Samavedam Sriramachari¹, H. Chandra²

¹Institute of Pathology - ICMR, Safdarjang Hospital Campus, New Delhi - 110029, INDIA ²Medico-Legal Institute, M. G. Medical College, Bhopal, INDIA

In any consideration of the "Biological Effects of Isocyanates", the worst-ever chemical disaster in Bhopal constitutes a unique landmark and role model in several respects.

On the midnight of 2nd December 1984, following a runaway chemical reaction, nearly 40 tons of methyl isocyanate (MIC) was released into the atmosphere over a densely populated area of Bhopal City in India. Over 3,000 people died and many more were condemned to long-term morbidity. Apart from MIC, the aerosol of toxic gases inhaled by the victims possibly contained a mixture of aqueous and thermal decomposition products, and a host of reactant chemicals and polymers of MIC. Detailed autopsy studies and chemical investigations were carried out, the salient features of which are given below:

Autopsy Studies - Gross Anatomy & Histopathology

Initial autopsy studies during the first four weeks revealed a characteristic "cherry red discoloration" of lung, the primary target organ, alongside massive pulmonary edema, emphysema and hemorrhages, generalized visceral congestion, cerebral edema, ring hemorrhages and anoxic brain damage.

Extensive pulmonary edema & exudative changes were observed during subsequent autopsy studies carried out victims succumbing one to four months post-exposure. Later studies from four months to one year and beyond revealed diffuse interstitial pulmonary fibrosis (DIPF).

Issue of Cyanide Toxicity

In over 100 autopsies of instantaneous and early deaths, we observed "arterialization of blood and a 'cherry red' discoloration of the lungs". This singular feature aroused a genuine suspicion of possible "cyanide toxicity", due to release of a certain quantity of HCN during "pyrolytic decomposition of MIC" (Blake and Ijadi-Maghsoodi, 1983). Although Union Carbide stoutly deprecated the possibility of HCN release, our subsequent chemical analysis confirmed an elevation of blood cyanide levels.

Initial investigations like "Blood Gas Studies", elevated 2-3 DPG levels and a negative spectroscopic evidence for carboxy-hemoglobin lent further support to an unknown type of "histo-toxic anoxia" due to MIC. While Union Carbide advocated that MIC does not cross the "alveolar-capillary barrier", our findings of elevated levels of blood cyanide and demonstration of MIC in the blood of victims are significant.

Evidence in support of Cyanide Toxicity

Direct evidence: In a study comprising 128 samples, a considerable elevation in the blood cyanide levels was observed in the living as well as dead persons exposed to MIC, as compared with the unexposed.

Indirect evidence: The dramatic "Clinical Response" to administration of I.V. Sodium Thiosulfate (NaTs) was followed by a few successful "Double Blind Clinical Trials".

Several clinical & epidemiological studies were carried and it was found that in many of the 'exposees' the urinary thiocyanate level was raised above the normal level of 1 mg%. Immediately following NaTs, there was a further increase, indicating the accumulation of the cyanide metabolites. Over 18,000 samples were monitored for the procedure over a period of 18 months of follow-up, when the metabolic disturbance tapered off.

Binding of MIC molecule to the free amino group of the end-terminal amino acids of hemoglobin and other tissue proteins

Further, in the annals of chemical disasters, the Bhopal episode appears to be the first instance, where the offending chemical/s have been tracked down to the bodies of the victims. The presence of MIC itself was established by the demonstration of "N-carbamoylated end-terminal valine residues of hemoglobin (Hb)" in the blood samples from cadavers and 'survivors'. Binding of MIC to the terminal valine residues of Hb or Myoglobin (carbamoylation) could explain the cherry red color transformation and functional disturbances, like impaired transport of carbon dioxide, release of oxygen and extreme muscle weakness.

Initially, a reduction of "free amino groups" of hemoglobin was demonstrated by the TNBS reaction. Subsequent chromatographic studies for demonstration of Methyl Valine Hydantoin (MVH) confirmed that MIC was bound to end-terminal valine residues. By mid-January 1985, we detected MVH in 19 out of 60 exposees and 7 out of 11 cord blood samples. Thus, it has been shown that MIC not only crosses the alveolar-capillary but even the placental barriers. Depending on the extent of binding and the continuous formation of new blood, the risk seems to be "contained".

"Methyl hydantoins" of ten other end-terminal amino acids were identified using G.C.-M.S. equipped with ion trap detector, in the several viscera, of victims who survived up to 16 months. Unfortunately, as mentioned earlier, the mechanism of random distribution of MIC throughout the body eluded us, because of lack of facilities to tackle S-carbamoylation of glutathione and several enzyme systems containing sulfhydryl groups. Later Bailie & Slatter(1991), demonstrated S-Carbamoylation in experimental material using a chemical ionization detector. However, the possibility of reversible S-Carbamoylation of SH groups of rhodanese, aldolase and cholinesterase, still remains unanswered!

Comparative Forensic Toxicological Investigation of the "tank Residue" and tissue extracts of cryo-preserved viscera from autopsied cases

A comparative G.C-M.S. study of the "incriminated tank-residue" and cryo-preserved autopsy tissue-extracts revealed an overlapping spectrum of MIC derivatives. Undoubtedly, the information is of considerable "evidentiary value". But more importantly, the high molecular weights, of some of the new compounds, may have a bearing on the controversial issue as to the maximum temperatures attained during the "run-away chemical reaction" which could have favored the release of HCN. Moreover, the possible toxicity of some of the unknown compounds with high molecular weight (MW) still remains an enigma.

Using G.C.-M.S (itd) technique we detected a total of 21 MIC-derived chemicals in the milieu-exterior as well as -interior systems. Four of these compounds had a MW greater than 150 indicating higher Melting Points and a high range of temperatures which operated during the "run-away-chemical reaction". Such information if confirmed might explain whether the reaction could have promoted pyrolysis of MIC and release of HCN.

Thus the scientific studies have provided acceptable and convincing answers to several issues raised by the Bhopal disaster involving methyl isocyanate.

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