HEPA Filters & UVC Light for Aerosol Management in Dental Clinics

By. Dr Shruti Maroo Rathi

HOME | COVID-19 & DENTISTRY |HEPA Filters & UVC Light for Aerosol Management In Dental Clinics

Author: Dr. Shruti Maroo Rathi (M.D.S Periodontics)


Several studies have now concluded that transmission of SARS-CoV-2, can occur via airborne routes as well. The virus remains viable in aerosols for up to 16 hours.[1], [2] It can be even spread by speech droplets of asymptomatic carriers which can linger in the air for 14 minutes [3]. Even WHO has now acknowledged that there is emerging evidence that the coronavirus can be spread by tiny particles suspended in the air. [4] This has once again stirred the debate on the existing view that HEPA filters and UV-C lights are not required for aerosol management in the clinic if there is a good ventilation system.

CDC and OSHA have proposed a hierarchical table[5] for aerosol management.

Hierarchy of Controls: Source [5]

Engineering controls (HEPA and UV-C) are favored over administrative and personal protective equipment (PPE) for aerosol management in the workplace because they are designed to remove the threat at the source before it comes in contact with the worker.

Therefore, in this article, we will be revisiting the basic information on aerosol management through HEPA filters and UV-C lights and the existing evidence in their support.

Ultraviolet – C

What is a UV-C Light? – In Simple Terms

On the electromagnetic spectrum, UV light comes between the visible light and X-rays. It is present in sunlight and has the potential of damaging our DNA which is why we slather up with sunscreen lotion. The lotion protects us from two kinds of UV light, UV-A (400 to 320 nm) and UV-B (320 to 290 nm), which are responsible for sunburns and skin cancers. There is a third kind, called UV-C (280 nm to 200 nm), which has a very short wavelength and is unable to cut through Earth’s atmosphere and reach our skin. This region is also called the germicidal region (peak activity 253.7 nm). Downs & Blunt in 1978 first reported the germicidal effects of UVC Lights. It can be recreated in a low-pressure mercury-vapor arc lamp, emitting around 254 nm; or more recently used xenon lamp technology.  

Mechanism of Action of UVC Lights against microbes?

UVC light releases high amounts of photons that are absorbed by ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) in cells and microbes. [6] This causes the dimerization of pyrimidine molecules which makes it very difficult for the nucleic acids to replicate. Even if replication does occur, it often produces a defect that prevents the microorganism from being viable.

Application of UVC Light

The most critical applications of ultraviolet radiation in aerosol management are the destruction of airborne microorganisms, inactivation of microorganisms present on surfaces or suspended in liquids and protection, and disinfection of products of unstable composition that cannot be treated by conventional sterilization methods.

Health Hazards of UVC Light?

UVC is less carcinogenic than UVB because of its more superficial penetration depth, but it has its own aftereffects. For instance photokeratitis and photoconjunctivitis[7], redness of the skin, erythema, or even malignant melanoma [8] with maximum effects occurring at 270 nm. Therefore, careful handling of the equipment is required to avoid accidental exposure. To avoid these side effects, FAR UVC lights have been introduced.

What is Far UVC Light?

Far-UVC light has a much shorter wavelength (207 to 222 nm), higher frequency, higher energy than UV-C, making its effect on killing microorganisms as efficient as conventional germicidal UV light.[9] Also, it does not cause the human health issues associated with direct exposure to conventional germicidal UV light.[10] This is because far-UVC is absorbed in the skin stratum corneum or the ocular tear layer & cannot reach living human cells in the skin or eyes. But far-UVC light can still penetrate and kill viruses and bacteria as they are extremely small.  

Another study reported that Far UVC light having a wavelength of 222 nanometers and a comparatively low dose of 2 mJ/ Cm3 is shown to inactivate all aerosolized SARS-COV-2. 

Continuous far-UVC light exposure in occupied public locations at ~3 mJ/cm2/hour would result in ~90% viral inactivation in approximately 8 minutes, 95% in ~11 minutes, 99% in ~16 minutes and 99.9% in ~25 minutes.[11]

UVC Disinfection against Corona Virus?

Darnell ME[12] in 2004 demonstrated that UVC light was able to inactivate SARS-CoV (Virus responsible for SARS).  Another study conducted in April 2020 concluded that SARS-CoV 2 virus can be inactivated with UV-C light at 254nm for 15 minutes at a distance of 3 cm. Another recent study by Buonanno, M concluded that Far-UV-C light is effective in the inactivation of coronaviruses HCoV-229E &HCoV-OC43 (virus responsible for the common cold). They also concluded that as all human coronaviruses have similar genomic size, a key determinant of radiation sensitivity, it is realistic to expect that far-UVC light will show comparable inactivation efficiency against other human coronaviruses, including SARS-CoV-2.

