evidence: Published in the Annals of the American Thoracic Society on 2 October 2020, the research found that wearing a face mask has a negligible negative effect on oxygen and carbon dioxide levels. [...] The researchers also tested the participants’ blood oxygen levels at five and 30 minutes while resting with a face mask on, and found there were no changes of clinical significance in either end-tidal CO2 nor oxygen saturation at rest. [...] ‘You’d be horrified if people are becoming hypoxic or hypercapneic [from wearing a face mask],’ he said.