Until recent decades, the mechanism of methyl isocyanate toxicity in humans was largely unknown or unclear. Methyl isocyanate and other isocyanates are electrophiles and are currently thought to cause toxicity by the alkylation of biomolecules. The mechanism of methyl isocyanate was previously suspected to be the carbamylation of hemoglobin, thus interfering with its oxygen-binding capability and causing hypoxia. However, experiments showed that when rats and guinea pigs were exposed to methyl isocyanate at concentrations above the median lethal concentration (LC, the concentration sufficient to kill 50% of the tested population), only 2% of hemoglobin molecules were carbamylated, suggesting that this is probably not the mechanism of toxicity.