Around 2000 people who have worked at Japan's wrecked Fukushima nuclear plant face a heightened risk of thyroid cancer, its operator says.
Tokyo Electric Power (TEPCO) said on Friday that 1973 people - around 10 per cent of those employed in emergency crews involved in the clean-up since the meltdowns - were believed to have been exposed to enough radiation to cause potential problems.
The figure is a 10-fold increase on TEPCO's previous estimate of the number of possible thyroid cancer victims and comes after the utility was told its figures were too conservative.
Each worker in this group was exposed to at least 100 millisieverts of radiation, projections show.
Although little is known about the exact health effects of radiation on the human body, the level is considered by doctors to be a possible threshold for increased cancer risk.
The Fukushima Daiichi nuclear plant became the site of the worst nuclear disaster in a generation after the massive tsunami of March 2011 destroyed its cooling systems.
The plant's reactors went through meltdowns that caused explosions in the buildings housing them, spewing radioactive materials into the air, sea and soil.
Tens of thousands of people were forced from their homes in a large area around the plant, where crews continue to clear debris and cool the reactors.
The fragility of the wrecked plant was brought into sharp relief again on Thursday with the discovery of steam in the roofless building around Reactor 3.
TEPCO said on Friday it still did not know exactly where the steam was coming from, although readings showed it was no more radioactive than expected and suggested it could have been accumulated rainwater.
The huge utility, which has faced frequent criticism for downplaying dangers and not being forthcoming about problems at the site, revised its method of estimating the level of radiation exposure among workers earlier this month.
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