Bought a pulse oximeter. Tested in Walmart. Purchased for $25 Had my wife carry it with her into the Walmart and periodically test her levels.   After 1 min with mask - 99 After 5 min with mask - 98 After 10 min with mask - 96 After 20 min with mask - 93 and feeling nauseous  After 25 min she was back in the car. Still at 93. She took off the mask and oxygen levels returned to normal within 5 minutes. So my plan now is to keep this in my pocket and offer to test anyone who gives me a hard time for not wearing a mask.  Then let them know that low oxygen levels can lead to mood swings, anger, and irritability. That they should remove their mask, breathe deeply, and calm down