4.1. Respiratory Protective Devices
Respiratory protective devices are commonly used to protect wearers from chemical, biological, and radioactive materials.
These devices have been classified by the US National Institute for Occupational Safety and Health (NIOSH), which sorts particulate filtering face-piece respirators (FFRs) into nine categories (N95, N99, N100, P95, P99, P100, R95, R99, and R100) [19].
The European Standard (EN 149:2001) classifies FFRs into three classes, FFP1, FFP2, and FFP3, with minimum respective filtration efficiencies of 80%, 94%, and 99% of the particles with a dimension up to 0.6 μm. An FFP2 is comparable to a N95 FFR [20].
The various indications of how to use FFRs is related to their different capacity and quality of filtration.
Surgical masks (SMs) can filter particles of 0.04–1.3 μm, and are commonly used to physically block particles such as droplets. Their principal limitation is due to poor quality of face fit and the consequent possibility of aerosol aspiration. The use of SMs protects the patient from saliva and respiratory secretion produced by healthcare workers.
The FFP1 and FFP2 masks are available with or without an exhalation valve, and FFP3 masks always have a valve. The FFP2 and FFP3 masks are the most appropriate barriers against aerosol because they provide a tight seal to the facial skin.
Considering that the air flow is filtered in the inhalation phase, but not filtered during exhalation (which is expelled from the valve), the infection risk is moved from operator to patient.
Masks with exhale valves are therefore not recommended for use in dentistry as this will increase the patient’s risk if the professional is infected with the virus, especially considering that the latter could be asymptomatic.

4.1.1. Indications
Wear a surgical mask in all eventualities where there is contact between the patient and other people less than 2 m away and for more than 15 min.
Wear an FFP2 or FFP3 mask in procedures with a risk of aerosol droplet production. If not available, use a surgical mask with a face shield [3].
Remove the mask after each aerosol risk procedure by touching only the strings.

4.1.2. Common Mistakes
Removing the mask by touching the possibly contaminated surface. The correct procedure is as follows: After removing the gloves, put on a new pair of gloves, and remove the mask by the strings.