Around 1800, French orphan Martin Fugate settled along Troublesome Creek near Hazard, Kentucky, and married Elizabeth Smith. Both unknowingly carried a rare recessive gene for methemoglobinemia, and four of their seven children were born with vivid blue skin. In a valley cut off from roads and railroads, descendants married into the local Combs, Ritchie and Smith families, and the trait resurfaced for generations. Yet many of the blue Fugates farmed, raised families and lived into their 80s and 90s, because their partial enzyme deficiency kept the condition at a steady, low level.

The episode explains the chemistry: iron in hemoglobin oxidizes from the ferrous to the ferric state, cannot carry oxygen, and makes the rest of the molecule cling to its oxygen, so blood turns chocolate brown and skin looks blue. The acquired form is more common, triggered by some antibiotics, local anesthetics like benzocaine, and nitrate-laced well water given to infants. It travels to Lurgan in Northern Ireland, where two blue brothers were treated in 1942, and ends with the counterintuitive modern antidote.

• Above 70 percent methemoglobin the condition is almost universally fatal, while 10 to 20 percent brings blue skin and mild symptoms
• The EPA limits nitrate nitrogen in public drinking water to 10 parts per million to protect infants from blue baby syndrome
• Standard pulse oximeters can read around 85 percent even as patients are starved of oxygen, so doctors turn to CO-oximetry
• Dr. James Deeny cleared the Lurgan brothers’ blue color with vitamin C and sodium bicarbonate, one in 12 days, the other in about a month
• Intravenous methylene blue rapidly restores the iron, but can cause serotonin toxicity alongside SSRIs