Risk of airborne infection and transmission-based precautions in Dental setting
Introduction
In the year 1993, the Centre for Disease Control (CDC) published Recommended Infection Control Practices for Dentistry. It was primarily based on health care precedent, theoretical rationale, and expert opinion, to reduce the risk of transmission of bloodborne pathogens among the Dental Health Care Personnel (DHCP) and patients . These universal precautions were based on the principle that blood and all body fluids contaminated with blood is infectious. It also mandates to assume all patients to be asymptomatic carriers of bloodborne pathogens. In the year 1996, the CDC revised the above recommendations and adopted the term “standard precautions”. Standard precautions apply to contact with 1) blood; 2) all body fluid secretions, and excretions (except sweat), regardless of whether they contain blood; 3) non-intact skin; and 4) mucous membranes (1). Hence, these precautions stand to embrace the standard of care provided to minimize the risk of disease transmission to the DHCW and patients through pathogens in blood and other body fluids, including saliva and respiratory secretions. Airborne transmission refers to the passage of micro-organisms from a source to a person through aerosols, resulting in infection of the person with or without the consequent disease. Airborne transmission is categorized into three types: 1. Specific air transmission, i.e., transmission caused by inhalation of aerosol particles under natural conditions (eg. Tuberculosis); 2. Priority air transmission, i.e., multiple transmission methods can cause transmission, inhalation of aerosol particles (eg. Measles and Chickenpox); 3. Opportunistic airborne transmission, i.e., natural conditions are mainly contact and droplet transmission, and under certain conditions, inhalation of aerosol particles causes diseases (eg. Influenza and SARS). The World Health Organization and Centers for Disease Control and Prevention (CDC) treat opportunistic airborne transmission as not airborne.
Only a few diseases (such as Tuberculosis, Measles, and Chickenpox) are recognized as ‘true’ airborne infectious diseases by American CDC Infection control guidelines for healthcare(!). and the WHO (4). But there is an increasing understanding that many other organisms like Bordetella pertussis, influenza virus, adenovirus, rhinovirus, Mycoplasma pneumoniae, Coronaviruses (SARS-CoV) & (SARS-CoV 2), Group A Streptococcus and Neisseria meningitidis where its acquisition, replication and/or colonization occur in the respiratory tract may also behave as ‘airborne’, given a favorable environment irrespective of their primary mode of transmission . Reports also suggest that in a pandemic or large, explosive outbreak situations, influenza and SARS CoV like diseases can become truly airborne. , , Mouth as a part of the oronasal pharynx also harbors bacteria and viruses from the nose, throat, and respiratory tract in the saliva and oral fluids. Therefore, any dental procedure that aerosolizes saliva has the potential to cause airborne contamination of the dental setting.
Airborne Transmission
Airborne transmission occurs through droplets and aerosols. Droplets are atomization of particulate liquid and solid particles when a person coughs or sneezes, or when water is converted into a fine mist through an aeration device or shower head(2)
Spatter refers to droplets usually greater than 100 μm in diameter, visible to the naked eye, with sufficient mass and kinetic energy dropping to the floor within seconds of expulsion. They may contain infectious bacteria and viruses harbored in blood, respiratory secretions, and saliva. They are capable of transmitting disease through direct contact with exposed mucous membranes of the eye, nose, and mouth or indirectly through fomites . HIV and HBV have been transmitted to health care workers by blood spatter. ,
Aerosols are a dispersive system of suspended solid or liquid particles in gas and those aerosols that contain pathogens are considered infectious. They contain particle size ranging from 0.001 to over 100 mm. Many have classified these droplets based on particle size, but it is generally accepted that i) large droplets (>20 μm) that follow a more ballistic trajectory, as they are too large to follow inhalation airflow streamlines and a short-range transmission (1m), ii) small droplets (< 5–10 μm) that follow airflow streamlines and remain suspended for longer periods, iii) intermediate droplets (10–20 μm) possess properties of both small and large droplets, settle faster than particles < 10 μm, and potentially carry a smaller infectious dose compared to large droplets. 9
The fate of Spatter and aerosol
The threshold size of large droplet fallout is 60–80 μm. Some of the exhaled large droplets (initial sizes of less than 60 μm) involved in droplet transmission can remain suspended in the air, but for a much smaller period than the air-change time scale of one hour in a typical room. Aerosols (particles less than 50 μm in diameter) are considered the greatest threat in airborne transmission diseases in dentistry. These can stay airborne and have the potential to enter respiratory passages around ill-fitted masks.9
A droplet nucleus is the airborne residue of a potentially infectious (micro-organism bearing) spatter from which most of the liquid has evaporated . Droplets that are less than 60 microns are more likely to evaporate forming droplet nuclei (<10 μm) before hitting an object in its trajectory, remain airborne, and participate in long-range transmission (2). In dry conditions, droplets evaporate quickly and can remain suspended in the air for a long time. There is evidence showing that relative humidity, size distribution, and travel distances of droplet nuclei can significantly influence infection risk in the indoor environment the transmission of respiratory diseases1. Early epidemiological and simulation studies of specific diseases have shown that the risk range of droplet transmission is within 3 feet (0.9144 m) around the patient, , but SARS investigations during the 2003 outbreak showed that the spread of droplets from SARS patients was even greater than 6 feet (1.8288 m) . Droplet nuclei are implicated in the transmission of Tuberculosis, SARS, Measles, and Herpes (4).
