About a year ago, a purchasing coordinator at my office asked me if I had a preference on which mask I would like to wear: the blue or the pink one? She was quoted a great price and wanted to stock up on our supply. “A blue mask” I said, knowing that it felt the lightest on my face and did not breakout my skin as much. “Are you sure you want a level I mask?” is the response I got from my dentist. I stood there perplexed and not knowing how to answer. 

My lack of scientific evidence for my choice of masks made me realize that I truly did not know much about differences in masks and which one is the best for my protection while working on patients.  Sadly, there are many like me, that do not realize that the level indicator on the box means something more than just a number. 

Let’s take a look at why it’s important to choose an appropriate mask per procedure and its risk level. 

Hazardous Airborne Particles

As dental professionals, we constantly expose ourselves to hazardous airborne particles during dental procedures. Composition of the airborne particles differ depending on the health status of the patient and the type of procedure being performed. With over 700 species of bacteria, fungi, and viruses present in a human mouth, these microorganisms can spread rapidly through splatter and aerosol when using high-powered devices such as air polisher, high-speed handpieces, air/water syringes and ultrasonic scalers.¹

Splatter are large particles of water, saliva, blood and other debris that travel a short distance and settle down quickly. Conversely, aerosols are tiny airborne particles or droplets that range in size from .5-10 microns in diameter² which are invisible to the human eye. These airborne particles cross contaminate with microorganisms found in saliva, nasopharyngeal secretions, biofilm, and blood.³ They are found in the air for a considerable amount of time after completion of a dental procedure. 

Experiments have found that highest concentration of dental aerosol is found 6 inches to up to 6.5 feet from the mouth of a patient. The highest level of contamination was found on a clinician’s mask. To prevent inhalation of hazardous airborne particles, it is crucial to select an appropriate mask based on design, fit, and filtration for each procedure and risk level. 

Proper Mask for the Job

Surgical Mask

Surgical masks, also known as medical masks, are designed to help block large-particle droplets, splashes, sprays, or splatter that may contain viruses and/or bacteria from reaching the clinician’s mouth and nose. Moreover, it helps reduce exposure of clinician’s saliva and respiratory secretions to others. 

Mask efficiency is assured by a tight fit around the nose and mouth, providing no gaps along the side, around the nose or under the chin that would allow air and droplets to bypass the filter medium. 

Surgical masks are categorized into three levels of protection by the American Society for Testing and Materials (ASTM) and regulated by Food and Drug Administration (FDA) in the United States. The three levels of protection are designed for a specific use depending on type of procedure and aerosol exposure level.

• Level 1 (Low barrier): Low level aerosol producing procedures:
  • Dental exams, exposing radiographs, cleaning tasks, laboratory work, taking impressions 
• Level 2 (Moderate barrier): Moderate level aerosol producing procedures:
  • Hand instrumentation, sealants, endodontics  
• Level 3 (High barrier): High level aerosol producing procedures:
  • Use of ultrasonic scaler and high-speed handpieces, performing surgical procedures 

ASTM levels are further graded on its performance in five criteria: 

• Bacterial Filtration Efficiency (BFE)- 
  • Percentage of particles filtered out at a pore size of 1.0 – 5.0 microns (μ). 
• Particulate Filtration Efficiency (PFE)
  • Percentage of particles filtered out at a pore size of 0.1 – 1.0 microns (μ).
• Fluid Resistance 
• Measures the ability of the mask to minimize the amount of fluid that can transfer from the outer layer to inner layer from a splash or spray
  • Higher resistance = higher protection. 
• Differential Pressure (Delta P) 
  • Measures the air flow resistance of the surgical mask (an objective measure of breathability)
  • Greater resistance = better protection but less breathability. 
• Flame Spread 
  • Measures ability to withstand exposure to a burning flame for three seconds 

Surgical masks rely on microfiber filter media technology in order to capture submicroscopic particles generated by high aerosol producing procedures, such as high-speed handpiece and ultrasonic scaler. 

