Meteorologists predict the wind direction will change today, taking dangerous emissions across Tokyo, 250 kilometres to the south.
The area immediately surrounding the Fukushima plant has been evacuated due to high levels of radiation.
Experts in the Philippines have also expressed concern that the radiation will reach their country.
Dr Romeo F. Quijano, professor at the University of the Philippines-Manila’s department of pharmacology and toxicology, advised Filipinos to be on alert.
”There is already a significant breach in the reactor core containment facilities, both immediate and secondary,” he said.
”There is no doubt that significant amounts of radioactivity had already been released into the open environment, exposing thousands of people within a several kilometres radius. It is highly probable that this radiation pollution will worsen in the next few days and will most likely reach the Philippines.”
Scientists in the US have downplayed the radiation, saying that only a ”minuscule” amount was detected at a California monitoring station yesterday.
The level of radiation registered in Sacramento, California, was about ”one-millionth of the dose” a person gets from rocks, bricks, the sun and natural background sources, and ”poses no concern”, the US Environmental Protection Agency and Energy Department said.
A similar level of the radioactive isotope, xenon-133, was detected in Washington state on Wednesday and Thursday, the agencies said. It was ”consistent with a release from the Fukushima reactors in northern Japan”.
Neither EPA or Energy Department monitoring systems found ”radiation levels of concern”.
President Barack Obama said yesterday his nuclear advisers did not expect ”harmful levels” of radiation would reach the US.