https://nypost.com/2020/05/06/two-boys-drop-dead-in-china-while-wearing-masks-during-gym-class/
Two Chinese boys dropped dead within a week of one another while wearing face masks during gym class, according to a report.
https://www.youtube.com/watch?v=QWuc2OFChfA&feature=youtu.be
Breathing in their own carbon dioxide, and not enough oxygen, I assume