Radiation, heat worries dismissed, then resurface.

Investigators at an eminent research clinic in Cleveland concluded last fall that excessive cell phone use could damage a man's sperm. Then last month, a major European study found that long-term cell phone use appeared to increase the risk of developing a head tumor.

Wait a second. Wasn't the cell-phone-can-hurt-you issue put to rest long ago?

In fact, no. In what is something of an oddity for a common consumer product, the mobile phone developed into the world's most popular personal electronics device without the scientific community ever unanimously declaring that it is fully safe to use.

Health concerns over the effects of radiation waves or the heat generated by cell phones in close proximity to the head continue to be shot down by respected scientists and then raised again by others, more than a decade after phones went into general use.

Regulators in this country and throughout the world say cell phones don't pose a health risk. And studies have piled up over the past 15 years concluding that wireless phones don't affect biology, human or otherwise.

But at the same time, many studies also have concluded cell phone radiation perhaps can produce biological effects, including possibly harmful ones.

An immense amount is at stake for the wireless industry if cell phones were ever found to be hazardous. About 1 billion of them were produced globally last year.

To the cell phone industry, the radiation issue appears settled.

"The overwhelming majority of studies that have been published in scientific journals around the globe show that wireless phones do not pose a health risk," according to a statement by CTIA-The Wireless Association.

Health regulators and the American Cancer Society agree, the CTIA noted.

Yet respected scientists continue to study the devices, and some are raising questions. The contentiousness of the issue goes beyond the research itself into who funds it. The wireless industry, including Schaumburg-based Motorola Inc., the world's second-biggest cell phone-maker, has long funded academic research on phone radiation.

A recent study by a team of Swiss academics found that industry-funded research was less likely to find that cell phone radiation could cause biological effects--results favorable to the industry's own products.

Worries about phone radiation started not long after the mobile market took off in the 1990s. The radiation issue died down, but still pops up when new studies are released.

For example, Ashok Agarwal, research director of the Cleveland Clinic's Reproductive Research Center, unveiled research in October that showed a significant decline in sperm health for men who use mobile phones frequently.

Agarwal was soon touring the morning TV talk shows, talking about potential fertility problems for hard-core cell phone users.

But there's a pingpong nature to phone radiation research. Six weeks after Agarwal's work was released, a study led by the Danish Institute of Cancer Epidemiology found no cause for worry.

It culled health data on 420,095 Danish cell phone users and found no evidence that phones were linked to a higher risk of leukemia or cancers of the brain or nervous system.

The Danish report is "epidemiological," meaning it involves studying disease patterns among people. Several such studies have been done in Europe, and most haven't found a higher tumor risk among cell phone users--but not all of them.

Last month, a pan-European study concluded that people who'd been using mobile phones for at least 10 years appear to have a slightly increased risk of a cancerous tumor on the side of their head where they held their phone.

There is an accumulation of research probing cell phone radiation--over 300 studies by the count of Henry Lai, a bioengineering professor at the University of Washington.

Just over half show radiation having some sort of potential biological effect--though not necessarily harmful--while just under half show no effect, Lai said.

Most of that research has been done in laboratories such as Lai's. He and fellow scientist N.P. Singh were the first in the 1990s to find that microwave radiation, which is akin to phone radiation, could lead to DNA damage in a rat's brain cells.

DNA carries genetic information, and many scientists believe that cancer is caused by some sort of DNA disturbance. A major European study, the "Reflex" report, found results similar to Lai's.

But several researchers also tried and failed to get the same outcomes as Lai and the Reflex study, said Dariusz Leszczynski, a biochemist who worked on the latter.

Leszczynski is head of radiation biology at Finland's Radiation and Nuclear Safety Authority, and a leading phone-radiation researcher.

Last year, his lab found that phone radiation caused changes in the activity of genes and proteins derived from human blood vessels. Cells essentially recognized radiation as a stress.

It's possible that such small biological effects might become significant for human health if exerted over a long time--such as decades of cell phone use, Leszczynski said in an e-mail interview.

But he's ready to admit there's no conclusive evidence from his work--or anybody else's--that cell phones are a health hazard.

To make matters murkier, scientists face a big challenge in explaining why phone radiation would cause biological effects such as DNA breaks.

Scientists agree that the low-energy radiation from cell phones can have a "thermal" effect, heating tissue. But they also agree that such a heating effect is simply too weak to actually hurt people.

So, the biological effects found by Lai and Leszczynski must be explained by something else. The problem, in addition to the conflicting lab results: lack of an accepted scientific theory.

