Contents of toothpaste - safety implications

Summary
A typical toothpaste contains an abrasive, humectant, binder, detergent, flavour, preservative and therapeutic agent. Apart from an unsubstantiated hypothesis linking the ingestion of silica abrasives with the development of Crohn's disease, toothpaste abrasives are considered safe for human use. The humectants, binders, flavours, preservatives and colourings are used routinely in the food and pharmaceutical industries and should pose minimal health risks when used in toothpaste. The flavours, colourings or preservatives may give rise to allergic reactions, but these are relatively rare. The detergent or essential oil flavours may produce localised mucosal irritation, but this is also rare. As ingestion of excessive amounts of fluoride toothpastes by young children has been implicated in dental fluorosis, parents should supervise tooth cleaning in order to minimise toothpaste ingestion. For the majority of people, toothpastes, when used properly, are safe and help to maintain dental health.

Introduction
The primary purpose of brushing the teeth with a dentifrice (dens - tooth, fricare - to rub) is to clean the accessible tooth surfaces of dental plaque, stains and food debris. Tooth cleaning with dentifrices dates back over 2000 years, while cleaning with toothpicks and brushes is an even older practice. Abrasive dentifrice materials came to be used when it was found that brushes, while facilitating the cleaning of soft deposits from teeth, were inadequate for the removal of harder deposits and stains. Dentifrices have been prepared in several forms such as powders, pastes and gels. The most popular forms are the pastes and gels with over 5 billion tubes used worldwide each year.

Several studies¹ have shown that toothpaste has a key role in helping to remove dental plaque - the major cause of dental caries and periodontal diseases. Apart from aiding cleaning of teeth directly, toothpaste has a role, arguably its most valuable role, in encouraging people to clean their teeth. Most people in the developed world use toothpaste largely for cosmetic reasons. Modern developments in toothpaste formulation have led to the addition of agents to provide therapeutic, as well as cosmetic, benefits. Frequently used, modern toothpastes can help prevent dental caries and limit the regrowth of dental plaque.

The exact composition of a particular toothpaste varies with each manufacturer, but a typical formulation is abrasive 10-40%, humectant 20-70%, water 5-30%, binder 1-2%, detergent 1-3%, flavour 1-2%, preservative 0.05-0.5% and therapeutic agent 0.1-0.5%.

Abrasives
For efficient tooth cleaning, the dentifrice requires a degree of abrasiveness. The common abrasives used include dicalcium phosphate dihydrate, insoluble sodium metaphosphate, calcium pyrophosphate, calcium carbonate, alumina trihydrate, magnesium trisilicate and, more recently, silica gels. These abrasives have a hardness ranging between that of dentine and a value below that of enamel in order to be safe but effective stain removers. Nevertheless, cervical tooth abrasion can occur, creating wedge-shaped notches usually near the gingival margin. In severe cases, pulpal pathosis and periapical lesions may result. However, factors other than toothpaste abrasivity play the major role in cervical abrasion e.g. injudicious tooth brushing with a hard-bristle toothbrush and excessive pressure.¹

The hypothesis associating Crohn's disease with the ingestion of silica from toothpaste has been reviewed.² Enteric lesions similar to Crohn's were reported in 1950 after mixing talc or fine sand in the feed of dogs or by injecting their intestinal lymphatics with silica dust.³ In 1969, silica and silicates were evaluated for acceptable daily intake by the joint FAO/WHO Expert Committee on Food Additives. After consideration of a large number of short- and long-term animal studies and observations in man, the committee concluded that the available data on orally administered silica and silicates substantiated their biological inertness. Therefore, no restriction was placed on daily intake. Silica and silicates occur ubiquitously in the environment and are contained in certain foods such as potatoes, milk and in the drinking water. While the hypothesis linking Crohn's disease with toothpaste ingestion should not be ignored, at present there is little substantiating scientific evidence.

Humectants and binders
Humectants are used in dentifrices to prevent loss of water and subsequent hardening of the paste when it is exposed to air. The most commonly used humectants are glycerol and sorbitol. Binders are hydrophilic colloids which disperse or swell in the presence of water and are used to stabilise toothpaste formulations by preventing the separation of the solid and liquid phases. Examples of binding agents used in toothpaste include the natural gums (arabic, karaya and tragacanth), the seaweed colloids (alginates, Irish moss extract and gum carrageenan) and synthetic celluloses (carboxymethyl cellulose, hydroxyethyl cellulose), with the latter now being used increasingly for economic reasons. These substances are used routinely in the food and pharmaceutical industries and should pose a minimal health risk when used in toothpaste. However, sorbitol may cause diarrhoea in large doses as it acts as an osmotic laxative. The FAO/WHO Expert Committee on Food Additives recommends that the intake of sorbitol be limited to 150 mg/kg/day. Therefore, the use of 60-70% sorbitol gel toothpastes by small children should be supervised by parents.