UV-C can be generated from a low-pressure mercury lamp or xenon lamps. It produces continuous UV-C with a peak wavelength of 254 nm and delivers a dose of 40 mJ/cm2. This dose is sufficient to inactivate viruses on perfectly flat and ideal surfaces. Studies have shown that as the distance from the lamp increases, effectiveness against microorganisms decreases sharply. In dental clinics, it is advisable to place the lamp close to the dental chair for effective disinfection.

Limitations of UVC Light technology:

1. It has been found that resistance of microorganisms to UVC may develop after excessive repetition (e.g., 80 cycles) of UVC irradiation.[13] This particular study was done on E.Coli bacteria. No study exists on its effect on the virus.

2. The effectivity decreases with distance, so it may require multiple positioning in the room. As the distance doubles, radiation weakens to only 1/4th energy.

3. Radiation does not reach in shadow areas effectively so it compromises the disinfection process.

4. Long hours of exposure to mild UVC can reduce the life of plastics, mica, and many other products. Dark-colored things may lose/fade their color and whiter things get yellowed.

5. The unpleasant pungent smell after the use of UV-C because of the production of Sulphur containing thiols.

HEPA Filters for Aerosol Management

What is HEPA filter?

HEPA is an acronym for High Efficiency Particulate Air Filter that has the efficiency to capture over 99.5% (European Standard) or 99.97% (US Standard) of all particulate with a diameter equal to 0.3 μm pollution.

The measurement of efficacy of HEPA filter is done by its ability to filter the percentage of 0.3 μm particles because this size particles are hardest to capture and is called MPS (Most penetrating particle size). The above definition often creates confusion that these filters act as a net and can filters only 0.3μm or above size particulates.

So how does a HEPA filter work?[14]

HEPA filters are based on the following principles: They trap the air pollutants in a complex web of fibers in 4 different ways depending on the size of the particle:

  1. Diffusion – For smaller sized particles (less than 0.3 microns)
  2. Interception – For Medium-sized particle ( between 0.3 – 1 micron)
  3. Inertial Impaction – Larger particles (larger than 1 micron)
  4. Sieving – Larger particles (larger than 1 micron)

Diffusion works really well under about 0.3 microns. Sieving and other mechanics that are more intuitive work well above 0.3 microns. The intersection of these mechanics is the hardest particle size to capture. Therefore, it is called as the weak spot of air purifiers.

How do I choose a HEPA filter?

There are three important variables to check for while buying an air purifier:

Grade of HEPA based on Efficiency: HEPA – H10-H12 filters only trap 85-99.5% of all particles that are 0.1 microns in diameter whereas H13 and H14 (medical grade) trap 99.95% and 99.995% of such particles, respectively.

ACH – How many times an air purifier can clean a room in one hour. It is usually 12 for aerosol generating procedures.

CADR – Clean Air Delivery Rate – the volume of air being purified in a given period of time. It is either measured as a cubic meter of air delivered in an hour or cubic foot of air delivered in a minute.

How to Calculate CADR for your dental clinic?

CADR is measured in cubic feet per minute (cfm)

Formula: [(ACH x L x W x H) / 60] cfm

Some Commonly purchased brands are: Sharp FP-J60 (396cfm), Sharp FP-J80 (480cfm), Bluestar, Philips etc.

How long should you use HEPA filter?

 It should be used through out during the procedure as well as in between the procedures.

Correct Positioning of HEPA Filters for Effective Aerosol Management?

Place HEPA unit within vicinity of dental chair, but not behind the dentist. Ensure that the dentist or the assistants are not positioned between the unit and the patient’s mouth. Position the unit to ensure that it does not pull air into or past the breathing zone of the dentist or assistant.

Can HEPA filters be washed and reused?

The general recommendation is that HEPA filters should be replaced, not cleaned as the process of cleaning might introduce some airborne pollutants in the clinic. Another reason is that during cleaning the fibers can create gaps in them which could be large enough for particles to pass through. Also, tapping off excess dust should be avoided as it may alter the seal between the filter and its frame and air can flow around the filter

How to clean washable HEPA filters?

There are no official standards or guidelines for cleanable HEPA filters. If a company claims that the filters are washable then it should be cleaned by rinsing under cold water without touching it. The filter should be allowed to completely dry before re-installing it otherwise the wet filters can lead to mold formation and growth, which will make the air purifier emit foul odors.

What is the general Filter Replacement Schedule?

  • HEPA Filters: Every 12 to 18 months but some companies claim a warranty of up to 2 years.
  • Carbon Filters need replacement after three to six months
  • Pre-Filters: should be cleaned every 30 days and replaced when worn out

As there is still a lack of proper guidance of handling the filters, the frequency to replace or clean the filters may be higher than that for ordinary use. These filters should be disinfected thoroughly and disposed of as medical waste as it will prevent secondary contamination.

Can HEPA Air Purifiers Capture the Coronavirus?

