In an attempt to establish context for reviewing the literature on dental aerosols, we begin this review by examining the reasons why definitions of aerosols vary widely. In general, aerosols refer to particles suspended in gas. Although aerosols may be generated from a multitude of events, such as combustion, evaporation, industrial work etc., we will focus on aerosols generated in the healthcare environment.

In 1934, Wells pioneered the concept that airborne infections can be transmitted either as droplets or as aerosols. According to his work, droplets are defined as those with particle sizes > 5 μm and typically carried on heavy colloids like mucus or saliva. Droplets cannot remain suspended in air for long or travel long distances, hence, they are spread by close contact with (typically 1 m ) and in the presence of, the host. However, according to Wells, droplets < 100 μm dry out before falling ≈2 m to the ground. When these droplets evaporate, they can be carried on airborne vectors and become aerosols. He estimated the particle size in aerosols to be < 5 μm (sometimes called droplet nuclei) and stated that these particles can stay airborne for long periods of time, carry viable pathogen as payload and settle on surfaces distant from the source (which is then referred to as a fomite). The vectors can be natural, namely, mist, fog, and vapor or anthropogenic, for example, smoke, dust, smog, and of particular importance to us, dental aerosol. However, in certain cases, for example, high ambient temperature or high airflow, large droplets can evaporate and acquire aerosol‐like properties. Because of their size, they can carry larger payloads than droplet nuclei (see below).

Aerosols have also been classified based on their deposition patterns. For example, using a semi‐empirical model, the International Commission on Radiological Protection (ICRP) estimated that particles between 1to 10 μm or < 0.5 μm are most likely to deposit in the tracheobronchial and pulmonary regions of the lungs, whereas particles ≤5 μm have the highest probability of entering the lower airways of the average adult during oral inhalation. Because the nose offers a greater filtration efficiency than the mouth, only particles ≤3 μm have a high probability of entering the lower airways during nose breathing. Particles with diameters between 1 and 3 μm or <0.5 μm have the greatest probability of entering the lung, thereby the highest potential of initiating an infection at this site. The Infectious Diseases Society of America (IDSA) has defined “respirable particles” as having a diameter of ≤10 μm and “inspirable particles” as having a diameter between 10 μm and 100 μm, nearly all of which are deposited in the upper airways. Other studies on infectious disease transmission indicate that droplets >5 μm are trapped in the upper respiratory tract whereas droplets ≤5 μm can be inhaled into the lower respiratory tract. In this review, we will use the 10 μm diameter to distinguish between aerosolized and non‐aerosol particles, because they have important implications for time of settling, penetration depth into airways and requirements for PPE.

Another important characteristic of aerosolized particles that impacts their definition is settling time. In still air, it has been estimated that particles 0.5 μm take 41 hours to settle over a distance of 5 feet, and that the time exponentially decreases as the size increases. For example, 1 μm sized particles take 12 hours to settle whereas 10 μm take 8.2 minutes and 100 μm take a mere 5.8 seconds. However, this characteristic is heavily influenced by the direction and velocity of air currents (such as those created by foot traffic, opening of doors, position and setting of room air circulation systems etc.), humidity, the forces of attraction/repulsion between aerosolized particles and the size of the agglomerates/coaggregates (see below). In the presence of turbulence, particles nearer the floor continue to follow the settling times described above, but other factors begin to influence those that are two feet or more above the surface, for example, particle impaction, electrostatic forces etc.

When vector particles and aerosol droplets collide with each other, they might coalesce or coaggregate, changing the particle size, in which case, the classifications described above do not apply anymore. In certain situations, these aggregates break down into numerous smaller conglomerates, generating a new generation of payload. Together, these collisions randomly create a heterogeneous mixture of large and small particles with highly variable electrical charges, aerodynamic diameter, diffusion dynamics, and terminal velocity. It is therefore unsurprising that, in real life scenarios, each aerosol responds in a highly variable manner to gravitational forces. Temperature and humidity of the environment, and the superimposition of new aerosol further impact aerosol dynamics.

The characteristics and behavior of aerosolized particles are important determinants of defining an aerosol, and for this reason, definitions have to be contextualized. For example, size and penetrability‐based definitions have important implications for selecting appropriate face masks, while settling‐characteristics‐based definitions are impactful in deciding nature and time of surface decontamination. Hence, studies on aerosol transmission must account for these confounding variables in order to be interpreted in the appropriate clinical context. As we shall see below, most studies on aerosol generating medical/dental procedures (AGM/DP) have used simplistic calculations, for example, estimating particle size to compute aerodynamic diameter (this has limited use outside of regular sized particles such as inhalable drugs) and applying Stokes’ law to calculate terminal velocity of a particle in a fluid (the assumptions of Stokes’ law fail for particles <1 μm).