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28 July 2020

Environmental disinfection, air conditioning systems and their relationship with the COVID-19 outbreak


The two main recognized routes of transmission of SARS-CoV-2 in humans are the respiratory, by direct transmission of respiratory secretions (respiratory drops) between people, and by direct contact with mucosa (eyes, nose or mouth), through the hands.

The respiratory drops containing the virus are generated by coughing, sneezing, or the speech of a sick person. On the other hand, it is known that SARS-CoV-2 can be transported through a conglomerate of droplets of different sizes that are accompanied by other fluids and substances, known as bioaerosols. Although it has not been confirmed that they are a form of airborne transmission of the virus, this route is considered a potentially dangerous mechanism of transmission, especially in the field of dentistry, with frequent aerosol-generating procedures. Therefore, specific preventive measures will be necessary to prevent the spread of the virus in the dental office (Figure 1). For the prevention of cross infection in the cabinet, different solutions are advocated, including the use of ultraviolet light, filtering and modifying the operation of air conditioning systems, air purifiers and the use of ozone. In this article we will try to solve the main doubts about them that arise in the sector. The ASHRAE (American Society Of Heating, Refrigerating and AC Engineers), the most important society in the air conditioning sector, and similar associations, promote pandemic control actions in their policies, deploying the technical resources necessary to address the challenges of the pandemic. , in relation to the effects of air conditioning on disease transmission. These strategies will be part of a set of measures to prevent the spread of the virus, along with others such as the use of PPE (Personal Protective Equipment) during work with possible exposure to it, as occurs in the dental field. Depending on the type of each clinic, there will be more or less feasible measures. 

The two parameters that will mainly define how to adapt dental consultations are: 

- The type of ventilation: natural, mechanical or hybrid;

- The type of air conditioning installation (A / C).

What type of ventilation is the most appropriate in our clinics?

 The basic principle is to increase the ventilation of our clinics, giving priority to the natural one. It is important for an expert to assess the individual characteristics of each clinic to determine if proper ventilation is available. Natural ventilation is adequate when there are spaces that allow air circulation and there are windows that can be opened. In this case, it must be properly ventilated after each treatment by keeping the cabinet door closed to prevent the spread of contaminants to other rooms. If no aerosols have been produced, the required ventilation time will be around 15 minutes, depending on the technical characteristics of the cabinet and the external climate. Ventilation will be performed once the patient leaves the room. On the other hand, if we have carried out a procedure that generates aerosols, the ventilation time would be at least 1 hour according to most authors, and it is also recommended irradiation with ultraviolet (UV) light of the C spectrum in the cabinet before ventilating, and once the patient and the professional have abandoned it. Some professional UVC lamp system may be generally recommended, such as those offered by the College of Dentists and Stomatologists of Catalonia (COEC). The power ranges are close to 180 W and serve to sterilize the air on a surface close to 8 square meters in the aerosolized area, in approximately 6 minutes (fig. 2). The more space, the time or power requirements will be greater. Ideally, the devices should be close to the aerosol production source, which is non-directional. Subsequently, it will be necessary to clean and disinfect the surfaces.

In closed spaces or with inadequate natural ventilation, if no other type of ventilation is available, the presence of air purifiers may be necessary. The main feature they must have is a suitable set of HEPA H14 or ULPA 15 filters (fig 3). Other recommended characteristics are the presence of UVC light, catalytic energy or captive ozone release in the purifier, and that it has an adequate air flow in volume and oriented to the area to be purified. The independent HVE (high evacuation volume) extraoral aspiration systems can count on these measures, being also provided with HEPA filters, and even, in some models, with an internal ultraviolet light unit. These systems can help minimize aerosols, but are not suitable for clinical air filtration. In addition, they require constant maintenance with periodic filter changes and have a high economic cost, so their use in the clinic can be debatable.

