According to the Eco-Dentistry Association (35), waste generated from the dental office includes 1.7 billion plastic sterilisation packets and >680 million plastic barriers for dental chairs, covers for light handles and drapes. Additionally, the use of items such as personal protective equipment, plastic disposable dental trays and plastic packaging of various dental products (syringes, polythene or polyvinyl chloride foils and dental materials) further exacerbate the existing burden (36).

Hence, evidence suggests that dentistry has a major influence on the environment and contributes to the carbon footprint; greater emphasis should be placed on changing these practices towards an eco-friendly and sustainable environment.

Focus on the environmental impact of dentistry has been limited, up until the past few decades. The majority of materials used in dental practice have a high potential to subsequently breakdown and become incorporated into the environment as pollutants. A major part of the discussions regarding environmental pollution in dentistry have been concentrated on amalgam and its potential health effects due to mercury toxicity (8).

Controversies surrounding amalgam have led to its replacement with resin-based restorative materials, which are considered inert or to have a reduced impact on the environment, which is in accordance with the mandates in the Minamata Treaty (37).

All the constituents of resin-based composites have the potential to act as environmental pollutants upon degradation. The environmental impact of these materials mainly originates from their contribution to the carbon footprint during the process of manufacturing, distribution, disposal, finishing and polishing and removing old resin-based restorations (9).

Resin-based composites can be classified on the basis of fabrication and use as follows: i) Direct placement in the oral cavity and activation: The conversion of monomers to polymers through this method is incomplete due to partial conversion. At best, only 60-75% of the monomers convert to form polymers. These levels can drop as low as 30% at the base of a restoration (10). ii) Indirect placement: This method involves the fabrication of appliances and restorations outside the oral cavity, in dental laboratories through either heat cure polymerisation or blocs/ingots for machined CAD/CAM. Restorations fabricated in this method have relatively higher degrees of conversion of monomers into polymers (38).

Resin-based composites have the potential to enter the environment by either chemical means (dissolution and breakdown of the material within the oral cavity), or physical means (through the process of milling or grinding, finishing and polishing and removing old/defective restorations). The illustrating in Fig. 2 depicts microparticulate waste derived from resin-based composite materials. However, most commonly, this release is a result of a combination of both processes (7). According to the U.S. Environmental Pollution Agency (USEPA) (39), potential contamination by dental composites occurs through its accidental discharge during the transportation of dental waste or any malfunction of landfills which can cause potential environment contamination of dental composites (11).

Denture base polymers can either be heat cured, auto-polymerised self-cured, light-activated or thermoplastic resin (40). However, this polymerisation has been often incomplete and residual monomer leaches out into the oral cavity, causing health hazards upon entering the gastrointestinal tract and the inhalation of these particles can cause asthma, anorexia, headache and drowsiness (41).

Polymers are widely used in orthodontics in elastomeric ligatures and chains, polycarbonates for esthetic brackets and myofunctional appliances. The amount of bisphenol A released is of minimal risk to humans; however, the effects of long-term exposure need to be considered (13). Bisphenol A is known to be a toxic chemical in the environment and recently, in animal models, it has been shown to induce carcinogenesis and mutagenesis (13,42).

In orthodontics, the release of bisphenol A can occur through three main ways: i) Through the peripheral margins of orthodontic brackets (43); ii) in bonded fixed retainers, a large surface to volume ration of the adhesives is exposed in the oral cavity and it allows aging, degradation and leaching of bisphenol A in an unpredictable manner (15); iii) upon the completion of orthodontic treatment, grinding and elimination of the adhesive with instruments is done which releases the components into the environment via aerosols (12).

Additionally, a recent in vitro study highlighted the detachment of microplastics from commercial clear aligners due to their mechanical friction. The majority of microplastics have a diameter ≥20 µm, which can be excreted from the gastrointestinal tract (17).

During the finishing and polishing of dental restorations, or when old, defective restorations are removed, secondary microplastics are released into the oral environment (18). These materials may contain additives such as bisphenol A, phthalates, some brominated flame retardants that give desirable properties in colour and transparency and also counteract the breakdown from ozone, bacteria, temperature and provide thermal and electrical resistance (19). However, they have been proven to be endocrine disruptors which alter the homeostasis of endocrine system, inhibit the action of natural hormones and alter its synthesis (44). The superficial layers of pit and fissure sealants remain unpolymerized on exposure to oxygen which causes leaching, particularly during the initial few hours post-placement, which gradually decreases (45).

