If you gab on a cellphone all day, your head is being zapped by waves of powerful radiation that could give you brain cancer. And the risk is even greater for kids.

That’s not the crazy talk of a paranoid loony — it’s the disturbing conclusion of a leading cancer expert who has worked with the National Academy of Sciences and shared in the Nobel Peace Prize in 2007.

Georgetown Prof. Devra Davis, a renowned epidemiologist, says studies from around the world have found that microwaves emitted by mobile phones, which act like tiny radio stations, penetrate the brain’s protective barrier, shred your cells’ DNA and set loose destructive free radicals, increasing your chances of getting cancer.

The damage is greatest among the youngest and most frequent users and those who keep the device pinned to their ears, she says.

“Years from now our grandchildren will look back and ask: Did we do the right thing and protect them or did we harm them needlessly . . . blinded by the addictive delights of our technological age?” Davis writes in her new book, “Disconnect.”

The bottom line: “You should not be holding a miniature microwave radio next to your brain for hours a day,” she told The Post.

Davis points to alarming studies in the US, Sweden, Greece, France and other countries.

A team at the University of Washington, for example, found that just two hours of radio frequency radiation splintered the DNA of brain cells in rats, making the cells like those found in malignant tumors.

Another study showed that rodents exposed to phone signals two hours a day for a week began leaking fluid from their brains.

In a third, rats that learned how to get out of a tank of water were given an hour of radiation and put back in. They became confused and swam around in circles.

Not all the studies were done on animals. Scientists in Moscow discovered that children between the ages of 5 and 12 who were regular cell users suffered from a host of learning and behavior problems.

Perhaps the most convincing evidence, Davis says, is the small-print warning that now comes standard with every new G3, BlackBerry and Palm: Don’t hold it against your body.

Or that England, Canada, Russia, Germany and Israel all officially discourage parents from giving their children cellphones because of the health risks, and France has banned sales to minors.

Still, Davis concedes that rock-solid proof of her theory is elusive — in part, she says, because brain cancer can take 20 years or more to develop, so it’s impossible to fully measure the problem.

“I’m concerned about the long-term effects,” she said. “If we insist on seeing proof that an epidemic is already under way . . . we will condemn ourselves and our families to lesser angels.”

Skeptics, such as Dan Krewski of the University of Ottawa and Swedish researcher Maria Feychting, claim she’s got it all wrong.

“Overall, we’re not seeing evidence of an increased risk,” Krewski told a TV reporter in May, when the results of what was hyped as a definitive study of the subject were published.

The Interphone study, run by the World Health Organization’s cancer research arm, brought together experts from 13 countries, including the US, Canada, Britain and Germany, to track 13,000 cellphone users for a decade at a cost of $24 million.

But the data was held up for five years while the researchers fought bitterly over what it meant.

Even the head of the study, Elisabeth Cardis, an epidemiologist in Barcelona, hedged.

“We have not demonstrated conclusively that there’s a risk, but I think it’s really important to note that that does not mean that there’s no risk,” she told a Massachusetts radio station. “We have a number of elements in the study which suggest that there might actually be a risk, and particularly we have seen an increased risk of glioma, which is one type of malignant brain tumor, in the heaviest users in the study.”

One problem, she added, was an ever-changing definition of “heavy user” — just 30 minutes a day in the Interphone study — and that so much of the information the group examined was old...