By Mark Kinver
BBC News science and nature reporter
Experts say it is still too early to know the full extent of the disaster
In April 1986, reactor number four at the Chernobyl Nuclear Power Plant exploded, spewing radioactive material across many parts of Europe.
Yet two decades after the world's worst nuclear accident, there is still no consensus on the full impact of the disaster.
Last week, a report by Greenpeace concluded that the impact on human health had been grossly underestimated.
It challenged UN figures that said up to 9,000 people would die from Chernobyl-related cancers. The environmental group's own research concluded that the death toll would be nearer 100,000.
It questioned the methodology used by the Chernobyl Forum, which published its final assessment of the 1986 explosion last September.
Headed by the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, the Forum was made up of eight UN agencies and a number of official bodies from the worst affected countries - Belarus, Ukraine and Russia.
In its initial press release, the Forum said that up to 4,000 people would die as a result of being exposed to radiation, but this was later revised to 9,000 to reflect findings within the 600-page report.
Even the revised figure of 9,000 was still significantly lower than previous official estimates. According to the World Health Organization (WHO), earlier numbers were the result of confusion surrounding the full impact of the disaster.
"The Greenpeace report is looking at all of Europe, whereas our report looks at only the most affected areas - approximately 6.5 million people in Ukraine, Belarus and Russia," says WHO spokesman Gregory Hartl.
Greenpeace campaigns director Blake Lee Harwood concedes there is a wide range of cancer death estimates, but says the environmental group's report is based on credible scientific observations.
"It is likely that the true human cost of the Chernobyl disaster will be many times greater than that estimated by the IAEA," he says.
GreenFacts, a Brussels-based organisation that produces short summaries of technical scientific reports for lay readers, has recently published a digest of the UN Chernobyl Forum's report.
The reason for doing this was because conflicting views among scientists were generating a level of uncertainty, says Stephanie Mantell, publications manager of GreenFacts.
"Because there are so many contradictory figures, a lot of people have unanswered questions, and they are still looking for answers," she says.
One answer could lie in the different ways in which raw data is being interpreted.
"What seems to be happening is that different agencies are taking a different view on these figures," observes Professor Tim Mousseau from the University of South Carolina, who has been researching the ecological impact of Chernobyl since 1999.
"The Greenpeace report appears to be taking the least optimistic view and focusing on the upper level of cancer figures, whereas the UN report is taking a much more optimistic view," he says.
Much of the reports' findings, he says, probably come from the same data. "Most of the past estimates have been based on research out of Japan on the effects of the atomic bombs - which is based on fairly high doses of radiation."
What has not been clear in the past, Professor Mousseau adds, is whether this research could be used in low dose exposure scenarios, such as the aftermath of Chernobyl.
"But new data shows that even exposure to low level radiation does have an impact on the cancer figures."
He says that applying improved modelling using this new data to Chernobyl reveals that the estimated cancer mortality goes up by 50% - from 9,000 to about 16,000.
Another analysis, which was recently published by two UK nuclear scientists for a group of Green MEPs, has gone widely unreported.
Torch (The Other Report on Chernobyl) predicts an extra 30,000 - 60,000 cancer deaths across Europe as a result of the 1986 accident.
It criticises the UN for focusing only on Belarus, Russia and Ukraine. Torch says this distorts the true picture because although the three nations were heavily contaminated, more that half of the radioactive fallout was carried to other European countries, including Sweden and the UK.
It is not only human health where there is a lack of consensus - the environmental consequences of Chernobyl are still open to debate.
"There has been such little investment in even basic research that we have no real idea of the full impact," Professor Mousseau said.
He and colleague Anders Moller recently published the results of a survey they carried out on other research into the ecological impact of Chernobyl.
They found that more than 20 species showed evidence of genetic damage as a result of being exposed to contaminants from the 1986 explosion.
Their findings, published in Trends in Ecology and Evolution, are described as the first systematic review of data sets from Chernobyl, and suggest genetic damage could be extensive.
Results from Professor Mousseau's own research on barn swallow populations support the idea that the radioactive legacy of Chernobyl is affecting ecology in the region, and possibly further afield.
"Barn swallows - a long distance migratory bird - appear to be particularly sensitive," he comments. "We know that they consume much of their antioxidant reserve during the period of migration, so when they arrive back in Chernobyl they seem to be particularly sensitive to contaminants.
"This is because of their lack of antioxidants which serve to protect them from the low level radiation."
He says the depressed antioxidants can be linked to mutations (partial albino) and defects in sperm within the birds, and that this may not be limited to birds within the immediate area.
"One 'take home' message from this can be that mutations are not fixed to one place and time, they are transmitted to future generations and to adjacent populations that have not been directly affected by the contaminants," Professor Mousseau observes.
Yet he also acknowledges that these findings cannot be applied to all species.
A fellow US-based scientist found that a number of rodents were contaminated to an unprecedented level, yet they were surviving in their surroundings with no apparent side effects.
And the fact there is very little human activity within the 30km "exclusion zone" surrounding the site of the nuclear disaster has led to a number of rare species setting up home there, including white-tailed eagles, black storks, lynxes and otters.
"Animals don't seem to sense radiation and will occupy an area regardless of the radiation condition," says Sergey Gaschak, a radioecologist based in the affected area.
As for the long term impact on wildlife, Professor Mousseau says: "We have no idea but we have to be sensitive to the possibility that there will be long-term biological effects.
"But at this point, we have such little information regarding the ecological and evolutionary impact of the contaminants that we cannot make predictions."
A paper recently published in the journal Nature also reaches the same conclusion regarding human health.
It said lessons learned from studies examining the aftermath of the US atomic bombs dropped on Japan in 1945 showed that 20 years was not long enough to gather a complete picture.
Professor Mousseau warns it may be several more decades before any authoritative assertions can be made.
"We really won't have a good idea of the death toll from Chernobyl for at least another 20, 30, or even 40 years."