Some oxiditation (1%) of Hb (Fe 2+) to metHb (Fe 3+) is normal, it gets reduced back by the enzyme Cytochrome b5 reductase. So far so good. But since metHb is

Some oxiditation (1%) of Hb (Fe 2+) to metHb (Fe 3+) is normal, it gets reduced back by the enzyme Cytochrome b5 reductase. So far so good. But since metHb is dysfunctional and cannot bind oxygen and you need levels of at least 10% metHb to be visible as cyanosis: Why didn't they exhibit oxygen-deficit related symptoms? Or can your body compensate with other mechanism if it's congenital? At least when it is visibly induced you most certainly will also suffer from symptoms like shortness of breath etc.