Compomers and resin-modified glass ionomers have better aesthetics than conventional glass ionomer cements. Compomers and glass ionomer cements can release fluoride. This property can be useful in cases where a patient has a higher risk of experience tooth decay in future. Fluoride is a mineral which helps strengthen our teeth and protects them from decay, and it is found in lots of dental products including toothpaste. Compomers and glass ionomer cements are able to release fluoride over extended periods, and this may help to reduce the risk of a tooth decaying further. However, such a property does not negate the need for excellent oral hygiene to prevent oral disease. Compomers are recommended for patients at medium risk of developing dental caries. There is conflicting evidence regarding the amount of fluoride compomers can release: Powers, Wataha and Chen (2017) state compomers do not release as much fluoride as glass ionomer cements because they have a lower concentration of fluoroaluminosilicate glass particles; there is supporting evidence to suggest compomers only release 10% of that of glass ionomer cement. On the other hand, Richard van Noort (2013) states that, due to recent developments, modern compomers are now capable of releasing the same amount of fluoride over the lifetime of the restoration as glass ionomer cements. Emerging evidence has shown that compomers and glass ionomer cements are able to absorb fluoride from the oral environment when their own fluoride stores are depleted, a process described as 'recharging'. The material can then release this stored fluoride when the fluoride concentration in the oral environment falls, thus exposing the teeth to fluoride for longer. This recharging ability is not as effective in compomers as it is in glass ionomers cements. Nevertheless, this can further prevent the risk of tooth decay. There is evidence to show compomers have no advantage over an amalgam restoration with a fluoride releasing bonding agent, which releases mercury and fluoride. Compomers undergo some shrinkage during the setting reaction, and the extent of this polymerisation shrinkage is similar to that of dental composites. Compomers absorb water more rapidly than dental composites due to the addition of hydrophilic resin monomers within the matrix (see "Composition" section above). As such, water equilibrium is reached within days rather than weeks, months or even years in the case of dental composite materials. This property has the advantage of compensating for the polymerisation shrinkage during the setting reaction, thus reducing any gap that develops at the cavity margins.