Radiation seen falling at Japan nuclear site: IAEA

VIENNA (Reuters) - Japanese data indicate that radioactivity levels fell over a six-hour period on Tuesday at a nuclear power plant damaged by last week's earthquake and tsunami, the U.N. nuclear watchdog said.

The International Atomic Energy Agency (IAEA) said about 150 people had received monitoring for radiation levels and that measures to "decontaminate" 23 of them had been taken.

It issued a statement after health experts urged governments in the Asia Pacific region to monitor radioactivity levels after Japan's quake-crippled Fukushima nuclear power plant exploded, sending radiation into the air.

The Vienna-based U.N. agency said a radiation dose level of 11.9 millisieverts (mSv) per hour was observed at the main gate of the Fukushima nuclear power plant at 0000 GMT on Tuesday.

Six hours later, the rate had declined to 0.6 millisieverts, the agency said in a statement. The IAEA uses the unit to measure doses of radiation received by people. It quantifies the amount of radiation absorbed by human tissue.

"These observations indicate that the level of radioactivity has been decreasing at the site," the IAEA said in a statement.

Exposure to over 100 millisieverts (mSv) a year is a level that can lead to cancer, the World Nuclear Association says. People are exposed to natural radiation of 2-3 mSv a year.

Earlier on Tuesday, the IAEA said Japan had told it radioactivity was being released "directly" into the atmosphere from the site and that a 400 mSv per hour radiation dose was observed, between the plant's reactor units 3 and 4.

That figure would be 20 times the annual exposure for some nuclear-industry employees and uranium miners.

"This is a high dose-level value, but it is a local value at a single location and at a certain point in time," the IAEA statement said.

"It should be noted that because of this detected value, non-indispensable staff were evacuated from the plant ... and that the population around the plant is already evacuated."