Tooth grinding using burs in rotary instruments is a critical process in many dental procedures, including preparing before dental restorations and cleaning up after orthodontic appliance removal [1,2]. This process generates a significant amount of debris particles, which are subsequently emitted into the air, forming aerosols. The particles may become contaminated with pathogenic microorganisms present in the saliva or blood of the infected patients, and even deposit in human respiratory alveoli after inhalation, posing potential health hazards to the dental team and other patients in the environment [3,4]. These infectious microorganisms include mycobacterium tuberculosis, human immunodeficiency virus (HIV), influenza viruses, hepatitis viruses, as well as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) responsible for the COVID-19 pandemic [5], [6], [7].

A study confirmed that SARS-CoV-2 enters human cells through a receptor called angiotensin-converting enzyme II (ACE2) [8]. Exposure to particulates, which increases ACE2 expression, indirectly elevates the likelihood of COVID-19 infection [6]. It is worth noting that the SARS-CoV-2 RNA has been shown to be detected in aerosol during ultrasonic scaling and tooth preparation in the dental operation room [9]. In addition, particle inhalation has also been also linked to other respiratory diseases [10]. According to the Centers for Disease Control and Prevention (CDC), occupational exposure to dust particles is a possible causative factor for idiopathic pulmonary fibrosis (IPF) among dental personnel [11].

In dentistry, there are generally two types of rotary instruments used to drive the bur. One type is an air turbine handpiece, commonly referred to as a high-speed handpiece, which operates at speeds of up to 600,000 rpm . The other is an electric gear handpiece, known as a speed-increasing handpiece, and may output speeds of up to 200,000 rpm [12]. The operation of a high-speed handpiece usually needs to be accompanied by the use of water coolant to maintain a balanced tooth temperature, while a speed-increasing handpiece may not [13,14]. Although data demonstrate that tooth grinding contributes to air pollution in the dental office irrespective of the presence or absence of cooling water, without cooling water, dry tooth particles of various sizes are released directly into the air [2]. However, under cooling water usage conditions, these particles tend to become trapped in water droplets, leading to an increase in material sizes [13].

Recent studies indicate that 100 µm is a more rational size threshold to distinguish droplets from aerosols, compared to the historical 5 µm [15,16]. Droplets (larger than 100 µm) fall rapidly to the ground within 2 m of the source, while aerosols (smaller than 100 µm) can remain suspended in the air for an extended period of time, and travel distance may exceed 2 m [16,17]. Despite aerosols are now widely well recognized as a significant mode of SARS-CoV-2 transmission [18], droplets should also be considered a potential infection threat [19], [20], [21]. As the droplets evaporate, they leave smaller particles of contaminated material (so-called droplet nuclei) [22], [23], [24]. These dry particles may re-airborne as dust or travel outdoors with the air, increasing the risk of consequent exposure through inhalation [5,6,25].

Particle size is one of the key considerations for evaluating the pathogenicity of aerosol. aerosol particles exhibit a higher pathogenicity within the diameter range from 0.5 to 20 µm [26]. The CDC defines particle with a diameter ≤ 5 µm as inhalable aerosol, which can penetrate the intricate network of small airways in the lungs and reach the alveoli [27,28]. In comparison with 5 µm particles, particulates with diameters of ≤ 2.5 µm are more readily absorbed into the bloodstream and have a prolonged tendency to accumulate in cardiopulmonary tissues [29]. Moreover, the smaller the particle size, the larger its specific surface area and the stronger its ability to adsorb pathogens [30,31]. In research done in Wuhan, China, particles containing SARS-CoV-2 were found to be primarily distributed in two size ranges: 0.25–1.0 and > 2.5 µm, while greater concentrations of the virus or viral RNA (expressed in copies/m3) were present in the 0.25–1.0 µm size region [32]. Furthermore, another critical factor is particle concentration. It may be significantly associated not only with an increased risk of chronic diseases such as pneumoconiosis, cardiovascular disease, and lung cancer, but also with the severity of COVID-19 [4,33]. Research conducted during the COVID-19 pandemic by the Harvard School of Public Health confirmed an association between increases in the concentration of suspended particles and mortality rates due to COVID-19 [34].

Previous research has focused on evaluating the distribution of aerosols and droplets generated by high-speed handpieces with water coolant [12,14,35]. However, to the best of our knowledge, no experimental reports have been published on measuring the size and concentration distribution of tooth particles produced by using a speed-increasing handpiece. Therefore, the objective of the present study was to quantify the particulate contamination in a real dental operation room from tooth grinding with a speed-increasing handpiece, run dry.