The results of the present study provide evidence that aerosol generation is an imminent consequence of carrying out dental procedures and constitutes a potential mechanism for the spread of infection. The spread of aerosols during AGPs represents a significant risk of exposure, primarily for the dental staff. The variables that were associated with a higher risk of exposure in the prediction scale were as follows: a distance of less than 78 cm; improper ventilation; the use of a high-speed handpiece and pneumatic scalers (in periodontics); the location of the patient, operator, and assistant; and, to a lesser degree, the intervention of the anterior region of the mouth. The majority of the aerosols generated during the procedures presented with droplet sizes ranging from 1 to 5 µm. This size has been associated with severe health consequences because the droplets can penetrate the lower respiratory tract to establish infection .
The association of these variables should be put into the clinic context, considering the reported transmission routes of SARS-CoV-2, either by direct contact or by air transmission [9, 10]. The proximity between the patient –possibly infected– and the dental staff determines a high risk of contagion depending on the adherence to the biosafety standards and personal protection elements recommended [8, 10, 13]. Particles of different sizes, mainly <5 µm (86%) were produced. Some of them may settle down due to gravity, whereas some could remain suspended in the air and enter the respiratory tract . The dispersion of expelled particles is not produced exclusively via airborne transmission or droplet mechanism, but by both methods simultaneously .
The permanence of these particles suspended in the air depends on the environmental conditions . The infectious range depends mainly on the time interval between its presence in the atmosphere until its settlement . Factors such as relative humidity, ambient temperature, and airflow have been closely related to the particle size and the time it takes to settle on a surface. During sample collection, conditions of 70% relative humidity and a temperature of 20°C, could favor the settlement of the aerosolized particles. Previous studies have shown that low relative humidity  and high ambient temperature  are related to a longer residence time of the droplet nuclei and droplets in the air . The two environmental conditions mentioned increase the tendency of the drops to pass to the vapor phase, which tends to decrease their size by drying. This results in an increase in the mobility and circulation of the particles in the air  thereby increasing the risk of spreading the infection in the operation site .
Poor ventilation demonstrated a high association with the amount of stained area. This is consistent with previous reports, which estimated that better ventilation substantially reduces the suspension time of the aerosols in the air . The positive influence of ventilation will depend on several conditions: first, on the amount of outdoor air that is available within the indoor space, defined as the ventilation rate; second, the direction of airflow from clean areas toward contaminated areas; and, finally, the distribution, which must cover all spaces while entering and leaving the clinical area . These characteristics will depend on the infrastructure and layout of the area . Although in this study, the experiments were carried out in six different clinical situations and twelve different dental unit locations, the extrapolation of the results should be done with caution, without neglecting the general vision of the clinical environment in which it will be applied .
The mass of the aerosol, as a possible important factor for the amount of viral load carried, determines the different settlement patterns with different possibilities of sizes and shapes of the droplets deposited on the surfaces . Sedimented droplets may facilitate the transmission of infection by fomites . Thick drops may be formed by splashes produced by the rebound of the pressurized water on some oral-dental structure or by the accumulation of oversized droplets on the operator's gloves or the patient's face and neck. Therefore, a mixture of aerosolized particles with particles that are not aerosols , which can contain saliva, blood, and microorganisms , might be formed. One must be vigilant about the adequate calibration of the water and air pressure in the dental unit and the maintenance of the hose system, where the instruments are connected to the unit. Furthermore, thick droplets may be formed by the phenomenon of coalescence or aggregation [13, 31], defined as a binary process in which two drops of the liquid merge to form a single drop. The factors that directly influence drop-drop interactions include Brownian motion, viscosity, density, interfacial contact area, diffusivity, surface tension, and concentration gradients; therefore, it is constituted as a phenomenon of the nature of the liquid [33-35].
As reported by Karimzadeh et al. , the SARS-CoV-2 viral load required to initiate COVID-19 disease may be less than 1,000 particles. In theory, taking into account the size of a viral particle that is in the range of 0.006–0.14 µm , a 1 µm drop could transport around eight viral particles. Hence, more than 120 drops (1 µm or larger than 120 µm in size) may contain sufficient viral loads for infection. Of the 1256 samples obtained in the current study, 664 presented with stained areas ≥120 µm, which makes transmission via generated aerosols biologically plausible during a dental procedure. However, other factors, such as the infectious capacity of the virions in the drops [20, 38], the inactivation potential of the virus, the saliva-water dilution ratio that varies between 1:20 to 1: 100 , the chemical composition of the drops, and the stability of their viability on different surfaces [39, 40] should be taken into account when evaluating the infectious potential of the aerosol.
The present study has two limitations. First, an in vivo model was not used to determine the amount of viable infectious virus in the aerosols and, second, the model used in this study was sensitive and could determine aerosols that have the capacity by size and weight to settle up to 30 min after the completion of the AGP [31, 41]. However, another model will be necessary to determine the amount and size of particles that remain suspended in the environment for a longer period.
Nonetheless, a significant contribution was made to the characterization of the size and settlement patterns of the aerosols generated by different instruments. More importantly, the broad need to maximize the biosecurity measures for the dental team and patient was demonstrated. The need to implement new clinical care strategies such as six-handed work to limit the traffic within the clinical area of non-team personnel during and after treatment ends, in addition to the exhaustive procedures that need to be performed as cleaning and disinfection measures of critical areas in a perimeter area of 200 cm in the vicinity of the patient's mouth and semi-critical areas up to 320 cm.
The proposed methodology was adequate to characterize the risk of exposure when performing the AGPs by using the aerosol contamination profiles. Furthermore, it helped to recognize the potential sources of contamination during a clinical procedure. It was possible to establish representative areas of environmental concentration of the aerosols that could be used as “level of action”, for the adoption of preventive measures. The findings of this study will contribute to evaluating the effectiveness of ventilation or extraction systems, new techniques, and PPE kits proposed to improve the clinical practice.
The production of aerosols is only one risk factor in a dental context, where a network of interactions is woven to determine the dynamics of the virus transmission. The relevance of these findings is based on their practical utility because public health decisions have been made regarding the protocols for the care of patients in dentistry. They involve the use of protective equipment, the measures of prevention and disease transmission control strategies, cleaning and disinfection protocols, and the need to raise awareness in the community about the risk of infection. However, so far, quantitative estimates are being made on aerosol dispersion, which is the basis for the recommendations established by health regulators . Hence, it is important to provide critical data to contribute to informed public health decision-making and the well-being of the population.