Brain tumor rates in Japanese atomic bomb survivors are often used to scare cell phone owners. The real story turns out to be far more reassuring.

FORTUNE - Tumors in the brains of Japanese civilians who survived Hiroshima and Nagasaki seem to tell a frightening tale, the story of how a cancer epidemic can remain hidden for several decades and then suddenly metastasize. The atomic bombings thus provide a perfect explanation for why U.S. brain cancer rates have dropped slightly in the past few decades even as Americans' cell phone use rose 500-fold.

Just wait, Britain's The Daily Mail warned mobile phone users: "the same slow development of problems occurred when the Hiroshima bomb survivors were tested: after ten years researchers found no evidence of brain cancer, but 30 years later many cases were found." Similar evidence from the atomic bombings has been cited in settings as varied as American city council meetings and debates on public radio.  After Fortune.com created this chart showing the apparent disconnect between cell phone use and brain cancer diagnoses, an article immediately popped up on the Huffington Post using the A-Bomb evidence to debunk it.

"To those who understand the long latencies involved, the absence of a general brain tumor epidemic at this time provides no comfort," wrote Devra Davis, author of a book on cell phones' cancer threat.  "Survivors of the atomic bombs that fell on Japan experienced no increase at all in brain cancer until four decades after the war's end."

It would be a compelling retort, if it were true. Fortunately for cell phone users, the real story from Japan is very different.

Claims that survivors "experienced no increase at all in brain cancer until four decades after the war's end," as Davis writes (or even just for 30 years, as The Mirror has it) are wrong on multiple levels.  Local tumor registries only began in 1958, so of course there are no records of tumors in the first 13 years after the bombing. Once the data began to be collected, an abnormal number of brain tumors quickly became visible.  This study, for instance, covers a period from 1961 and 1974 (from 16 to 29 years after the bombing) and found a five-fold increase in brain cancer in some groups of survivors.

Dale Preston, a leading expert in radiation induced cancers, says there's no sign of a "sudden spike of risk" in any of his research on cancer in bomb survivors. In fact he suspects tumors were already increasing in frequency in the 1950s, before the records were being kept. He is skeptical about drawing parallels between atomic bomb radiation and cell phone radiation (since the A-Bombs deliver ionizing radiation in one big dose, while cell phones deliver non-ionizing radiation in thousands of small doses.)

And then there's the chief scientist of the Radiation Effects Research Foundation in Hiroshima, who says that he knows of no evidence that's identified a sudden increase in brain cancer after 30 or 40 years, or for that matter any other time frame. The main problem is that so few survivors developed brain tumors that he doesn't see how anyone could suss out any sort pattern. The Radiation Effects foundation is a joint project the American and Japanese governments that has been meticulously accumulating and analyzing data on the surviving civilians' health following the bombings.

The grim fact that the A-bombs caused some people to get fatal brain cancers soon after exposure fits with the pattern of other radiation-induced cancers, which tend  to increase gradually in the affected population, as different people develop tumors at different rates. The idea that the atomic bombs caused brain tumor rates to shoot up in a sudden spike also conflicts with other the tumor-causing pattern seen in medical radiation treatments: this study found huge variation in the onset of radiation-induced brain cancer, estimating an average lag time of 18 years, plus or minus 10 years.

In other words, if a huge wave of brain cancer is on its way, scientists would expect to see the tide beginning to rise.  Thus the lack of any uptick in brain cancers is "comforting," says David Savitz, a Brown University professor and former editor of The American Journal of Epidemiology.

Still, conclusively disproving the link between cell phones and brain cancer will only come with another decade or two in which phone use plateaus while tumor rates continue to be flat or down.

Hopefully the evidence will continue to mount. In the current issue of Bio-Electromagnetics a study of brain cancer in England found that "the increased use of mobile phones between 1985 and 2003 has not led to a noticeable change in the incidence of brain cancer in England between 1998 and 2007." For now the very good news for the billions of people who talk on cells phones is that the real scientific evidence on radiation-induced brain cancer indicates that (1) scientists would expect to see some sign of an impending epidemic if one were really on its way and (2) So far they don't.