Amitai et al1 examined neuropsychological performance in 45 university students exposed to carbon monoxide from old kerosene stoves used in their dormitories. Venous blood carboxyhemoglobin (COHb) levels measured at the end of the testing period showed that except for 4 measurements all COHb levels were below 0.06 (normal, <0.01), although the ambient air concentration of carbon monoxide ranged from 17 to 100 ppm. The scatterplot of COHb levels less than 0.06 vs carbon monoxide concentrations shows a rectangular distribution and not the expected linear relationship. The significant correlation between these 2 variables is artificially created by the 4 values of COHb above 0.06.
Bleecker ML, Lindgren KN. The Mere Presence of Low Levels of Carboxyhemoglobin Is Not Causal Proof for Altered Neuropsychological Performance. Arch Neurol. 1999;56(10):1299-1300. doi: