Author response to: Personal protective equipment impairs pulmonary gas exchange causing systemic hypercapnia–hypoxaemia and cerebral hyperperfusion-induced cephalalgia. Issue 12 (5th November 2021) - James, Osian P; Stacey, Benjamin; Hopkins, Luke; Robinson, David B T; Luton, Oliver; Williams, Ian M; Egan, Richard J; Bailey, Damian M; Lewis, Wyn G