Key process steps in managing dental solid waste involve collection from different dental facilities, storage, segregation into infectious and non-infectious categories, further separation into polymer-rich, metal-rich, and ceramic-rich constituents, followed by appropriate end-of-life treatments. A number of methods, such as low/high temperature thermal treatments, autoclaving, incineration, chemical methods, mechanical methods, landfilling, etc., are used to dispose different types of dental waste, with the chosen approach depending on the costs involved, infrastructure, and environmental impact. Infectious waste must be disinfected first and pathogens, viruses, and bacteria removed before disposal; human tissues, sharps, and cultures must be disposed of after treatment and untreated waste sent to landfills. In dentistry, waste from dental amalgam, mercury waste, silver- and lead-containing waste are fairly common and pose severe health risks as well as environmental hazards. A brief overview is provided next of the main waste management approaches with a focus on key features and limitations.

Incineration involves the burning of waste at high temperatures (200–1000 °C) in closed environs. The waste undergoes combustion (pyrolysis) in the presence (absence) of air/oxygen, reducing combustible and organic waste to inorganic end products while significantly reducing the waste volumes of incombustible constituents. Basic requirements for incineration include reasonably high calorific value of the waste (>2000 Kcal/kg), high combustible content (>60%), non-combustible solids content less than 5%, and low moisture content (<30%). However, incinerators are known to be highly polluting, emitting toxic, hazardous gases along with generating toxic ash residues [111]. Other thermal waste treatments include the use of microwave, infrared, and plasma treatments in waste treatment [112]. 

Specific challenges to incinerating dental waste can be attributed to its complex composition profile, which includes a variety of polymers, mercury amalgams, metals, ceramics, composites, nanomaterials, etc. These materials have very significant differences in their thermal degradation behaviors and can generate different end products depending on the heat treatment process. A very wide variety of polymers are used in dentistry for restorative, regeneration, or preventive treatments [114]. While the primary focus in emerging dental polymers is on the resultant properties, e.g., mechanical, thermal, water solvation, and bio-functionality (antibacterial capability, bioactive delivery, remuneration, etc.) [115], little attention has been paid to determining their high-temperature thermal degradation, generation of toxins and dioxins, role of metals, and their eventual destruction.

The incineration of biomedical waste, including dental waste, has been identified as one of the highest known sources of dioxin-based emissions [116] and mercury emissions [117]. These pollutants are present in the atmosphere as gases as well as microparticulates [118]. Once released into the atmosphere, these can get transported over long distances, causing significant environmental damage [119]. Exposure to dioxins and furans is known to cause gastrointestinal, neurologic toxicity, hepatic, and dermal issues in humans as well as immunologic toxicity and adverse reproductive effects in animals [120]. If mercury-containing items are sent to an incinerator, mercury-based vapors will enter the global distribution cycle, thereby contaminating the environment. Mercury is known to be a potent neurotoxin; mercury levels exceeding permissible levels can lead to chronic fatigue, loss of appetite, and dizziness [121]. Mercury toxicity in the elderly population has been linked with Alzheimer’s disease as well as irreversible organ damage [122].

Inorganic compounds present in incinerated ash residue include potentially toxic heavy metals, metalloids, ceramics, and other oxides [123]. Various metals and metalloids contain biologically essential elements, such as cobalt, copper, chromium, zinc, and manganese, and non-essential elements like arsenic, cadmium, lead, etc., some of which are toxic to humans, plants, or animals at high concentrations [124]. Various ceramics present in dental waste require very high temperatures (>2000 °C) for processing; as incineration temperatures are too low for their degradation, these precipitate out as ash residue [125]. The ash materials derived from the incineration of hazardous medical waste are generally disposed of in landfills after the solidification and/or stabilization process. A serious concern with respect to developing nations is the uncontrolled incineration of medical waste without flue gas treatment and extensive release of toxic emissions [126]; such practices should be avoided to the extent possible for the health and safety of the populace near waste management facilities.

Despite the advantages of the incineration process, up to 25% of residues may be generated in the form of bottom ash and fly ash from the non-combustibles present in waste [127]. As the dioxins, volatiles, and heavy metals produced during incineration tend to concentrate in these residues, these are classified as hazardous waste and should be disposed of carefully [128]. Water washing has been used as a pretreatment method to remove soluble substances such as chlorides from these residues [129]. The washing of ceramic-rich sludge produced during incineration has also been used to remove impurities and improve the efficiency of downstream processing [130]. The reutilization of these waste products further enhances the environmental sustainability of dental waste management and needs to be implemented on several fronts.

