps up every time bacteria encounter sugar, the frequency of sugar exposure matters more than the total amount. Sipping a soda over two hours bathes your teeth in acid repeatedly, while drinking it in one sitting limits the exposure to a single acid attack. The same logic applies to snacking: fewer episodes of eating means fewer drops in pH throughout the day.
Brushing twice daily with fluoride toothpaste removes the bacterial film (plaque) that produces acid. Flossing or using interdental brushes cleans the surfaces between teeth where cavities commonly form and where your toothbrush can’t reach. Chewing sugar-free gum after meals stimulates saliva flow, which helps neutralize acid and wash away food particles.
Tap water in most U.S. communities contains added fluoride, which provides a low, steady level of protection throughout the day. If you drink primarily bottled or filtered water, you may be missing this benefit, something worth discussing at your next dental visit.