ASTM tests for Bacterial Filtration Efficiency (BFP) with a droplet size of 3 microns containing living Staphylococcus aureus, whose particles range in size of 0.6-0.8 microns. The Particle Filtration Efficiency (PFE) test with nonliving particles sized from 0.1-1.0 microns. In order for a mask to receive a surgical/medical grade, a minimum of 95% filtration in BFP and PFE is required. Moderate or high protection masks must have a bacterial filtration of 98% or greater. For airflow resistance, which is measured in mmH₂O/cm^2, , ASTM requires a Delta P of less than 5.0 for moderate or high barrier masks. (Refer to Table 1)

Table 1

TEST	ASTM Level 1	ASTM Level 2	ASTM Level 3
Bacterial Filtration Efficiency (BFE) (%)	> 95	> 98	> 98
Particle Filtration Efficiency (PFE) (%) @ 0.1 µg	> 95	> 98	> 98
Dental P Differential Pressure-Breathability (mm H2O/cm2)	< 4.0	< 5.0	< 5.0
Fluid Resistance (mm Hg)	80	120	160
Flammability	Class 1	Class 1	Class 1

For procedures that generate moderate to high aerosols, it is best to choose an ASTM Level 3 mask that provides the highest BFP and PFE filtration, Fluid Resistance and Differential Pressure.  

Center for Disease Control and Prevention (CDC) guidelines suggest changing masks between patients and when it becomes wet from breath or splash; wet masks allow microbial penetration, making the mask ineffective. 

Now that we understand the importance of choosing an appropriate grade level mask depending on procedure and aerosol production, what about times when airborne infection isolation precautions are necessary, such as COVID-19 pandemic? 

Pandemic Time! Now what? 

Surgical Respirators: N95 Masks & KN95

Surgical respirators are approved by the National Institute for Occupational Safety and Health (NIOSH) and cleared by Food and Drug Administration (FDA) as a medical device.

In the Guidelines for Infection Control in Dental Health-Care Settings-2003, CDC states that air-purifying surgical respirators, such as N95, are suitable to wear when airborne infection isolation precautions are necessary.⁴ The benefit of using an N95 respirator is its tight fit around the nose, mouth, and chin area and its ability to filter particles down to 0.1 microns with 95% efficiency. SARS-CoV-2, the virus that causes COVID-19, is about 0.125 microns in diameter. 

NIOSH categorizes the respirators into two classifications: its ability to protect against oils and percentage of airborne particles being filtered during testing. (Refer to Table 2)

The ratings to show how well the mask protects against oils are rated as N, R, or P. In occupational settings, such as dentistry, an N rated respirator is adequate for protection against aerosols, as its Not resistant to oil. For industries that work with vapors and oils, such as construction workers that deal with welding and oil-based paint, respirators under the category of R, somewhat Resistant to oil, and P, strongly Resistant to oil (oil Proof), are necessary. 

Respirators that filter out 95 percent of airborne particles are given a 95 rating. Therefore, an N95 respirator filters out 95 percent airborne particles, but is not resistant to oil. Respirators that filter out at least 99 percent of airborne particles have a 99 rating and the ones that filter out 99.97 percent of airborne particles receive a rating of 100. For example, construction workers that deal with welding or oil-based paints would require to wear an R/P 95 or greater respirator. Natural disaster response and recovery teams would require wearing P100 respirators.

Table 2

NIOSH RESPIRATIOR CLASSES            

              Oil Resistance                                                                                         Filtration Percentage

	95	99	100
Not Resistant to Oil (N)	N95	N99	N100
Somewhat Resistant to Oil (R)	R95	R99	R100
Strongly Resistant to Oil (P)	P95	P99	P100

KN95 Masks

Due to mask shortages during the COVID-19 pandemic, CDC allowed KN95 masks to be imported to the US from China. KN95 masks are the Chinese equivalent of an N95 mask and should filter out at least 95% of small airborne particles. Recent tests by CDC have revealed, however, that many imported KN95 masks are either counterfeit or do not meet minimal NIOSH standard for protection.  

It is difficult to identify counterfeit masks; therefore, it essential to look for correct markings and contact the manufacturer to ensure the product is theirs. Moreover, check out FDA’s updated list of Chinese KN95 manufacturers that did and did not pass quality assurance test. 

If you have a KN95 mask, simply recognize that you might not have as much protection as you have thought. 

Prior to the use of respirators, fit testing and annual training in the use, maintenance, and care are required. Furthermore, written safety standard using the Occupation Safety and Health Administration (OSHA) guidelines must be in place. 

Cloth masks

For proper allocation and supply of adequate masks to health care workers during the COVID-19 pandemic, CDC recommended for general population to use cloth masks to help reduce spread of the virus. 

In their early release regarding effectiveness of cloth masks to protect against COVID-19, CDC stated that cloth masks are not adequate for health care workers. However, in community settings, “cloth masks may be used to prevent community spread of infections by sick or asymptomatically infected persons.”⁵ They strongly encourage the public to be educated about the correct use of cloth masks. 

