Methemoglobinemia resulting from exposure in a confined space: Exothermic self-polymerization of 4,4'-methylene diphenyl diisocyanate (MDI) material.

A worker attempting to remove solidified material inside a confined space (storage tank) suffered severe methemoglobinemia and almost died. The tank contained liquid 4,4'-methylene diphenyl diisocyanate monomer that had solidified after an equipment power failure caused excessive heating. Wearing a full-face elastomeric air-purifying respirator and Tyvek® coveralls, the worker used pneumatic air hammers to break up the solid material. After two tank entries totaling slightly less than one hour, the worker complained of headache and dizziness and within two hours of exiting the tank, he was admitted to the hospital in severe respiratory distress. During his eight-week hospital course, he suffered a cardiac arrest among other complications. An investigation into the cause of the worker's illness used onsite gas chromatography-mass spectrometry which identified aniline and p-toluidine vapor within the tank, attributable to overheating that led to formation of the solid material. Both are well-known causes of methemoglobinemia, and had the initial characterization of the confined space atmosphere adequately identified the hazards present appropriate engineering controls and personal protective equipment could have allowed the tank entrant to work safely in the space.