At the peak of the coronavirus epidemic in America, hospitals needed to triage patients. Only the sickest were admitted. Others were sent home to self-monitor. One measure used to determine the severity of an individual’s illness was his blood-oxygen level. The devices typically employed to do this, known as pulse oximeters, are easy to use. They clip onto a fingertip like a clothes peg. Regrettably, they record some darker-skinned patients as being healthier than they really are. This may have resulted in people who needed hospital treatment being denied it.