Face shields are highly effective at preventing large droplet contamination of the operators face and mask, but they will not reduce aerosol penetration, so a good fitting mask is imperative. The advantages in this form of PPE can quickly be lost with poor attention to doffing and donning procedures especially when not undertaking AGPs.
In the world before Covid-19 the majority of the dental profession had never heard of the words ‘aerosol generating procedure’(AGP) in relation to the use of low and high-speed dental handpieces, ultrasonic scalers, air polishers, and triple syringes. In late 2019 the Covid-19 virus brought with it the risk of potentially fatal respiratory infection via droplet and aerosol spread (Meng et al., 2020). To counter the risk of droplet infection the majority of international emergency dental care guidance almost universally recommended the use of face shields to protect the clinical team (Cochrane, 2020). Up until now the dental team had mainly been using safety glasses as eye protection, so what additional information can we learn from the research relating to face-shields?
To simplify the literature review process when there are some many non-peer reviewed unindexed preprint papers available on line I limited my search to a review by Roberge to locate some of the primary literature (Roberge, 2016). 8 studies were extracted from the review for further appraisal, 6 were simulation studies (Shoham et al., Bentley et al., 1994, Rusin et al., 2002, Lindsley et al., 2014, Brady et al., 2017, Weber et al., 2019) , one observational study of personal protective equipment (PPE) usage by operating theatre staff (Herron et al., 2019) and a further review (Jones et al., 2020).
The simplest of the simulation studies sprayed fluorescent dye at various combinations of glasses, goggles, and a face shield. As one would expect the face shield plus FFP2 mask effectively blocked all droplet contamination to the eyes, nose and mouth (Shoham et al.).
A dental simulation study using similar fluorescent dye demonstrated aerosols penetration of a single layered face mask behind a face shields to enter the nose indicating the importance of a well-fitting mask to protect against aerosol that can flow round a face shield (Bentley et al., 1994). Weber and co-workers conducted a far more sophisticated simulation study using a total of 39 participants who performed 74 experiments, involving 10–12 experimental trials for each healthcare activity. Their first interesting finding was that with face shields, facemask contamination was higher among non-AGPs rather than AGPs. The authors concluded that:
‘The lack of correlation between the fluorescein mass on the face shield and fluorescein aerosol concentration suggests that projected droplets or transfer upon contact by contaminated gloves were more likely the source of contamination than particles suspended air.’
A further simulation study by Brady also concluded that improper doffing and reuse of masks increased infected droplet transfer to the hands from 7% to 15.5% (Brady et al., 2017), the 3 to 5 times higher droplet transfer as opposed to nuclei transfer (dehydrated droplet) may be associated with the fluid resistant surface of the FFP2 mask holding the droplets on the surface. Rusin demonstrated 3300 to 6600 fold increase in transfer efficiency of virus (PRD-1 phage) when comparing a porous surface with a fluid resistant surface (Rusin et al., 2002).
Continuing on the theme of cross contamination Herron observed 1036 operating theatre staff on their technique of applying masks and face shields and found only 18% (190/1034) of surgically scrubbed staff fully complied with the CDC guidelines on the application of a face mask, and face-shields were worn correctly 3.6% of the time (37/1034). The most recent review of PPE for infectious diseases prior to Covid-19 focused on lessons learned from Ebola, SARS, and MRSA, and in line with the above studies concluded that face shields were susceptible to being by-passed by splash or touch (Jones et al., 2020)
From this rapid review we can conclude that face shields are highly effective at preventing large droplet contamination of the operators face and mask. The lack of seal round the periphery however will not prevent aerosol contaminating the inside of an improperly fitting face mask in simulation studies. The two most interesting observations however relate to the major source of contamination of the mask coming from the operator’s hands, and the reduced contamination during AGP procedures. I would hypothesize that the reduced contamination during the AGPs could be attributed to the more focused workflow keeping operators’ hands more closely confined to the surgical site and increasing awareness of hand contamination. As with all these reviews there is a need for more high quality long-term real-world studies to evaluate the effectiveness of enhanced PPE usage.
Disclaimer: The article has not been peer-reviewed; it should not replace individual clinical judgement, and the sources cited should be checked. The views expressed in this commentary represent the views of the author and not necessarily those of the host institution. The views are not a substitute for professional advice.
BENTLEY, C. D., BURKHART, N. W. & CRAWFORD, J. J. 1994. Evaluating spatter and aerosol contamination during dental procedures. J Am Dent Assoc, 125, 579-84.
BRADY, T. M., STRAUCH, A. L., ALMAGUER, C. M., NIEZGODA, G., SHAFFER, R. E., YORIO, P. L. & FISHER, E. M. 2017. Transfer of bacteriophage MS2 and fluorescein from N95 filtering facepiece respirators to hands: Measuring fomite potential. J Occup Environ Hyg, 14, 898-906.
COCHRANE. 2020. Recommendations for the re-opening of dental services: a rapid review of international sources [Accessed 30th May].
HERRON, J. B. T., KUHT, J. A., HUSSAIN, A. Z., GENS, K. K. & GILLIAM, A. D. 2019. Do theatre staff use face masks in accordance with the manufacturers’ guidelines of use? J Infect Prev, 20, 99-106.
JONES, R. M., BLEASDALE, S. C., MAITA, D., BROSSEAU, L. M. & PROGRAM, C. D. C. P. E. 2020. A systematic risk-based strategy to select personal protective equipment for infectious diseases. Am J Infect Control, 48, 46-51.
LINDSLEY, W. G., NOTI, J. D., BLACHERE, F. M., SZALAJDA, J. V. & BEEZHOLD, D. H. 2014. Efficacy of face shields against cough aerosol droplets from a cough simulator. Journal of occupational and environmental hygiene, 11, 509-518.
MENG, L., HUA, F. & BIAN, Z. 2020. Coronavirus Disease 2019 (COVID-19): Emerging and Future Challenges for Dental and Oral Medicine. Journal of Dental Research.
ROBERGE, R. J. 2016. Face shields for infection control: A review. J Occup Environ Hyg, 13, 235-42.
RUSIN, P., MAXWELL, S. & GERBA, C. 2002. Comparative surface‐to‐hand and fingertip‐to‐mouth transfer efficiency of gram‐positive bacteria, gram‐negative bacteria, and phage. Journal of Applied Microbiology, 93, 585-592.
SHOHAM, S., ACUNA-VILLAORDUNA, C. & COTTON, M. Comparison of Protection against Ocular Contamination with Disposable Eyewear Products [Online]. [Accessed 30th May].
WEBER, R. T., PHAN, L. T., FRITZEN-PEDICINI, C. & JONES, R. M. 2019. Environmental and Personal Protective Equipment Contamination during Simulated Healthcare Activities. Ann Work Expo Health, 63, 784-796.
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