Landfilling is one of the most commonly used disposal approaches for managing solid waste in developed as well as developing nations across the globe [131]. In addition to requiring large areas of land for dumps, the generation of leachates in the form of highly concentrated organic/inorganic liquids raises serious environmental and contamination issues. Residues from other waste management techniques, such as composting, recycling, and incineration, are also disposed of in landfills [132]. Leachate is generated from waste moisture, rainwater, and as a byproduct of waste degradation and can contain a wide range of highly toxic and potential carcinogenic toxic macro/micropollutants [133,134,135]. Landfill leachate contaminates nearby aquatic systems, thereby impacting the quality of waterways with detrimental effects on human health, flora, and fauna [136,137].

The European Union Landfill Directive has established the principles and rules for controlling landfill leachates, stipulating a leachate treatment system and leachate confinement at landfill sites [138]. Measures need to be taken to decrease seepage, lower interaction between the leachate and the landfill, as well as leachate harvesting wherever feasible [139]. Landfills are also known to emit methane, hydrogen sulfide, greenhouse, and other noxious gases affecting air quality, causing health issues and environmental damage [140]. Leachate composition includes inorganic macro/micro materials, dissolved organic matter, heavy metals, xenobiotic chemicals, and small amounts of mercury, lithium, cobalt, etc. Elements such as As, Cd, Pb, Hg, and Ni are known to have a wide spectrum of toxicity, including neurotoxic, teratogenic, hepatotoxic, mutagenic, and nephrotoxic effects [141]; Cd, As, and Cr are considered to be carcinogenic [142]. When a number of discarded mercury-based products are dumped in landfills along with other waste, mercury is released during waste decomposition and can be transported over long distances or become a part of leachate [143]. Proper monitoring of landfills, leachate treatment, and risk assessment are essential to prevent ecological harm as well as to avoid leachate toxins from contaminating groundwater, surrounding soil, and the environment [144].

While incineration and landfilling have been used for managing traditional dental/healthcare sector waste, the presence of novel dental materials in dental waste, their degradation behaviors, waste treatment, and associated environmental effects raise new concerns and need to be addressed separately.

A wide variety of nanomaterials (1–100 nm in size) are being used in various aspects/applications of dentistry, such as copper-based nanomaterials, including copper-coated metals, copper amalgam alloys, glass ionomer cements, nano copper-Ca₂SiO₄, nickel-titanium-copper alloys, etc., and zinc oxide nanoparticles, silver-based alloys, etc. End-of life nanomaterials are typically referred to as nanowaste [145]. Nanowaste is an emerging problem, as little is known about the sustainable waste management options for the diverse range of nanomaterials with distinct characteristics [146]. The fate and extent of nanoparticles in incineration plants are poorly understood. During waste incineration, the behavior of nanoparticles can be quite complex, as these can behave like gases when airborne creating no sedimentation or be carried along with larger particles and diffuse as nanopollutants [147,148]. Nanowaste needs to be treated as a separate category of waste as uncertainties due to extremely small sizes, a variety of shapes, chemical reactivity, and biocompatibility makes it significantly different from standard waste types [149]. The liberation of nanoparticles in dental waste, their chemical and mechanical interactions, and toxicity can lead to environmental damage and serious risk to the health and safety of health workers [150]. To better understand the risks from nanowaste, the behavior of nanomaterials in traditional waste management processes, i.e., incineration, recycling, wastewater treatment, and landfilling, needs to be investigated in great detail and evaluated for sustainable treatment of nanowaste [151].

The three main constituents of resin-based dental composites are an organic matrix composed of resin monomers, inorganic fillers, and coupling agents. A very wide variety of organic monomers/polymers are used in dental applications. While developing these resin-based materials, focus is always on the requisite dental applications and their basic/key requirements [152]. Little is known about the high-temperature (up to 1000 °C) degradation behavior of these materials during incineration, as such high temperatures are never encountered during the utilization/application of these materials [153]. Similarly, there are negligible experimental, theoretical, or modeling investigations of their degradation and/or leachate behavior when buried in a landfill as a mixed waste. This points to a major gap in knowledge for the sustainable management of dental waste.

The pyrolytic decomposition of polymeric waste into char, gasoline-based fuel, and synthetic gas is a key industrial approach, wherein the hydrogen generated from polymer degradation is used for the hydrogenation of unsaturated intermediates during thermal cleavage of polymer chains [154,155]. During thermal treatment, most polymer degradation was found to reach completion by 450–550 °C [156,157]. Metals present in resin-based dental waste can have a catalytic influence on the polymer degradation behavior as well as thermal stability [158]. There is also a strong likelihood of the generation of harmful particulates and toxic emissions [159]. Dioxins are highly stable and may require high temperatures (>700 °C), excess oxygen, and long residence times (>2 s) for destruction. Once these enter the human body, these tend to be absorbed in the fatty issue and stored for a long time [160].

