The Problem

A major issue for dentists at the moment is that they cannot see more than 1 person per hour due to COVID.

I have heard from a number of dentists that they have been advised that they should leave rooms for at least 1 hour in-between patients and then deep clean & sanitise the room prior to seeing the next patient.

The one hour comes from the many UV-C air cleaners that have appeared on the market that have a maximum air volume flow rate of 250m3/hour and a typical dental surgery being 60m3 volume room (250/60 = 4.16 air changes per hour).

The Centre for Disease and Control have published a table B1 which provides details of air change rates to remove airborne contaminants:

However, this table comes with some serious caveats:

Values apply to an empty room with no aerosol-generating source. With a person present and generating aerosol, this table would not apply. Other equations are available that include a constant generating source. However, certain diseases (e.g., infectious tuberculosis) are not likely to be aerosolized at a constant rate. The times given assume perfect mixing of the air within the space (i.e., mixing factor = 1). However, perfect mixing usually does not occur. Removal times will be longer in rooms or areas with imperfect mixing or air stagnation.213 Caution should be exercised in using this table in such situations. For booths or other local ventilation enclosures, manufacturers’ instructions should be consulted.

When you consider these caveats the chances of the room being safe within one hour is very unlikely. For a start the table states at 4 air changes per hour it will take 69 minutes (i.e. more than an hour) to clear the air – in perfect conditions – which it then goes on to say does not exist!

These principles are based on air clearance time calculations. These are commonly used in industry to control emissions such as paint spraying in booths. However there is a very serious mistake in applying these recommendations to a dental surgery. A paint booth has no furniture or benches iin the way of air flow; moreover, in a paint booth the air flow is introduced over a very large area in the floor (often a floor grid 3 or 4 metres long by 1 to 2 metres wide) and goes out through a similar huge area in the ceiling, designed so the air travels upwards consistently through the whole space (“laminar flow”). In a surgery with benches and furniture, and where the ventilation system does not have large inlet and outlet areas on opposing surfaces of the space, the air change will not be consistent – near to the ventilator there will be hundreds of air changes per hour, but further away the air will hardly move at all so will not be cleaned effectively. A large fan to stir the air helps, but still there will be stagnant zones unless the whole room is re-engineered to work like a paint booth, hardly a practical proposition.

Even with large fans to stir the air in the room, the imperfect mixing of the air within the room is a serious problem. The HSE document identifies that the contaminant “is most likely to linger and form eddies close to all the walls”.

The American Conference of Governmental Industrial Hygienist (ACGIH) Industrial Ventilation manual provides details on imperfect mixing with relevant calculations and worked examples. There are many variables such as:

• where the clean air comes from
• where the contaminated air is taken away
• room temperature
• external ambient temperature
• air volume flow rates
• obstructions to air flow

Making an assessment of all these factors is challenging even for the most expert hygienist, and even then gives only an approximate result.
Summarising the above, in the absence of special design of the room to achieve laminar flow, the use of a simple ventilator, aided by fans to stir the air, relies on the fresh or cleaned air being brought into the room to simply dilute the contaminated air. But diluting COVID does not make it safe, it only reduces the level. The level of virus that one patient can yield during their visit is unknown, comprising not only their exhaled breath but also the aerosol generated by drilling etc, which will of course vary greatly. Additionally, the acceptable level of virus for a dentist to breathe safely is as yet unknown, so it is not possible to calculate the necessary dilution ratio for safety. Putting that uncertainty together with the unknowns of air movement and mixing in the surgery, means that in reality there is currently no rational basis to calculate a number of air changes required to make a given surgery safe after the visit of an infected patient.

Based on the above – unknown levels of contaminant, imperfect mixing of the air, eddy currents and dead patches etc etc, a realisticaly room clearance time for this type of application at the air movement rates published is more likely to be in the hours before it is safe to go back into the room and start the deep clean.

Such a fallow time between patients will destroy most dental practices.

The solution

However, there is a way to reduce this fallow time. We suggest the below simple 8-step approach to keep everyone safe and healthy:

STEP 1

Have patients wait outside the dental surgery, ideally in their car. When it is their turn to come in call them on their mobile to tell them to come into the surgery.

Ideally the route from the car to the surgery will have no touch points (open doors etc).

STEP 2

Prior to entering the premises, they must wear a simple face covering that extends across both nose and mouth.

STEP 3

Upon entering the premises, they must either wear disposable gloves or be provided with touch free hand sanitiser upon entry to the premises.

STEP 4

Once hands are sanitised, they are to be escorted direct to the dental chair.

Ask them to sit on the chair keeping their mask on.

STEP 5

The dentist positions the local exhaust extraction system adjacent to the patients breathing zone and switch the unit on. Only once the unit is on can the patient remove their face mask.

