Dental caries, or cavities, result from the destruction of tooth structure caused by a prolonged acidic environment. Acidity is measured using the pH scale, where lower numbers indicate greater acidity. When the oral pH drops significantly, it initiates a chemical process that dissolves the hard, protective layer of the tooth. Understanding the precise pH level that triggers this dissolution is fundamental to preventing tooth decay.
The Critical Threshold for Enamel Erosion
The specific point at which the tooth’s outer layer begins to dissolve is the critical pH, which is 5.5 for dental enamel. Below this threshold, the mouth’s environment becomes acidic enough that the protective minerals within the tooth are chemically vulnerable. The regular resting pH of saliva is near neutral, typically between 6.7 and 7.4. When acidity descends to 5.5, the balance shifts from a stable state to one of mineral loss, initiating the physical destruction of the tooth structure.
Sources of Oral Acidity
The primary reason the pH drops below the critical threshold is the metabolic activity of oral bacteria, particularly Streptococcus mutans and Lactobacillus species. These microorganisms thrive within dental plaque, a sticky film on tooth surfaces. When a person consumes simple sugars or fermentable carbohydrates, these bacteria rapidly metabolize them. This fermentation produces organic acids, such as lactic acid, which are released into the plaque biofilm. The concentrated presence of these acids overwhelms the mouth’s natural neutralizing agents, causing the localized pH to plummet.
The Demineralization Process
The chemical reaction defining the onset of a cavity is called demineralization, triggered precisely when the pH falls below 5.5. Dental enamel is the hardest substance in the human body, composed primarily of crystalline calcium phosphate known as hydroxyapatite. This mineral structure gives the tooth its rigidity and strength.
When acid attacks the enamel, hydrogen ions chemically react with the hydroxyapatite crystals. This reaction breaks down the mineral structure, causing calcium and phosphate ions to leach out of the enamel. This loss of ions dissolves the tooth’s crystalline structure, making the enamel porous and weaker. The initial stage appears as small, chalky white spots on the tooth surface, indicating lost density. If acidic conditions persist, the continuous leaching of minerals eventually creates a microscopic defect that enlarges into a cavity.
Saliva and Natural Restoration
The body’s natural defense against acid attack centers on the function of saliva. Saliva acts as a buffering agent, containing components like bicarbonate and phosphate that neutralize the acids produced by plaque bacteria, raising the pH back toward a neutral level. This neutralization minimizes the duration the oral environment spends below the critical pH of 5.5. Once the pH is restored, the mouth enters a phase called remineralization, where saliva, supersaturated with calcium and phosphate ions, redeposits them back into the weakened enamel structure, repairing early damage. Fluoride significantly enhances this repair process by helping to form fluorapatite, a more acid-resistant mineral structure.