Figure 2  Visualization of (a) methemoglobin (MetHb) and (b) carboxyhemoglobin (COHb) values of COVID-19 patients reported in the literature. The green shaded areas refer to the reference ranges for healthy individuals in case of MetHb (0.67 ± 0.33% for healthy non-smokers [15]) and COHb (0.5–1.5% for healthy non-smokers [16]). Different subjects reported in the case reports are marked with “#”, and red lines with arrows indicate the development of subject-specific values during the course of the disease.