Detergents, flavours, preservatives and colourings
Detergents lower the surface tension and therefore help loosen plaque deposits and emulsify or suspend the debris removed from the tooth surface during cleaning. Detergents also contribute to the foaming property of dentifrices, an effect which appeals to consumers. The commonly used detergent in toothpaste is sodium lauryl sulphate. Flavours constitute only a minor part of a dentifrice, but are important components for consumer acceptance. The flavour of a toothpaste is usually a blend of several components. The principal flavours used are peppermint, spearmint and wintergreen modified with other essential oils of anise, clove, caraway, pimento, eucalyptus, citrus, menthol, nutmeg, thyme or cinnamon. The humectants and some of the binders in toothpastes can act as nutrients for various micro-organisms. Microbial contamination of dentifrices is restricted by a low water activity and by the inclusion of preservatives such as benzoates. Colouring agents are also added to dentifrices. These include titanium dioxide for white pastes and various food dyes for coloured pastes and gels.

Contact sensitivity or mucosal irritation by dentifrices is relatively rare. Occasionally, the flavours, colourings or preservatives can cause allergic reactions in some individuals.⁴ These may include desquamation and oedema of the lips and tongue, perioral dermatitis, angular cheilitis, gingivitis and intra-oral ulceration. Some toothpastes can cause mild irritation of the oral mucosa which disappears after use and is usually attributable to the detergent or essential oil flavours. This again is quite rare as the majority of people prefer the more flavoured dentifrices as the tingling sensation makes the mouth feel fresh and clean, albeit for only a few minutes.

Therapeutic agents
The use of fluoride dentifrices is beneficial in the prevention of dental caries.⁵ Most dentifrices today contain 0.1% (1000 ppm) fluoride, usually in the form of sodium monofluorophosphate (MFP); 100 g of toothpaste containing 0.76 g MFP (equivalent to 0.1 g fluoride). The concentration of fluoride in toothpaste is limited in Australia to a maximum of 1000 ppm by a recommendation of the Standard for Uniform Scheduling of Drugs and Poisons No. 6 (NHMRC, 1992). The suggested toxic dose of fluoride ion is 5 mg fluoride per kg body weight.⁶ For a 10 kg child, this corresponds to approximately half the contents of a 90 g tube of toothpaste. Therefore, young children should not be allowed unsupervised access to fluoride dentifrices.

A review of fluoride benefits and risks by the U.S. Public Health Service⁷ concluded from more than 50 human studies that no evidence existed showing an association between fluoride and cancer. However, there is evidence of an increase in the prevalence of mild dental fluorosis.⁸ The consumption of excessive amounts of fluoride during enamel formation may result in dental fluorosis. This is a continuum of changes in the enamel varying from fine white lines in mild cases to very chalky, opaque enamel which breaks apart soon after tooth eruption. Since fluoride appears to affect the activity of the ameloblast, especially during the late secretion or early maturation of enamel, excessive fluoride intake is of concern during the first 7 years of life. Toothpastes have been identified as a significant source of fluoride for the young child. The NHMRC has reported in its review of the effectiveness of fluoridation, that fluoride from toothpaste accounts for up to 53% of the total fluoride intake of children aged two years.⁹

For the purpose of enhancing the safe use of fluoride dentifrices by children, several measures should be taken to minimise the risk of developing dental fluorosis. Parents should be advised to supervise tooth cleaning closely using only small (pea-size) quantities of toothpaste. Manufacturers should be encouraged to market a low fluoride dentifrice (e.g. 400-500 ppm fluoride) for infant use. This level of fluoride in toothpaste, given that all other sources are constant and low, should result in a total fluoride intake which does not exceed the recommended upper limit of 0.07 mg/kg of body weight for a child between 2 and 7 years of age.¹⁰ A low fluoride, sorbitol-based toothpaste designed specifically for children is available (Colgate Junior Toothpaste) and contains 0.304% MFP (400 ppm fluoride). Data from several independent studies indicate that, although a dose-response relationship does exist for fluoride levels in toothpaste and caries, use of a 400 ppm fluoride-containing paste by children under 7 years of age instead of the standard 1000 ppm fluoride paste should not increase their caries risk. There is currently no glycerol-based, 400-500 ppm fluoride toothpaste available in Australia.

In addition to claims of the anticaries activity of fluoride, new therapeutic dentifrices are being promoted to the public and the dental profession for the control of dental plaque and gingivitis. Triclosan (2,4,4'-trichloro-2'-hydroxy diphenyl ether), an antimicrobial agent used extensively in deodorants, soaps and other dermatological preparations, is the active agent in these new dentifrices. In various clinical studies, brushing with a 0.3% triclosan-containing dentifrice when compared with a control paste resulted in significant reductions in dental plaque formation and gingival inflammation.¹¹ From animal toxicity, mutagenicity, teratogenicity and carcinogenicity studies, as well as pharmacokinetic studies in man, it has been proposed by the major toothpaste companies that 0.3% triclosan-containing toothpastes are safe for human use. However, the U.S.A. Food and Drug Administration has stated that new drug approval will be required for a toothpaste containing triclosan because the ingredient has not been used in any intra-oral drug or cosmetic preparation previously.

Conclusion
Dentifrices have evolved and improved over the last 2000 years. The most significant improvement was the introduction of fluoride in the 1960s resulting in the development of toothpastes with anticaries efficacy. For the majority of people, modern toothpastes, when used properly, are safe and help to prevent dental caries, dental plaque formation and gingival inflammation. However, the use of fluoride-containing pastes by young children should be closely supervised by parents to restrict fluoride ingestion.