Microparticulate waste is generated chairside when resin-based composites undergo clinical grinding through the use of high-speed rotary and abrasive burs and discs. This is usually performed while removing defective and/or old restorations or while shaping, finishing and polishing the restorations. These wastes can also be generated from laboratory processes, such as the milling and grinding of pre-polymerized resin blocks to design indirect restorations such as crowns, inlays, onlays and implant abutments (7). In comparison of the amount of waste produced, it was noted that indirect restorations generated significantly higher quantities of microparticulate wastes (46). These wastes are removed from the oral cavity by means of a suction tip or an aspirator, which is released into the sewage wastewater, and eventually, into the environment (21).

The American Dental Association recommends changing a toothbrush every 3 to 4 months, or more often if the bristles are visibly matted or frayed (47). In the US market, approximately one billion toothbrushes are thrown away each year and 50 million pounds of toothbrushes are added into the landfills annually (36). Considering the world's population of 7.53 billion individuals, ~29.4 billion toothbrushes are discarded each year. On average, a plastic toothbrush weighs ~20 g; it can thus be calculated that the whole of humanity produce 600 million kg of plastic toothbrush waste in only 365 days (47).

Among the currently available toothbrushes, including manual plastic and bamboo toothbrushes and electric toothbrushes, the greatest contribution to the overall environmental impact has been observed by traditional plastic manual toothbrushes (23) as they are made from polypropylene plastic and nylon, which are derived from non-renewable fossil fuels (48). The manufacturing process of the nylon bristles releases nitrous oxide (36). Nitrous oxide is a greenhouse gas, which is 310-fold more potent than carbon dioxide (49). The toothbrush handles are relatively larger, made of polypropylene plastic which is neither biodegradable, nor recyclable (23). The amount of derived plastic waste in landfills further increases over time.

In the Indian scenario, the mean yearly release of microplastics from toothpastes into the environment was gauged as 1.4 billion grams per year, which poses a significant environmental threat (24). In order to systematically assess the effects of toothpaste on the environment, two main aspects must be considered:

i) Toothpaste packaging: The toothpaste tube is primarily comprised of high-density polyethylene with the cap comprised of polypropylene. Of note, ~4,000 empty tubes of plastic need can become converted into one ton of plastic (24).

ii) Contents in toothpastes: Toothpastes are open-use consumer products, which are intended to be washed off and ultimately end up in the drains. Active contents of toothpastes are remineralizing agents (50), antibacterials (51) and lately, also probiotics (52). Numerous personal care and cosmetic products, such as toothpastes, soaps and gels, include microplastics to enhance the scrubbing and cleansing action (25). In toothpastes, they can be added to improve the whitening properties and act as a polishing aid (26). These microplastics which eventually accumulate in marine bodies through the drainage system and in turn within the tissues of aquatic life as shown in Fig. 3. The constituents include polyethylene (27), cellophane, polypropylene, polyvinyl chloride and polyamide in varying concentrations (28). The most widely prevalent microplastics are polyethylene and toothpastes contain up to 1.8% of this component (29). A previous study demonstrated various changes in marine organisms, such as the complete or partial absence of contacts in the filaments of gills, the presence of hemocytic infiltration in gills or mutation of ciliated epithelium to squamous epithelium within digestive organs and severe necrosis was noted in the gonads (30).

On average, each individual uses approximately three million miles of dental floss each year (53). Plastic forms a major component in the preparation of dental floss and its packaging.

Dental floss contains plastic. Traditional dental floss is comprised of non-recyclable and non-biodegradable materials, such as synthetic waxed nylon and teflon that add on to singe use plastic waste that pollutes marine bodies. Additionally, it may be coated with pre- and polyfluoroalkyl substances to allow passage between teeth; however, these chemicals are released while decomposing, which is toxic to the environment. It is also inadvertently consumed by marine life, which can cause starvation, poisoning or suffocation. Corals, that form an essential part of the marine ecosystem have floss entangled within them.

Dental floss with plastic packaging typically comprised of polypropylene plastic have the worst carbon footprint and highest environmental impact compared to other interdental cleaning aids. The handle alone formed 49% of the overall carbon footprint.

Mouthwashes contain antimicrobials, such as triclosan [5-chloro-2-(2,4-dichlorophenoxy) phenol]. The use of triclosan is not highly regulated due to its low acute toxicity and safety. The continual exposure of on marine life to triclosan leads to its accumulation within tissues and acute and chronic toxic effects, such as increased catabolic activity within tissues. Concentrations >34.5 nmol/l triclosan in river biofilms have been shown to exert inhibitory effects on periphytic algae (31).

