  Wearing face mask is an effective measure to reduce the risk of COVID-19
infections and control its transmission, thus its usage survey is important for
better policy decision to mitigate the epidemic spread. Current existing
worldwide surveys are mostly self-reported, whose accuracies are hard to
guaranteed, and may exaggerate the percentage of face mask wearing. Therefore,
we collected busking videos with a large amount on YouTube from December 2019
to December 2020, mainly from South Korea, and reported an objective
investigation of face mask use in the crowds outdoor. It is found that the face
mask wearing rate has an obvious positive correlation with effective
reproductive number (Rt) in the South Korea, which indicates that the people in
South Korea kept sensitive to the COVID-19 epidemic. The face mask wearing rate
in South Korea is higher than some other countries, and two rate droppings in
June and September also corresponds to the temporary remission in 2020. This
study shows significant potentials to utilize public big video data to make an
accurate worldwide survey of face mask use with the support of deep learning
technology.
