MORE than 2,000 people have died; up to 200,000 are sick, many of them blinded; the rest of the 670,000 population of Bhopal in central India live on with the lingering effects of the gas that enveloped much of the city on Monday when Union Carbide’s pesticide plant leaked a poison cloud. These are the grim statistics of the world’s worst man-made disaster. How did it happen?

Until last Sunday night, the potential damage that methyl isocyanate (MIC) could cause had only been guessed at. What was known was that even in minute quantities it killed rats and inflicted discomfort on humans. In an experiment in West Germany in 1974, four volunteers were briefly exposed to a cocktail of two parts of the chemical to one million parts of air and suffered eye irritation. When the dose was increased to 21 parts per million, they found it unbearable.

The best guesses about MIC’s effects in much greater concentrations depended until last Sunday on the evidence about one of its ingredients, the gas phosgene. At 5.30am on December 19, 1915, the German gas corps sent clouds of it, smelling like new-mown hay, across the trenches of the British front at Ypres.

The soldiers who inhaled it at first felt nothing more than mild irritation of the throat and eyes. But as the phosgene gas met moisture in the lungs, it broke down into carbon monoxide and hydrochloric acid. The soldier’s bodies responded by producing fluid which gradually filled the lungs – leading, in the worst cases, to “dryland drowning”.

At Bhopal an estimated 12,000 people lived across the road from Union Carbide’s plant. Nobody told them about the dangers of MIC. Last Sunday night the plant’s alarm system, which might have given some last-minute warning, failed to go off for three hours. The back-up system also failed.

When the disaster happened, the two men sent to deal with the leaking MIC were equipped with hosepipes. When they could not cope, they ran away.

As a result of these shortcomings, and crucial failures of safety equipment at Bhopal, the precise effects of MIC gas and its degree of deadliness compared with phosgene are no longer matters of guesswork. In the first world war phosgene took, on average, two days to kill its victims: last Sunday MIC took, on average, three minutes.

What remains in doubt are the long-term effects on the estimated 200,000 people who survived.

The first sign that something was badly wrong at the plant came at about 11pm. The outside temperature was no more than 17C. Yet in the control room the gauges were monitoring 3,840 gallons of liquid MIC, stored in one of three tanks, at 38C.

The hotter MIC gets, the more gas it gives off. Its boiling point is between 39C and 44C, depending on purity. The gauges showed that the temperature was still rising – and as more and more gas was given off, so was the pressure inside the tank.

The two men with the hosepipes fled, as did most of the rest of the nightshift. A lone supervisor, Shakeel Ahmed, struggled alone for 45 minutes to stem the stream, but all 3,840 gallons of the liquid MIC turned into gas and escaped. Outside help did not arrive until 1.51am. Ahmed is now in hospital, seriously ill and under police guard.

The gas, twice as dense as air, hugged the ground like fog. It rolled slowly and silently in the northwestern breeze, across the road and into the ramshackle homes.

Most of those who died were asleep. Those who awoke before their lungs filled with water and they drowned could not smell the gas because it is so vicious it instantly destroys the nostrils. Those who were not blinded could not see the gas because it is colourless.

The gas continued its silent passage into Bhopal proper, reaching the bus station. In all, it covered 25 square miles, leaving thousands of dead animals and blinded, choking people. Many suffered heart attacks. Indian scientists from the Council of Scientific and Industrial Research, who made an on-the-spot survey, said: “The people of Bhopal didn’t have a chance. The gas moved like a phantom.”

It was later estimated that about 5,000 died in the immediate aftermath of the disaster and another 15,000 or so subsequently. A total of 574,304 people received compensation from a $470m settlement paid by the company. The site remains heavily contaminated.

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