It is generally accepted that cloth masks do not provide adequate filtration from coughs and sneezes as surgical or N95 masks. Cloth masks help reduce the distance droplets from sneezes and coughs travel. Effectiveness of each cloth mask depends on factors such as material type, number of layers, fit, frequency of adjusting, facial hair, face shape, and moisture level of the material from breath. 

When choosing or making a cloth mask, several things should be considered. 

1. 1. Choose a fabric with a high thread count. A tighter weave would provide a better filtration than a looser weave through which a light can clearly be seen. 
   2. Use two or three layers of fabric 
   3. Fabrics made of more than one type of thread may provide a better filtration and might be more comfortable to wear. For example, a combination of cotton and silk or cotton and flannel.
   4.  Assure that the mask fits well and seals around the face. 

Cloth mask can be disinfected and maintained by washing in hot water with soap or preferably in a washing machine at 60 degree Celsius. 

You might have seen some people use bandanas instead of a mask. That method of protection should be the last resort since it does not provide a tight fit around the nose and chin area.  Furthermore, the material its usually made of tends to have a loose weave and will provide very little filtration. 

COVID-19 Virus

It is crucial to understand the fundamental principles of how viruses behave. The COVID-19 virus is around 0.125 microns in size. Physics demonstrates that such small viral particles bond to larger droplets or aerosols and never travel alone. This is possible due to a physical phenomenon called “Brownian motion,” which is a random, uncontrolled movement of particles in a fluid/gas as they constantly collide with other molecules.⁶

Moreover, electrostatic attraction, when particles are drawn to charged fibers in the mask and attract oppositely charged particles, help capture large and small particles from the air. Therefore, the uncontrolled movement of the particles and electrostatic attraction in the mask increase the chance of the virus being captured by the mask fibers as they are attached to larger droplets or aerosols.⁷ 

Recommendation

The fundamental principles of viral behavior demonstrate that health care precautions for COVID-19 pandemic are designed around stopping droplets, since there is not much evidence for aerosol spread of COVID-19.⁸ 

Health care workers with high frequency and probability to risk of exposure, are recommended to wear a surgical respirator, such as N95 or greater.   

In a dental setting, if N95 respirators are not available, it is safe to wear ASTM Level 3 masks with a face shield. By design, surgical masks are effective against droplet transmission, blocking splashes and large-particle droplets. It is crucial to have the mask tightly fit around the nose, cheeks, and chin area. A loose fit between the surface of the mask and the face, would not provide complete protection from germs or other contaminants. 

A tight fit could be achieved by wearing a Level I or II mask on top of the Level 3 mask or using an adjustable hook on the “ear saver” to tighten the elastic straps of the mask. 

For the general population, cloth masks are recommended to help reduce spread of the virus. It is further strongly encouraged to continue practicing safe habits of physical distancing and frequent hand hygiene. 

Conclusion

As for me, after my extensive research on the importance of choosing an appropriate mask for each procedure and its risk level, I now confidently request ASTM Level 3 masks to be purchased to prevent inhalation of hazardous airborne particles during patient care.

 

References: 

1. Raghunath et al. (2016) Aerosols in Dental Practice- A Neglected Infectious Vector; BMRJ, 14(2): 1-8, 2016; Article no.BMRJ.24101. Retrieved from https://pdfs.semanticscholar.org/b890/dab11db9cb13386a3d17345b6678adc47aa8.pdf?_ga=2.84696601.1678563823.1587049335-1916324727.1586541002

2. Jacks, M.E. (2002) A Laboratory Comparison of Evacuation Devices on Aerosol Reduction. Journal of Dental Hygiene: 76(3):202-6. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/12271865

3. Sawhney, A., et al. (2015). Aerosols: How Dangerous They Are in Clinical Practice. Journal of Clinical and Diagnostic Research : JCDR, 9(4), ZC52–ZC57. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4437160/

4. Kohn, W.G., Collins, A.S., Clevelant, J.L., Harte, J.A., Eklunt, K.J., Malvitz, D.M. Center for Disease Control and Prevention. Guidelines for infection Control in Dental Health-Care Settings-2003. MMWR 2003; 52 (Report No. 17). Retrieved from http://www.cdc.gov/mmwr/PDF/rr/rr5217.pdf

5. Center for Disease Control and Prevention (CDC):

https://wwwnc.cdc.gov/eid/article/26/10/20-0948_article

6. Wikipedia

7. https://www.nap.edu/read/11637/chapter/4#26

8. World Health Organization (WHO)

https://www.statnews.com/2020/06/09/who-comments-asymptomatic-spread-covid-19/

9. Occupational Safety and Health:  https://www.fws.gov/policy/e1242fw14.pdf