In turn, when natural ventilation is inadequate, mechanical ventilation and hybrid ventilation are usually installed, in addition to these. The hybrid behaves like the natural one under favorable pressure and temperature conditions, and as mechanical when these conditions do not exist. The mechanics works both in the insufflation of air and in its extraction. It is very important that the inlet and outlet of the air flows are separated by at least 2 meters to avoid contamination of the clean air. If we have these vents, it is advisable to preferably install high-efficiency filters. For example, a F9 class fine filter is capable of retaining between 70% and 80% of 0.1 micron particles; however, a class F7 filter will only do this between 50% and 60%. As an optional complement to these, it is sometimes possible to place a HEPA filter for final filtering in critical installations. A filter disinfection system can ultimately lower maintenance costs4.

Ideally, if mechanical or hybrid ventilation is available, it is to avoid air recirculation, trying to extract all the stale air without reincorporating it into the system. In the event that the ventilation is double-flow mechanical, in which the inlet and outlet flows go through the same circuit, it is not feasible to cancel this recirculation if we want to keep the ventilation working. In a double flow mechanical ventilation there is a flow of clean air that goes from outside the building to the interior of the building and another of stale air that is expelled by mechanical valves and fans. This apparently watertight system can suffer contamination in the heat exchangers, which are devices that help save thermal energy in both winter and summer. Sometimes, in these exchangers a leak of the stale air can occur that would mix with the clean contaminating it, so we recommend that they be periodically checked to inspect that they do not experience such leaks. If you have rotary enthalpic exchangers, they should be turned off because, due to their design, contamination of the inlet air is very possible. 

Can we use air conditioning in dental clinics?
The HVAC, equivalent to the English term Heating, Ventilating And Air Conditioning (HVAC), consists of creating conditions of temperature, humidity and cleanliness of the air suitable for comfort and maintenance of the quality of the indoor air within the inhabited spaces. It comprises three fundamental factors: ventilation, winter heating or air conditioning, and summer cooling or air conditioning. In relation to these, and especially to refrigeration, we should talk about air conditioning systems (A / C), this being a concept that should really reflect the sense of air conditioning at all times of the year.

According to ASHRAE, buildings' heat, ventilation, or A / C (summer cooling or air conditioning) systems should not be stopped as a measure to prevent the spread of SARS-CoV-2, because other problems with health 5.

The use of A / C does not exempt from continuing to provide as much fresh air as possible from the outside, being useful to open windows even if there is some thermal discomfort. In centralized A / C through ducts, recirculation must be canceled to avoid the spread of contagious particles to other rooms, and it is appropriate to assess the placement of accessory HEPA-type filters in ventilation systems. In multisplit A / C or individual splits, air can be used perfectly without the need to cancel recirculation, since air movement is limited to the room where the equipment is located.

Nor should we forget a very important pillar such as the maintenance of the system itself, which must be carried out periodically by an expert professional. If air recirculation has been bypassed and fresh air has been used entirely for ventilation, a more exhaustive protocol should not be created in normal duct cleaning and maintenance procedures. Regarding the outdoor air filters present in modern systems, it is not necessary to replace them more frequently, but only when the pressure or time limits are exceeded, always according to scheduled maintenance.

Is it advisable to use ozone as an environmental disinfectant in clinics?

We think that it should be avoided as a disinfecting method if there are human beings in its vicinity, since it is potentially toxic if it is used in the quantities necessary to be a viricide. Likewise, the European Chemical Agency does not accept it as a biocide, so its use should not be recommended. The advantages of ozone for its defenders are that in a very short time it reaches the critical level necessary to inactivate the virus and can disinfect shaded areas, but it takes about 30 minutes to dissipate6. If you still want to use it, do it at the end of the day, with a timer, and never with people inside the closed compartment where it is ozonized.

Original article from Maxillaris: https://www.maxillaris.com/maximas-20200529-Desinfeccion-ambiental-sistemas-de-climatizacion-y-su-relacion-con-el-brote-de-COVID-19.aspx

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