Ventilation and SARS-CoV-2 in dentistry

Current evidence suggests that transmission of Covid-19 occurs primarily through direct, indirect or close contact with infected persons.

Infected secretions, including saliva and respiratory secretions or droplets, are expelled when an infected person coughs, sneezes, talks, shouts or sings. It is known that people with no symptoms can infect others; it is not clear to what extent this occurs.1
The consensus remains that Covid-19 is mostly spread by droplets, but the World Health Organisation (WHO) agrees that there may be evidence of the spread of Covid-19 by small airborne particles (aerosols).1
Infected secretions can fall on objects or materials, producing fomites (contaminated surfaces). Consequently, surface disinfection of the area surrounding the patient operative zone is critical. All hand touch surfaces should be cleaned at least twice daily. Aerosols can remain suspended in the air.

Ventilation

For this reason, ventilation of dental surgeries and local decontamination units (LDUs) is important. Ventilation can be achieved naturally, e.g., by using a window, or mechanically, e.g., a wall unit extracting air from the room and venting it outside. As stated in HTM 01-05:2 “Good standards can be achieved without resorting to unreasonably complex or expensive ventilation systems”.
Suitable ventilation of the room will keep air contamination to a minimum. This is particularly important due to the potential aerosol risks.3 Air changes per hour (ACH) is a measure of the air volume added to or removed from a surgery/LDU divided by the volume of the room. The recommendation for dental surgeries/LDUs is about 10 air changes per hour (ISO 14644-1 – dirty room in a hospital central decontamination unit).4 An average of 6-12 ACHs is recommended for the dental surgery.3 A single air change can remove over 60% of airborne contaminants, and after five air changes only about 1% of the original contamination remains.5
Mechanical air removal devices (e.g., extraction fans) specify the amount of air removed and from this the ACH rate can be calculated. It is important that ventilations systems are maintained in accordance with the manufacturers’ recommendations.
Heating ventilation and air conditioning (HVAC) systems will filter the air as well as controlling the humidity and temperature. HVAC systems may have a role in decreasing the spread of infection in indoor spaces by increasing the rate of air change, decreasing the recirculation of air and increasing the use of outdoor air. High-efficiency particulate air (HEPA) filters have shown good performance with particles similar in size to the SARS-CoV-2 virus (70-120nm).6 The manufacturer or supplier should be consulted on the filtration efficiency of any system intended for use in a dental surgery. For further useful general information on ventilation, please refer to the Health Protection Surveillance Centre (HPSC) guidance document.7
Split air conditioners and fans, which heat or cool a room, recirculate air and do not provide ventilation. They are not suitable for healthcare systems unless ducting, filtration and extraction are included (Figure 1). This is because healthcare settings require air changes and micron filtration (removes 99% of bacteria, moulds and viruses).
A split air conditioner consists of an outdoor unit and an indoor unit. The outdoor unit is installed on or near the exterior wall of the room that you wish to cool. This unit houses the compressor, condenser coil, and the expansion coil or capillary tubing. The indoor unit contains the cooling coil, a long blower and an air filter.

FIGURE 1: Example of a split system that is acceptable if fitted with extraction and filtration. 

Installing a ducted air conditioner within a practice is best undertaken as part of a building project; however, if you have a suspended ceiling (like many health centres) then they can be easily retrofitted without significant disturbance (Figure 2). Some advantages of this system include the fact that air changes and diffusion of microbes in air can be measured and controlled (Figure 3).
It is not necessary to buy sophisticated air cleaning/‘sterilising’ systems that are intended mainly for hospital and not community use. There is no conclusive evidence that these systems will add substantially to the ability of a dental practice to resume ‘routine practice’. Most of the air disinfection systems procurable during the pandemic require maintenance, are expensive and do not heat/cool the circulating air. UV radiation, which is used in some of the advertised ‘air sterilising’ systems, must be contained so that it cannot harm dental staff. UV light can be dangerous and may lead to cancer and cataracts.
The use of ‘dental foggers’ or other surgery fumigation systems is not necessary if the dental surgery has an adequate ventilation system. The potential health risks of some of these systems in areas of poor ventilation have not been assessed. It is important that surgery fumigation is only carried out after a thorough cleaning of the premises.
Most transmissions occur at close range. The distinction between droplets and aerosols may be a moot point from a dental point of view as the droplets can vary in size from very large to very small. However, there will be greater emphasis on ventilation in indoor locations if it becomes apparent that aerosols are resulting in a higher number of infections than is considered likely at present.
This document refers to the treatment of patients without any signs or symptoms of Covid-19. Further information on dealing with the pandemic can be obtained on the IDA website – www.dentist.ie.

FIGURE 2: Example of ducted air conditioning system. 

FIGURE 3: Installation on HSE site that satisfied microbial and ventilation requirements. 

References

  1. World Health Organisation. Transmission of SARS-CoV-2: implications for infection control prevention precautions. Scientific Brief, July 9, 2020. Available from: www.who.int.
  2. Department of Health, UK. Decontamination in primary care dental practices (HTM 01-05). 2013. Available from: www.gov.uk.
  3. World Health Organisation. Considerations for the provision of essential oral health services in the context of Covid-19. Interim guidance, August 3, 2020. Available from: apps.who.int.
  4. ISO 14644-1:2015. Cleanrooms and associated controlled environments – Part 1: Classification of air cleanliness by particle concentration. Available from: www.iso.org.
  5. Health Protection Scotland. Covid-19 Annex 1: Infection prevention and control in urgent dental care settings during the period of Covid-19. April 2020. Available from: www.hps.scot.nhs.uk.
  6. European Centre for Disease Prevention and Control. Heating, ventilation and air conditioning systems in the context of Covid-19. June 22, 2020. Available from: www.ecdc.europa.eu.
  7. Health Protection Surveillance Centre. Guidance on Non-Healthcare Ventilation during Covid-19, V1.2 15/10/2020. Available from: www.hpsc.ie.