Observational study of the adherence to the correct use of face masks by the general population in open public places during two periods of the COVID-19 pandemic in four Spanish cities

Background: One of the steps adopted to mitigate the pandemic due to SARS-CoV-2 is the use of face masks by the general population. For a face mask to be effective it should cover the nose and the mouth. We wanted to measure the correct use of the face mask by the general population in open public spaces through direct observation. Methods: We conducted an observational study of the proper use of face masks among the general population in open public places in Bilbao, Santander, Oviedo and Zaragoza from 16th to 26th July, 2020 and from 23rd January to 2nd March, 2021. Sampling for convenience; compliance of the proper use of a mask was evaluated when adults completely covered their mouth and nose. The type of mask and its improper use was registered using a standardized form. The results were obtained using frequency distribution, Pearson's chi-squared test and multivariate logistic regression analysis. Results: A total of 5,464 observations were documented. The overall compliance was 89.5%. We observed that the compliance in 2021 (94.7%) was 10.9 percentage points higher than in 2020 (83.8%) (p<0.001). The main cause of non-compliance was the incorrect placement of face masks (64%); 36% were without masks. The nonreusable face masks were most commonly worn (54.1%). We observed a significant increase in use of high-efficiency face masks in 2021 (27.1%) versus 2020 (13.7%). Conclusions: In all the cities where the study was conducted we observed an increase in compliance of the proper use of face masks as well as an increased usage of high-efficiency masks. The main cause of non-compliance was incorrect placement.