"Nobody has identified a route that would allow [magnetic fields] to interact with a biological system except through the heating effect," said Ben Greenebaum, an emeritus physics professor at the University of Wisconsin-Parkside in Kenosha.

"That doesn't means it doesn't exist," said Greenebaum, who's also president of the Bioelectromagnetics Society, a group for those who study radiation and biology. "It means no one has come up with it."

Greenebaum is in the camp that thinks that while research hasn't shown phone radiation is a health hazard, he won't discount the possibility. "I never say never."

But there are skeptics who will say "never"--such a discovery would be too contrary to the principles of physics.

"I think it's impossible," said Robert Adair, an emeritus physics professor at Yale University and a member of the National Academy of Sciences. The whole notion isn't even "recognized by good biologists."

Many studies on the biological effects of weak radiation are "subject to errors that follow from faulty processes," Adair wrote in a Web piece last fall for the American Council on Science and Health.

The notion that cell phone radiation could have a biological effect is tantamount to a scientific hypothesis like cold fusion, he wrote.

Cold fusion caused a stir years ago with the prospect of limitless energy, but it turned out to be an illusion, the result of poorly controlled experiments.

Adair's bluntness and reactions to it are emblematic of the contentiousness the radiation issue has created among some of those who study it.

"[Adair] has dug himself a hole and keeps digging," said Martin Blank, a physiology professor at Columbia University.

Adair has staked out a position, even though evidence of biological effects is mounting, said Blank, whose own work concluded that cell phone radiation caused cellular changes in fruit-fly larvae.

The contentiousness isn't limited to the radiation issue itself: Industry influence on radiation-related publications has also been a hot potato.

For instance, Blank wrote a missive in 2004 to the newsletter of the Bioelectromagnetics Society, castigating its editor for bias.

The editor, a Motorola scientist, had been quoted in a British science magazine saying that further phone radiation research was a waste of money--there simply were no health effects.

The society formed a committee to look into Blank's complaint. It found that while "some single reports" caused "strong concern" there was no "systematic bias" in the newsletter.

The bias issue surfaced again last year, including in a study led by researchers at two Swiss universities.

They reviewed 59 laboratory studies involving mobile-phone radiation, 68 percent of which reported some sort of biological effect.

Their conclusion, published in the journal Environmental Health Perspectives: Compared with studies done by public agencies or charities, research exclusively funded by industry was less likely to report that radiation caused a statistically significant biological effect.

The Swiss academic study came out a couple of months after a report on possible funding bias on the Web site Microwave News, which has tracked radiation issues for many years.

The site's publisher, Louis Slesin, is a pot-stirring independent voice, or an advocate of the view that radiation risks are being soft-peddled--depending on who's describing him.

With Lai's help, Slesin analyzed 85 lab studies that examined whether radiation had caused harmful changes to DNA in animals or cell cultures. Forty-three of them found a biological effect; 42 didn't.

But studies funded by industry or the Air Force were considerably more likely to find no effects than studies without such funding, Slesin concluded. (The Air Force is also a beneficiary of weak radiation, Slesin argues.)

Slesin singled out the scientific journal, Radiation Research, saying it published primarily cell phone-related research, much of it funded by Motorola, that showed no biological effects.

"It almost appears that Radiation Research is a house organ of the Motorola Corporation," Slesin wrote.

Radiation Research's editor, Sara Rockwell, in an interview with the Tribune, said: "I'm trying to think of a response that's printable. It's totally untrue, of course."

Rockwell, a Yale University radiology and pharmacology professor, criticized Slesin's methodology, contending that some studies he counted as industry funded also got money from non-industry sources such as public agencies.

Many of the Motorola-funded studies cited by Slesin were done by Joseph Roti Roti's lab at Washington University School of Medicine in St. Louis.

A radiation oncologist, Roti Roti has been investigating weak radiation issues since 1994. He's published at least 25 papers, primarily finding no biological effects. Any effects he found, he couldn't replicate.

Roti Roti said Motorola was one of the few places he could get funding for his work; the federal government has put relatively little money into his field.

Motorola had little say over his research, he said. "They never said how we should do the experiment or what we should write. Otherwise, I wouldn't take the money."

Roti Roti said he started with a "huge team" of scientists when he began looking at phone radiation. "They gradually lost interest because they were finding no effects," he said.

"I've wasted--I've invested more than 10 years of my career doing this. Believe me, I've tried to find [effects]."

To Roti Roti, the issue is all but settled, but that doesn't mean it will go away. "I think it will go on forever."

Indeed, some cancers can lay hidden for 20 years before blooming. But cell phones--as a mass-market device--haven't even been around that long, making any 20-year latency a tough issue to research.