Some advantages of shields:
Some disadvantages of shields:
- There are varieties of shields that are very small, therefore they cover less of the face than masks.
- Such small masks possibly may be collecting pathogens by guiding them towards the face when worn wrongly.
- Lingering aerosol particles can be inhaled more easily 'sideways'.
Some advantages of masks:
- For a sideways seal the N95 type establishes a better seal than surgical masks.
- Easier to carry around in a bag.
Some disadvantages of masks:
- Subjectively people claim it to be harder to breathe or talk with.
- Disinfection for reusable masks may be more involved, if possible at all for some variants of masks.
A publication by JAMA compares masks versus shields (https://jamanetwork.com/journals/jama/fullarticle/2765525)
In this there are a few references of studies conducted before the COVID-19 outbreak that evaluates protection by either cloth masks or shields:
Rengasamy S, Eimer B, Shaffer RE. Simple respiratory protection: evaluation of the filtration performance of cloth masks and common fabric materials against 20-1000 nm size particles. Ann Occup Hyg. 2010;54(7):789-798. doi:10.1093/annhyg/meq044
Summary: Fabric materials against aerosols are higher than for N95 respirator filter media but were in the range found for some surgical masks.
Lindsley WG, Noti JD, Blachere FM, Szalajda JV, Beezhold DH. Efficacy of face shields against cough aerosol droplets from a cough simulator. J Occup Environ Hyg. 2014;11(8):509-518.
Summary: Face shields are effective, but lingering aerosol particles can easily be inhaled.
The main difficulty in the comparison seems to be the variety of masks and shields in use, ranging from self made to factory produced from a variety of materials. Also how masks are worn and also the fact that people sometimes touch these protective devices with unwashed hands on the inside may be a factor reducing protection.
Various institutions, including the CDC (https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover-guidance.html) and John Hopkins Hospital (https://www.hopkinsmedicine.org/health/conditions-and-diseases/coronavirus/coronavirus-face-masks-what-you-need-to-know), do not recommend choosing face shields over face masks, when possible.
It may be that the combination of masks and shields (either using both or a manufactured variant including both) is the best option aside of more advanced respiratory protection equipment. For very exposed medical workers the combination has been put down as mandatory, e. g. in situations where endotracheal intubation is carried out on a patient tested positive for COVID-19; some modifications to this are in use, e. g. safety goggles instead of face shields.