The acidity or alkalinity of the environment in the mouth is measured using the pH scale, which ranges from 0 (highly acidic) to 14 (highly alkaline). Tooth decay begins when the oral environment becomes too acidic, causing the destruction of the tooth structure. A neutral pH is 7.0, and the natural resting pH of the mouth is typically slightly above this, generally around 6.7 to 7.4. When the mouth’s pH drops, the acidic conditions start to dissolve the minerals that make up the teeth, initiating the process of decay.
The Critical pH Threshold
The point at which the oral environment shifts from being protective to destructive is known as the critical pH. For the hard, outer layer of the tooth, the enamel, this threshold is accepted to be pH 5.5. When the pH level falls below 5.5, the surrounding fluid is no longer saturated with the minerals needed to maintain the tooth structure. This marks the highest pH level at which there is a net loss of minerals from the enamel.
The critical pH is not a fixed, universal number, as it is influenced by the concentration of calcium and phosphate ions present in saliva. Other parts of the tooth structure, which are less mineralized than enamel, have a higher critical pH and are more susceptible to acid attack. Dentin, the layer beneath the enamel, and cementum, which covers the root, begin to demineralize at a pH closer to 6.2 to 6.7. This difference means that once the protective enamel layer is breached, decay accelerates more quickly in the softer underlying tissues.
The Mechanism of Enamel Demineralization
Tooth enamel is primarily composed of crystalline structures called hydroxyapatite. These crystals are made of calcium and phosphate ions, existing in stable equilibrium with ions present in the surrounding saliva. When the pH drops below the critical threshold, the concentration of hydrogen ions (acid) increases dramatically. These excess hydrogen ions actively pull calcium and phosphate ions out of the hydroxyapatite structure in the enamel.
This chemical process is termed demineralization, as minerals dissolve out of the tooth. The mouth constantly cycles between demineralization and remineralization, the natural process of rebuilding the tooth structure. During an acidic attack, the rate of mineral loss significantly outpaces the rate of mineral repair. This leads to a net loss of tooth material and eventually forms a visible lesion or cavity.
Identifying Sources of Acidic Attack
The pH in the mouth drops below the critical threshold through two primary mechanisms: internal microbial acid production and direct external acid exposure.
Microbial Acid Production
The first and most common cause is the metabolic activity of oral bacteria, particularly species like Streptococcus mutans. These bacteria consume fermentable carbohydrates, such as sugars and starches, left on the tooth surface. They then produce organic acids, mainly lactic acid, as a byproduct of their digestion. This localized acid production quickly lowers the pH within the dental plaque biofilm adhering to the teeth.
Following a single exposure to a sugary food or drink, the pH can remain below the critical level for 20 to 50 minutes. Repeated snacking or frequent consumption of carbohydrates prevents the mouth from recovering, leading to an almost continuous state of acid attack.
Direct External Acid Exposure
The second mechanism is the direct introduction of highly acidic foods and beverages, a process known as dental erosion. Common culprits include carbonated soft drinks, sports drinks, fruit juices, citrus fruits, and vinegars, which often have pH levels far below 5.5. Unlike bacterial acid, which is localized in plaque, these external acids bathe the entire tooth surface, causing widespread mineral loss. The combination of a low pH and chelating agents, such as citric acid, found in many of these items can intensify the erosive effect by binding to and removing calcium from the enamel.
Restoring pH Balance and Prevention
Saliva is the primary natural defense against acidic attacks, possessing a powerful buffering capacity. It is rich in bicarbonate, phosphate, and calcium ions that neutralize acids and raise the pH back to a neutral level. The bicarbonate system is especially effective, reacting with excess hydrogen ions to restore balance in the oral cavity. Saliva also contains the calcium and phosphate ions necessary for remineralization, helping repair early damage.
Prevention strategies focus on minimizing the duration of the acidic state. After consuming acidic foods or drinks, rinsing the mouth immediately with water helps wash away acid and speed up pH recovery. Delaying toothbrushing for at least 30 minutes after an acid attack is recommended, as brushing softened enamel can cause physical wear.
Fluoride is a powerful tool in prevention, incorporating into the tooth structure to form fluorapatite. Fluorapatite is significantly more resistant to acid dissolution than natural hydroxyapatite, with a critical pH threshold of approximately 4.5. This means the tooth can withstand ten times the amount of acid exposure before demineralization begins. Dietary modification is also effective; limiting the frequency of fermentable carbohydrate intake and confining acidic beverages to meal times reduces the total time the teeth spend below the critical pH.