#MaskOn! #MaskOff! Digital polarization of mask-wearing in the United States during COVID-19.

The coronavirus disease 2019 (COVID-19) has caused an unprecedented public health crisis worldwide. Its intense politicization constantly made headlines, especially regarding the use of face masks as a safety precaution. However, the extent to which public opinion is polarized on wearing masks has remained anecdotal and the verbal representation of this polarization has not been explored. This study examined the types, themes, temporal trends, and exchange patterns of hashtags about mask wearing posted from March 1 to August 1, 2020 by Twitter users based in the United States. On the one hand, we found a stark rhetorical polarization in terms of semantic antagonism between pro- and anti-mask hashtags, exponential frequency increases of both types of hashtags during the period under study, in parallel to growing COVID-19 case counts, state mask mandates, and media coverage. On the other hand, the results showed an asymmetric participatory polarization in terms of a predominance of pro-mask hashtags along with an "echo chamber" effect in the dominant pro-mask group, which ignored the subversive rhetoric of the anti-mask minority. Notwithstanding the limitations of the research, this study provides a nuanced account of the digital polarization of public opinion on mask wearing. It draws attention to political polarization both as a rhetorical phenomenon and as a participatory process.