A clinical observational analysis of aerosol emissions from dental procedures

Aerosol generating procedures (AGPs) are defined as any procedure releasing airborne particles <5 μm in size from the respiratory tract. There remains uncertainty about which dental procedures constitute AGPs. We quantified the aerosol number concentration generated during a range of periodontal, oral surgery and orthodontic procedures using an aerodynamic particle sizer, which measures aerosol number concentrations and size distribution across the 0.5–20 μm diameter size range. Measurements were conducted in an environment with a sufficiently low background to detect a patient’s cough, enabling confident identification of aerosol. Phantom head control experiments for each procedure were performed under the same conditions as a comparison. Where aerosol was detected during a patient procedure, we assessed whether the size distribution could be explained by the non-salivary contaminated instrument source in the respective phantom head control procedure using a two-sided unpaired t-test (comparing the mode widths (log(σ)) and peak positions (D(P,C))). The aerosol size distribution provided a robust fingerprint of aerosol emission from a source. 41 patients underwent fifteen different dental procedures. For nine procedures, no aerosol was detected above background. Where aerosol was detected, the percentage of procedure time that aerosol was observed above background ranged from 12.7% for ultrasonic scaling, to 42.9% for 3-in-1 air + water syringe. For ultrasonic scaling, 3-in-1 syringe use and surgical drilling, the aerosol size distribution matched the non-salivary contaminated instrument source, with no unexplained aerosol. High and slow speed drilling produced aerosol from patient procedures with different size distributions to those measured from the phantom head controls (mode widths log(σ)) and peaks (D(P,C), p< 0.002) and, therefore, may pose a greater risk of salivary contamination. This study provides evidence for sources of aerosol generation during common dental procedures, enabling more informed evaluation of risk and appropriate mitigation strategies

Atomization, combustion and control of charged hydrocarbon sprays

The atomization of electrically insulating liquid fuels has been investigated using the charge injection method with consideration of its usefulness as a spray combustion technique. This requires electrostatic spraying systems that atomize unadulterated commercial grades of fuel oil sufficiently finely while being able to operate robustly in the ionized combustion environment. As a natural consequence of the liquid charging mechanism, the atomization quality improves with flow rate because of the dual action of aerodynamic shear and the higher specific charge that can be achieved in the liquid jet. At moderate charging conditions, sprays of insulating liquids are similar to those of semiconducting liquids, with a core of larger drops surrounded by a sheath of much smaller companions. More highly charged sprays are more homogeneous, and stable combustion of kerosene and diesel oil has been achieved for the first time at practically useful flow rates. Flame stability improves with atomization quality and a stable flame seat is formed without the need for a pilot flame for an atomizer of 150-mm orifice diameter for a kerosene flow rate of 0.5 ml/s and a specific charge of 3.0 C/m3. Spray manipulation of both cold and combusting sprays using DC electric fields has been demonstrated, and the effectiveness of the technique suggests that optimization of the combustion process is possible by applying AC electric fields

Electrohydrodynamics of charge injection atomization: regimes and fundamental limits

The dynamics and controlling mechanisms of a charge injection electrostatic atomization method for insulating liquids have been investigated using a sharp electrode as a current source, which is placed inside the atomizer and held at a high electrical potential. The method is applicable to highly insulating liquids such as hydrocarbon oils and oil-based solutions, and potential uses of this technology include combustion systems and high-quality spray coating/deposition applications. Subcritical and supercritical flow regimes of atomizer operation are delineated by different types of electrical breakdown. The subcritical flow regime is limited by an insulation failure of the liquid hydrocarbon itself and occurs inside the atomizer. This regime, although of interest to understand the fundamental nature of the electrohydromechanics, does not produce finely atomized sprays. Atomization performance in the supercritical flow regime is characterized by the maximum spray specific charge being limited by a partial discharge in the gas outside the nozzle, around the liquid jet as it emerges from the nozzle. In this case, finely atomized sprays are possible, and with no electrical limitation on the upper flow rate limit. The maximum spray specific charge achievable in the supercritical regime has been found to increase at larger nozzle exit velocities and also for smaller orifice diameters. These two factors, combined with the increased aerodynamic shear acting on the charged liquid jet as it emerges from the orifice at higher velocities, enhances the destabilization of the charged liquid jet to produce finely atomized and well-dispersed sprays of insulating liquids

Design issues concerning charge injection atomizers

Two versions of a charge injection electrostatic atomizer design, useful for producing charged sprays of highly insulating liquids, have been subjected to systematic changes of their physical and electrical characteristics. The aim is to enhance the atomization of these liquids by maximizing the amount of charge that can be introduced into the spray. Spray current measurements showed that atomizer internal geometry is the key parameter and a "point-plane" geometry was found to be optimum. The "point" is the charge injection site, the tip of a needle electrode, coaxial with the "plane," conceptually a disk which contains the atomizer orifice. An important requirement is to place the electrode tip as near as possible to this "plane," while maintaining sufficient current. A ratio of unity of the distance between the tip and the plane, and the orifice diameter, was found to be optimum, and independent of orifice diameter. The orifice length-to-diameter ratio was varied and was found to have little measurable effect on the electrical performance of the atomizer. An investigation of the atomizer electrical characteristics was also performed, by placing large resistances between the metal nozzle body and ground and also placing a ring electrode around the liquid jet as it emerged from the orifice. We found that electrical modifications to the atomizer can provide significant improvements in the spray current and hence the atomization quality