Of particular interest as infective agents are aerosol particles in the 0.5 to 10 μm diameter range (median particle diameter 5 μm. It is important to understand the role-play of the droplet sizes in disease transmission, particles of < 5 μm readily penetrates the airways down to the alveolar space, highly capable of initiating a lower respiratory tract LRT infection. Particles of diameter (a) < 10 μm penetrate up to the glottis, beyond which the penetration diminishes; (b) > 20 μm will probably impact respiratory epithelial mucosal surfaces or be trapped by cilia before reaching the lower respiratory tract. Older experimental studies and some recent field observations of influenza cases consistently associate aerosol-based transmission with a more severe form of the illness. , , Therefore, particles of size < 10 μm are of concern, because of their significant qualitative differences including prolonged suspension time, penetration to deeper regions of the airways, and specific requirements for Personal Protective Equipment (PPE).
Knight estimated the times taken for particles of various diameters to fall 3 m (10). Particles of diameters: 1-3 μm remained suspended almost indefinitely; 10 μm took 17 min; 20 μm took 4 min, and 100 μm took 10 sec to fall on the floor. ‘Naked’ viruses, bacteria, and fungal spores (i.e. without associated water, mucus, or pus droplets) range in approximate size from 0.02 to 0.3 μm, from 5 to 100 μm and from 1 to 10 μm, respectively (23). The amount of solid matter in a droplet ultimately determines its minimal size limit and its infectivity. Infectious agents disseminated in mucus or saliva remain viable for a longer duration of time.
Risk of airborne transmission in Dental setting
The microbial load of dental aerosol may vary widely depending upon its source. The common sources are; the operating site (patient's saliva, respiratory secretions, plaque, tooth), water from the DUWLs, and/or contaminated dental instruments and devices. Qualitative and quantitative analysis of the makeup of dental aerosols would be extremely difficult, and the composition of aerosols probably varies with each patient and operative site. However, it is reasonable to suppose that components of saliva, nasopharyngeal secretions, plaque, blood, tooth components and any material used in the dental procedure, such as abrasives for air polishing and air abrasion, all are present in dental aerosols
Micik and colleagues performed a series of interesting experiments to study the dynamics of dental aerobiology in late 1960 and 1970s, recognizing the relevance of particles consisting of or conveying microorganisms, irritants, allergens, or other toxic substances that can be atomized into the air during dental procedures as a potential source of disease transmission. , , The results of their research showed that dental procedures incorporating the use of water sprays or rotary instruments generated aerosols with significantly greater bacterial count(24).
These splatters landed six inches to four feet from the mouth of the patient and easily encompassed the area occupied by the dentist and his assistant. Dye spatter was clearly and immediately evident on the filter paper disks. Mists could be seen forming as clouds of moisture that gradually settled out and coated the paper disks with fluorescent dye in less than one minute, within a few feet of their origin. The finest aerosols were not apparent in the light streaming through the room. Aerosols were detected as a fine powder like fluorescence that gradually coated the white filter paper disks on surfaces 2 feet or more from the dental chair over several minutes. The disks attached to the face shield also showed evidence of significant airborne dispersion. Less intense spatter and aerosol dispersion were found on the paper disks attached to the assistant. Continued to coat new filter paper disks applied to the surfaces of the operatory for at least 10 minutes despite a room air exchange of one every four minutes. significant fluorescent dye spatter on paper disks applied to the upper surfaces of the operator’s arms, lower neck. region, chest, and face shield. the fluorescent dye in aerosols penetrated our single-layered face masks behind the face shields to enter the nose (27). In a CDC report,
Sources of air contamination in a dental setting
Patient activities
Sneezing Produce 40, 000 droplets projected to several meters which can evaporate to produce droplets of 0.5 to 12 mm in diameter. ,
Coughing Produce about 3000 droplet nuclei
Talking 5 minutes of talking approximate one cough
Dental procedures Polishing with cups
Air spray
Air turbine handpiece without water coolant Similar to coughing (24)
Air turbine handpiece with water coolant An air turbine handpiece, when used with air-water spray coolant, atomized 20 times greater numbers of bacteria than with air spray alone.24
Ultrasonic scaling Amounts to a maximum of aerosol generation (24, 27, 28)
Polishing with a bristle brush
Air-water spray
Note The procedures involving air-water sprays generated aerosols with the greatest percentage of particles with a diameter of 5 microns or less (24).
Dental aerosol experiments were conducted in various dental setups. It was noticed that in a single closed or multiple chaired clinic, the aerosol required 10 minutes (27) to 2 hours to clear away from the operatory depending on the air changes and that the aerosol contaminated the whole room in a single closed room and traveled to areas distant from the treatment zone in multiple chaired clinic
Virus was recovered from all handpieces after they were treated with surface wipe-down and internal saline flushing in spite of the anti-retraction valve. Demonstrated that HSV can be retained and can survive in the dental handpiece.