PPEs are prepared from various synthetic polymers, including high- and low-dentistry polymers which are neither recyclable, nor biodegradable. High-density polymers include polyester, polyvinyl chloride and polyvinyl alcohol, which sink into deep marine sediments (57). On the other hand, low-density polymers, such as polyethylene, expanded polystyrene and polypropylene float on sea water (58).

Disposable masks comprise of three layers where the inner layer is made up of hydrophilic soft fibers, the middle layer is made of melt-blown filter and the outer layer is made of non-woven hydrophobic fibers. The main filtering layer is the middle layer, which is manufactured by the entanglement of micro- and nanofibers (59).

Gloves were mostly made up of polyvinyl chloride, latex and nitrile. Face shields are made using a wide variety of materials, including polyethylene terephthalate glycol, acetate, polyvinyl chloride and polycarbonate (60). The fate of these microplastics ranges from either floating along the ocean currents, or sinking into the sediments and becoming a part of the geological record (61).

Various practices can be adopted by the dental team to make environmentally friendly choices. To begin with, further attention should be directed towards purchasing products with minimal plastic packaging and the use of reusable plastic containers should be encouraged. Non-critical and semi-critical items, such as paper towels, cotton rolls or wool rolls made from recycled or partially recycled materials can be used. Containers or packaging with polyvinyl chloride should be avoided, whenever practical. Digital platforms for the maintenance of records and the use of both sides of pages with single spacing can reduce the amount of paper used in the office.

The use of biodegradable bags made out of vegetable starch should replace plastic bags as natural materials, such as vegetables are degraded readily and completely and serve as natural soil fertilizers, as compared to plastics. To disinfect the dental operatory, the Occupational Safety and Health Administration (OSHA) recommended reusable nitrile gloves should be used instead of disposable examination gloves. Furthermore, the use of gloves should be limited as per requirement, and injudicious use should be restricted.

Instruments that require sterilization in an autoclave should be packed in metal autoclave cassettes or reusable cloth or fabric bags, instead of plastic backed paper. This alternative is more cost-effective, protects practitioners and patients and does not overburden the environment (14). Instead of using plastic suction tips, metal or autoclavable suction tips can be used. Special considerations should be made towards incorporating a container for recyclable items and effort to ensure the correct disposable of recyclable items to designated centers, rather than adding on to landfills (7).

The concept of ‘green dentistry’ incorporates the four R's: Reduce, reuse, recycle and rethink. The use of trays for the application of fluoride should be substituted by the use of fluoride varnish. If the need for trays is mandated, such as for the preparation of dental impressions, metal or reusable trays should be used instead. Clinical recommendations should be followed to avoid overbuilding resin-based composite restorations so as to avoid further reduction and generation of microparticulate waste (62). Consideration towards repairing defective restorations, as compared to complete removal. In the case that the removal of resin-based restorations is deemed necessary, a water spray should be then used in conjunction to reduce the generation of microplastics (9). The development of measures is required to allow the complete conversion of monomers, such as placing the curing light as close to the adhesive as clinically possible during polymerization to allow the maximum direct conversion of the monomer and reduce the amount of bisphenol A released (14). Clinicians are advised to perform a pumice prophylaxis after bonding to the surface of these resin adhesives (43). Rinsing of the oral cavity is recommended after bonding orthodontic brackets within the first hour of placement to prevent the potential leaching of monomers (14). Brackets comprised of ceramic release lesser amounts of unpolymerized resin than polycarbonate brackets (16). The use of manual toothbrushes with replaceable heads and bamboo toothbrushes have a marked positive environmental impact in terms of their effect on climate change, resource use, ozone depletion and accumulation in marine bodies (23). Additionally, they use >97% less plastic than traditional plastic toothbrushes. However, the higher purchase costs of bamboo toothbrushes may pose as a barrier for consumer use (23). Alternatives to dental floss include silk and candelilla wax floss, silk and beeswax floss with packaging comprised of glass and aluminum dispensers or in carbohydrate or bioplastic boxes (36). It is essential to advise patients to not flush down used dental floss, as it can cause severe clogging in the sewer systems by entangling with other debris, hair or other particles.

The present review aimed to provide a comprehensive overview of the environmental impacts of plastic use in dentistry, from restorative practices to oral hygiene products, and their contribution to microplastic waste. However, the present review has certain limitations, such as a lack of quantitative data on the specific contributions of dental practices to global plastic pollution, limited focus on the regional variations and how local regulations and practices may influence this generation and management of plastic waste in dentistry.