In-depth investigations need to be carried out on the thermal degradation behavior of resins used in various dental applications towards identifying suitable and optimal routes for managing their end-of-life waste. While there are challenges galore in the incineration of these products, landfilling such resinous waste is not a good option either. These organics are most likely to be non-biodegradable and are likely to persist in landfills for long periods of time. Significant research needs to be carried out in this field towards environmentally sustainable waste management.

Cordeiro et al. [161] investigated the recycling of zirconia waste powder generated during the manufacture of dental prostheses. These powders were calcined at 500 °C and de-agglomerated in a rolling mill to produce micron-sized powders. These were later sintered in the temperature range of 1300–1500 °C and their strengths and mechanical properties were determined. These sinters were found to be suitable as an alternative low-cost and high-strength material in ceramics.

A wide variety of ceramics are used in a range of dental applications, e.g., glass ceramics; leucite-reinforced glass ceramics based on SiO₂, Al₂O₃, and K₂O; ZrO₂-reinforced lithium silicate glass; lithium disilicate glass; etc. [162,163]. Ceramics are known as highly stable refractory oxides that require very high temperatures for their degradation. For example, the melting and boiling points of SiO₂ are 1713 °C and 2700 °C, respectively; the corresponding data for ZrO₂ are 1850 °C and 4409 °C. With incinerations of dental waste typically taking place at temperatures below 1000 °C, various ceramics present in dental waste will be completed unaffected by the heat treatment. While the polymeric parts in the waste will degrade at these temperatures, the ceramic components are likely to be decoupled as loose fractions. Most of these are likely to end up in the ash residue as waste products or slag.

A few additional aspects of dental solid waste are considered next. As a specific example, researchers consider the case of single-use plastic (SUP) waste generated in clinical dental practices. The adoption of single-use plastics is a relatively recent development, the role of which became significant during the coronavirus (COVID-19) pandemic, especially as personal protective equipment (PPE) for cross-infection control.

To highlight the enormity of the issue, Martin et al. [164] reported on the SUPs (PPE, face masks, gloves, etc.) generated from oral healthcare and clinical dental settings in the UK and established baseline data/volumes of SUPs used. Most of the SUP waste was disposed of via either landfill or incineration with adverse environmental effects. Appropriate legislation may be necessary for limiting the environmental damage from SUPs without compromising patient safety during clinical care.

While some energy can be recovered from waste incineration, little effort has been made towards resource recovery from dental waste. Recovery of nickel from orthodontic implants using the hydrometallurgical route has been reported [165]. On the other hand, electronic waste is also a complex waste containing polymers, metals, ceramics, and hazardous elements, wherein a variety of recycling approaches have been developed to process waste and extract copper, precious metals, rare-earth elements, etc. [166,167].

Toxicity in dental settings is another serious issue of concern. Toxicological studies are carried out to screen dental materials in terms of their biocompatibility and possible adverse effects. There is convincing evidence for the toxic effects of mercury amalgam on human health, such as anorexia, weight loss, weakness, fatigue, etc. [168]. Resin-based composites contain organic matrix, fillers (SiO₂, Al₂O₃, glass, etc.), ceramic particles, and coupling agents; these are considered to be good alternatives to amalgams. Saliva enzymes, chewing, thermal changes, dietary changes, and oral microorganisms can cause the degradation of composites and release of monomers in the body [169]. Being soft and flexible, bisphenol A (BPA)-based monomers are commonly used in root canal sealers, adhesives, composites, and sealants; these are associated with increased incidences of developmental disorders, breast cancer, and diabetes [170]. Titanium is one of the most commonly used materials in dental implants due to its strength, biocompatibility, and stress resistance. In the oral cavity, titanium implants can undergo chemical reactions in the body causing corrosion, wear, and the release of titanium particles in the body [171]. The presence of toxic elements in dental solid waste needs to be handled very carefully.

Efforts are also being made to manage dental solid waste from the perspective of the circular economy. Waste management plays a key role in the circular economy by determining the order of waste treatment hierarchies, e.g., prevention > preparation for reuse > recycling > energy and material recovery > sanitary landfilling. Although the delivery of high-quality care is the top priority in healthcare, recycling programs and waste minimization can play a significant role in enhancing the economic and environmental sustainability of organizations. Some of the key steps in the circular economy model include, among others, establishing a green team, quantitative determination of waste production, waste minimization, safe reutilization, recycling, and reprocessing [172].

Another challenge to the sustainability of healthcare waste management practices that needs to be addressed is the example of ‘sustainable washing’. Some researchers/operators might label themselves as sustainability experts without adequate qualifications, experience, research publications, and/or project reports. Without appropriate training, sustainable washing of qualifications could limit the extent to which various environmental issues are likely to be addressed [173]. There is also a need for incorporating interdisciplinary pathways, different knowledge and skill bases, resources, and perspectives.