The five cities with the top five iodine-131 detections in drinking water are:
Philadelphia/Queen - 2.2 picocuries per liter
Chattanooga - 1.6 picocuries per liter
Philadelphia/Belmont -1.3 picocuries per liter
Oak Ridge/371 - 0.63 picocuries per liter
Philadelphia/Baxter - 0.46 picocuries per liter
Source: U.S. Environmental Protection Agency
* Iodine-131 is produced by fission of uranium atoms during operation of nuclear reactors and by plutonium (or uranium) in the detonation of nuclear weapons.
* Iodine released to the environment from nuclear power plants is usually a gas. It emits beta particles during radioactive decay.
* Anywhere spent nuclear fuel is handled, there is a chance that iodine-131 and longer-lived iodine-129 will escape into the environment.
* Nuclear fuel reprocessing plants, which dissolve the spent fuel rods in strong acids to recover plutonium and other valuable materials, also release iodine-129 and -131 into the airborne, liquid, and solid waste processing systems.
Source: U.S. Environmental Protection Agency
While a cloud of radiation from Japan's maimed and melting nuclear plant snaked across the Pacific and curled along America's jet stream, Chattanooga collected the second-highest radioactive iodine-131 level measured in drinking water anywhere in the U.S.
Radioactive iodine-131 is easily absorbed by the human thyroid and can cause thyroid cancer.
But authorities say there is good news.
They say levels measured so far in Chattanooga's drinking water - although spiked - is not high enough to be dangerous.
And unlike most radioactivity types, iodine-131 is short-lived. Its half-life is eight days, meaning half of the radioisotope's radioactivity decays in that time.
"The results being reported are well under the levels of health concerns," said Tennessee American Water Co. spokeswoman Kim Dalton. "We will continue to follow the situation closely."
Iodine-131 is produced by the fission of uranium atoms in nuclear reactors and by plutonium (or uranium) in the detonation of nuclear weapons.
The radioactive plume that prompted Chattanooga's findings swirled through the region on the jet stream early in the Fukushima-Dai-ichi Nuclear Plant crisis. The Japanese plant was crippled March 11 when a 9.0 earthquake and tsunami hit the island nation.
The Chattanooga water sample showing radiation was collected March 28, according to U.S. Environmental Protection Agency's nationwide radiation monitoring system, known as RadNet. The finding was posted in the RadNet database April 8.
Since then, U.S. and Japanese authorities have said more radiation has been released, but new sampling data has not yet been posted because of the time it takes to collect and analyze samples, according to EPA and state officials.
The EPA's maximum allowable amount for iodine-131 in drinking water is 3.0 picocuries per liter, according to EPA spokeswoman Dawn Harris-Young. Chattanooga had 1.6 picocuries per liter.
A picocurie is a tiny fraction of a curie. A curie is a measurement of radioactive decay.
The RadNet database on Wednesday afternoon listed 69 drinking water samples collected and analyzed across the nation since March 28. More than half - 42 - found no detection of iodine-131.
RadNet also is sampling air, rainwater and milk. No Chattanooga-area samples have been posted in rainwater databases. Earlier this month, officials detected air samples near Sequoyah with radioactive isotopes.
Milk samples taken in Athens, Tenn., and Knoxville on March 31 showed no detection for iodine-131, according to the databases.
With greater radiation releases in Japan in recent days, Mike Stafford, director for the nuclear and radiological protection division at Oak Ridge National Laboratory, acknowledged that officials may see higher sample readings.
But he said the duration of spikes shouldn't last much longer if Fukushima conditions stabilize. He also said radiation may not show up again in the same places across the U.S.
"You'll see patterns that will be linked to the jet stream and where there was a rain right before [the sample was taken]," he said.
While Chattanooga's drinking water sample measured 1.6 picocuries per liter, the highest reading on one of Oak Ridge's five sample spots registered 0.63 and was collected the same day as Chattanooga's. Another of Oak Ridge's sample spots, collected the following day, showed no detection.
Meg Lockhart, a spokeswoman for the Tennessee Department of Environment and Conservation, said the level detected in Chattanooga water is so low that even an infant would have to drink 600 liters of water to receive a dose equivalent to one day's background radiation in nature.
EPA, too, has stressed that levels seen to date are low and are not expected to cause health effects.
But EPA's website acknowledges risks from too much exposure. Radioactive iodine can cause thyroid problems, but it also can help diagnose and treat thyroid problems, according to EPA.
Still, the agency says physicians must maintain "a fine balance between the risks and benefits of using radioactive iodine because this small, additional exposure may tip the balance in favor of cancer formation."
The National Academies of Science has said there is no safe level of exposure to radiation.
Neither EPA nor state and local authorities will acknowledge where the Chattanooga drinking water sample was taken.
"We do not divulge that information," Harris-Young said.
Previously she and TDEC officials indicated they thought the samples were taken from water taps at Tennessee American Water Co.
Dalton, Tennessee American's spokeswoman, seemed to deny that by questioning where that information came from.
"We have been in contact with the regulatory agencies to determine the location of the samples taken," Dalton wrote in a e-mail.