  It is often accepted a priori that a face mask worn by an infected subject is
effective to avoid the spreading of a respiratory disease, while a healthy
person is not necessarily well protected when wearing the mask. Using a frugal
stain technique, we quantify the ballistic droplets reaching a receptor from a
jet-emitting source which mimics a coughing, sneezing or talking human: in real
life, such droplets may host active SARS-CoV-2 virus able to replicate in the
nasopharynx. We demonstrate that materials often used in home-made face masks
block most of the droplets. We also show quantitatively that less liquid
carried by ballistic droplets reaches a receptor when a blocking material is
deployed near the source than when located near the receptor, which supports
the paradigm that your face mask does protect you, but protects others even
better than you.
