I'm a medical toxicologist, I'm surprised this tiny case report got picked up by The Guardian, and that it got on the hacker news front page, it's bread and butter stuff. Sulfhemoglobin is actually even less dangerous than methemoglobin, since it shifts the oxygen-hemoglobin dissociation curve right. We all have low levels of methemoglobin produced in our own bodies, but it can increase from certain exposures. Symptoms are usually very mild, but rarely it can become severe. It's typically treated with methylene blue, if treated at all. If anyone has questions for me, I'll check back later in the day.