Three industrial accidents produced disastrous toxic clouds in the Ukraine, India and Italy.

The Ukraine is home to 52 million people, some of whom live on the great rolling plains, in small farming communities growing wheat. As cities grew, so did the need for more food and more power: power to heat and light homes and to fuel industry. Nuclear power was favoured: cheap, efficient and modern.

In the north lay Chernobyl, an ideal site for a nuclear power station, not too far from the great city of Kiev and beside a vast water supply. Here a dreadful accident happened, which changed the history of nuclear power.

On 26 April 1986, the core of the huge reactor exploded, blowing the plant wide open and starting a poisonous fire which burned for ten days. As it burned it spewed out radioactive dust containing caesium, strontium and plutonium: some of the most dangerous substances known to science, which are invisible and long-lasting, and cause cancer.

People tens, hundreds and even thousands of miles away were and still are affected by this terrible disaster. On 29 April, three days after the accident, the prevailing wind spread the poisonous cloud over Belarus and towards Scandinavia. No-one seemed prepared or able to say just how dangerous it was.

Ten days after the accident, it was confirmed that Britain, 2,400 kilometres away, was also under the cloud. Eventually the whole world was affected by the cloud from Chernobyl, blown by winds high in the atmosphere.

Back in Belarus, the people closest to the disaster were eventually moved. The land immediately around the power station and for a hundred kilometres beyond was devastated: a bleak landscape of abandoned homes. Nearly 135,000 people were moved, to start new lives in Russia and Crimea. The impact of this disaster was enormous. When the reactor blew, it released radiation into the ground as well as the air, and from there into the underground water that flows into rivers and lakes. The water that people drink was poisoned, as were the crops which were sold as food in markets throughout Belarus and the Ukraine. Children are particularly sensitive. Ten years on, they are still tested for thyroid and gland abnormalities. Many have cancer.

Technology also failed in an even poorer part of the globe: in India. In 1972, the American chemical company Union Carbide set up a plant to make pesticides to help farmers in Bhopal, the capital of Madyha Pradesh. People migrated in from the countryside and set up homes right against the factory. They were desperate for work, but tragically unaware of the potential for disaster.

The factory had huge tanks of a chemical called methylisocyanate. On 2 December 1984, water that should have been flushing pipes got past valves and barriers to pour into the tanks. The reaction was violent and produced a deadly cloud that enveloped both the factory workers and the shanty town. Old people and babies died. Other people awoke coughing, vomiting and blinded by the toxic cloud. Tens of thousands of people tried to flee. Hospitals immediately overflowed, recording a death every minute.

In all, 12,000 died, and half of Bhopal's million people were injured, many for the rest of their lives. The accident cost Union Carbide 470 million dollars and the factory is now deserted. Bhopal is still poisoned and a ghostly shadow of a terrible past.

A world away and ten years earlier, there was an ominous precedent in northern Italy. A factory was set up in Seveso, north of Milan, to make weedkiller. In July 1976, on a hot weekend, a batch of chemicals was left half-processed. The usual cooling had been turned off. The heat and pressure built up inside the reactor and blew it open. A poisonous cloud billowed out for 6 kilometres to the south-east, covering the town and countryside. A white dust covered everything. Birds fell dead, cats and rabbits died and people were developing sores, dizziness and headaches. Something catastrophic had happened.

The deadly cloud engulfed 750 acres and nearly 6,000 people. It wasn't until nine days afterwards that ICMESA officials admitted that dioxin, one of the most poisonous chemicals known, was in the fallout. Dioxin doesn't dissolve and is very difficult to clear up. In the following months, food crops perished. People, especially children, developed liver and kidney damage and respiratory, heart and eye problems. It took seven years for topsoil to be removed and incinerated, houses demolished and rubble sprayed to contain the poison and buried in plastic-lined pits.

Operating industrial plants which produce potentially dangerous products can never be absolutely safe. When they're badly operated, the effects of disasters can last for centuries.