The extraction system takes the contaminated air away from the persons and filters it through a H14 HEPA filter.

I am not a dentist but Dr Sekhon is. Please take a few minutes to watch his interview in the attached link.

With the recent WHO report that suggests the virus can hang in the air for many hours, capturing at source is even

more important before it has a chance to spread into the environment, especially in a room where there is not an even distribution of ventilation air flow from bottom to top or side to side.

STEP 6

The dentist and technician (both wearing suitable RPE such as the WorkSafe AirHood*) carryout their work, ensuring the correct positioning of the exhaust hood.

AirHood being worn. Fabric cuff can be tucked into clothing to prevent dentists exhaled air being blown onto the patient

The AirHood will protect the dentist and technician from breathing in any harmful substances in the air. The power unit is fitted with a H13 or optionally H14 HEPA filter to remove 99.95% of particle size contaminants at 0.3m. Note not all “HEPA” filters filter to this level of effectiveness.

“PAPRs reduce the aerosol concentration inhaled by the wearer to at least 1/25th of that in the air, compared to a 1/10th reduction for FFRs and elastomeric half facepiece air-purifying respirators.”
CDC

A PAPR is an air-purifying respirator that uses a blower to force air through filter cartridges or canisters and into the breathing zone of the wearer.

Loose-fitting PAPRs have several advantages over tight-fitting non-powered air-purifying respirators, including:

• o A fit test is not required for PAPRs with loose-fitting headgear such as hoods.
• The positive pressure in the hood always gives a positive barrier to inflow of contaminated air, whatever the fit.
• PAPRs with loose-fitting headgear can be worn with a limited amount of facial hair.
• Because the face is completely free, there is no discomfort, irritation or bruising as created by tight-fitting goggles and mask
• No pressure or weight on the head so no headaches.
• Some models offer cartridges for particulate and vapor protection.
• Hooded PAPRs offer complete splash protection for the face and eyes.
• PAPR systems have assigned protection factors (APF) of at least 25 (and up to 1,000 in some cases, as described above). This compares with a N95 respirator of an APF of just 10 when fitted correctly!
• The AirHood PAPR has loose-fitting headgear and patients can see the face of the HCP, providing for better interpersonal communication.
• The PAPR components can be cleaned, disinfected, re-used, and shared.
• The AirHood PAPR use only HEPA filters, which have a greater filtration efficiency against the smallest pathogen particles compared to N95 FFRs.

A PAPR may be less taxing from a physiological/breathing resistance perspective than other respirators.

STEP 7

Once the dentist is done, the patient can re-apply their face covering and the extraction can be removed.

STEP 8

The patient can be on their way – leave the surgery.

The arms of the chair (and any other points touched) can be disinfected prior to the arrival of the next patient.

The equipment:

Extraction unit:

It is important to use the right type of extraction unit to control the hazardous substance. Critical features include

• The hood – too small and it will not encompass the breathing zone, too big and it will get in the way!
• The filter – it has to be able to filter out the hazardous substance. It needs to be a minimum H13 or preferably H14 HEPA rated.
• The air flow rate – too low and you will not capture the air. Too much and it will be noisy.

The new local extraction unit from Geovent has been trailed and tested by dentists. It can minimise the release of COVID and other harmful substances such as Zirconia dust and Monomer toxic fumes being released into the atmosphere.

Also, following the above practice will minimise the fallow time between patients and eradicating the need to deep clean the entire room.

WorkSafe AirHood:

This is a new form of Personal Air Powered Respirator, developed especially for front line health care workers such as dentist.

Fitted with high efficiency HEPA filters, it only delivery clean air to the wearers breathing zone.

“The National Institute for Occupational Safety and Health (NIOSH) announces the publication of an interim final rule external icon to update the regulatory requirements used by NIOSH to test and approve air-purifying particulate respirators for use in the ongoing pandemic.

With this rulemaking, parallel performance standards are added to existing regulatory requirements for powered air-purifying respirators (PAPRs) to allow for the approval of a new class, PAPR100. This new class of PAPRs may be better suited to the needs of workers in the healthcare and public safety sectors needing protection against COVID-19.”
Centre for Disease Control and Prevention

Unique to the AirHood is the fabric cuff. This is designed to be tucked into the wearers clothing so that the exhaled air does not flow across the patient thus keeping everyone safe.
Other PAPR units are available however the AirHood unit has several unique features including:

• Silencer to reduce noise levels in the hood to aid easy communication
• Easy wipe clean
• Protects both wearer and patient
• No facial obstructions
• Able to wear glasses, eye loupes, head light etc without any impairment of their function or the wearer’s comfort