For the control of opportunistic airborne transmission in health care setups, WHO has recommended a ventilation rate of at least 288 m3/h per person.[15] In addition to natural ventilation, the mechanical ventilation system is also necessary to achieve such a high ventilation rate. However, natural ventilation is dependent on environmental conditions and building structure, which make it difficult to achieve such a high-level ventilation rate all the time. So, a HEPA filter can act as a supplementary measure to remove airborne SARS-CoV-2.[16] Liu et al [17] concluded that the peak concentration of SARS-CoV-2 aerosols appears in two distinct size ranges, one between 0.25 and 1.0 μm, and the other in size larger than 2.5 μm. The HEPA filter can effectively remove both these aerosol size ranges.

Newer technology in Air Purifiers for Aerosol Management

1. Photoelectrochemical oxidation (PECO) The main advantage, in addition to physical filtration, is the oxidation of organic matter that takes place via photoelectrochemical reaction on the surface of a nano-coated filter. PECO technology removes and efficiently destroys organic matter, bacteria, viruses, mold, and Volatile Organic Compounds by converting them into their trace elements. This technology process claims to be able to destroy organic material 1000 times smaller than what a HEPA filter can capture.[18]

2. HEPA filters with inbuilt UVC – This combination has certain advantages for instance; UVC will sterilize the air that passes by it before passing into HEPA filter. As a result, the life of HEPA filters increases as well it serves as a back up in case of some damage to the filter. It also protects the filter maintenance staff by inactivating microbes before they are exposed to it.

Disadvantages include: Firstly, Production of Ozone in poor quality purifiers, secondly, more power consumption & high maintenance charges, and thirdly, dust can build-up on UVC lights and so it can reduce its effectiveness.[19]

In Conclusion: The air purifiers with HEPA filters and UV-C lights are portable, affordable, and effective and have the potential to reduce the exposure of dentists as well as staff members to virus-laden aerosols and serve as a useful supplement to other protective procedures.

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[1] Doremalen N. et al. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. N Engl J Med 2020.

[2] Fears AC et al. 2020 Comparative dynamic aerosol efficiencies of three emergent coronaviruses and the unusual persistence of SARS-CoV-2 in aerosol suspensions med Rxiv 2020.04.13.20063784;

[3] Stadnytskyi V. et al The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission Proc Natl Acad Sci 2020.

[4] https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions

[5] Dental Aerosol Management. California Dental Association

[6] Budowsky EI, Bresler SE, Friedman EA, Zheleznova NV. Principles of selective inactivation of viral genome. I. UV-induced inactivation of influenza virus. Arch Virol. 1981;68(3-4):239-247.

[7] Balasubramanian D. Ultraviolet radiation and cataract. J Ocul Pharmacol Ther. 2000;16(3):285-297.

[8] Setlow RB, Grist E, Thompson K, Woodhead AD. Wavelengths effective in induction of malignant melanoma. Proc Natl Acad Sci U S A. 1993;90(14):6666-6670.

[9] Narita, K. et al. 222-nm UVC inactivates a wide spectrum of microbial pathogens. J Hosp Infect (2020).

[10] Buonanno M, Ponnaiya B, Welch D, et al. Germicidal Efficacy and Mammalian Skin Safety of 222-nm UV Light. Radiat Res. 2017;187(4):483-491.

[11] Buonanno, M., Welch, D., Shuryak, I. et al. Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses. Sci Rep 10, 10285 (2020).

[12] Darnell ME, Subbarao K, Feinstone SM, Taylor DR. Inactivation of the coronavirus that induces severe acute respiratory syndrome, SARS-CoV. J Virol Methods. 2004;121(1):85-91.

[13] Alcantara-Diaz D, Brena-Valle M, Serment-Guerrero J. Divergent adaptation of Escherichia coli to cyclic ultraviolet light exposures. Mutagenesis. 2004;19(5):349–354.

[14] Lee JH, Kim JY, Cho BB, et al. Assessment of air purifier on efficient removal of airborne bacteria, Staphylococcus epidermidis, using single-chamber method. Environ Monit Assess. 2019;191(12):720.

[15] WHO Natural ventilation for infection control in health-care settings. 2009. https://www.who.int/water_sanitation_health/publications/natural_ventilation/en 

[16] Zhao BB, Liu Y, Chen C. Air purifiers: A supplementary measure to remove airborne SARS-CoV-2 [published online ahead of print, 2020 Apr 24]. Build Environ. 2020;177:106918.

[17] Liu Y., Ning Z., Chen Y. Aerodynamic characteristics and RNA concentration of SARS-CoV-2 aerosol in wuhan hospitals during COVID-19 outbreak. Mar 10 2020.

[18] Rao NG, Kumar A, Wong JS, Shridhar R, Goswami DY. Effect of a Novel Photoelectrochemical Oxidation Air Purifier on Nasal and Ocular Allergy Symptoms. Allergy Rhinol (Providence). 2018;9:2152656718781609.

[19] Medical Advisory Secretariat. Air cleaning technologies: an evidence-based analysis. Ont Health Technol Assess Ser. 2005;5(17):1-52.

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