Radiation reached around 20 times normal levels in the capital Tuesday morning, the Tokyo Metropolitan Government said, while offering the assurance this reading posed no immediate risk to human health and that the public should remain calm.

"I received a report this morning that there was an important change of data," Gov. Shintaro Ishihara said at a news conference. "I heard that it will not immediately cause health problems."

He said the metropolitan government would continue to give minute-by-minute updates.

At a metropolitan facility in Setagaya Ward, "a very small amount" of iodine and cesium, substances generated by a nuclear reaction, were detected in floating dust particles between midnight and 7:12 a.m.

At a facility in Shinjuku Ward, a maximum hourly level of 0.809 microsievert was detected at around 10 a.m., but the hourly level went down to 0.151 microsievert after 11 a.m.

These figures compare with 0.035 to 0.038 microsievert detected Monday, and 50 microsieverts absorbed when one takes a chest X-ray.

The metro government said an exposure level of more than 100,000 microsieverts per hour would endanger health.

While the highest level, 0.809 microsievert, was observed around 10 a.m., the average between 10 a.m. and 11 a.m. was 0.496 microsievert per hour.

Even if one is exposed to that level for a year, the total would amount to 4,344 microsieverts. That compares with the 2,000 to 5,000 microsieverts per year that exist in nature, the metro government said.

Ishihara instructed officials Monday to monitor radiation levels amid the crisis occurring at the Fukushima No. 1 nuclear plant.

In adjacent Ibaraki Prefecture, radiation at one stage reached 5 microsieverts per hour, 100 times higher than usual, the prefectural government said.

In Kanagawa Prefecture, the radiation level was 10 times higher than usual.

In the city of Saitama, the amount shot up about 40 times higher than usual.

In Ichihara, Chiba Prefecture, the amount of radiation showed a two- to fourfold increase, the prefectural government said.

The amount of radiation rose to 1.318 microsieverts per hour — a figure 33 times greater than usual — in Utsunomiya, Tochigi Prefecture, the prefecture said.

The science ministry said it had asked prefectures to monitor radiation levels as frequently as possible.

Information from Kyodo added