COLUMBIA, Md. - Scientist Kwok Chan switches on a cell phone held against a model of the human head and gently lowers a radiation probe into the honey-colored goopy liquid simulating the blood and brain tissue inside a person's skull.
Chan's computers in this Federal Communications Commission laboratory then measure how much radiation Americans absorb when they talk on their cell phones.
Now, amid the Christmas sales rush, manufacturers are revealing for the first time the radiation levels of nearly every phone sold to consumers so infatuated with the gadgets they use them when they walk, drive, even eat.
It is a disclosure that could alert many of the nation's 97 million cell phone users to an issue they largely have ignored: conflicting evidence about whether the devices, which unlike regular phones work by beaming radiofrequency energy through the air, pose any health threats.
But will the radiation disclosures suggest one cell phone brand is safer than another? Or that people who spend hours pressing them against their ear are at risk of brain cancer?
Or will consumers even bother to read the levels?
''People are going to look at that information and say 'huh?''' said Dave Berndt, a financial analyst at Boston's Yankee Group who specializes in wireless technologies.
Federal health officials insist, and published medical studies agree, that so far, there is no real evidence cell phones cause brain tumors or other health hazards - beyond causing car crashes when people gab while driving.
At the same time, no government or health organization yet gives cell phones a definitively clean bill of health.
In fact:
-The Food and Drug Administration just ordered new studies after industry-sponsored test-tube research discovered cell phone signals might cause genetic damage in human blood cells, which in turn might spur cancer growth.
-British health officials last week began distributing leaflets warning that children should limit cell-phone use to emergencies, because children's still-forming skulls and brains could be more vulnerable if the phones ultimately prove risky. The leaflets also disclose each phone model's radiation level and advise on ways users can minimize exposure.
-Some small studies raise health questions, including one controversial finding that tumors were more likely near the ear where patients held their cell phone. But it has been attacked as scientifically flawed.
The conflicting opinions have prompted at least one wireless company to post store signs recommending that parents consider pagers instead of cell phones for children. Metrocall, a nationwide reseller of wireless service, also recommends using earphones instead of holding cell phones to the head.
While taking no position on the science, ''We decided if we were going to err, we wanted to err on the side of the angels,'' said Mike Scanlon, senior vice president at the Alexandria, Va.-based company.
Many phone manufacturers fear that listing the radiation number will prompt consumers to buy only the lowest-emitting phones even though every phone must meet federal radiation safety limits.
Consequently, manufacturers will not make it easy to compare levels before buying. The numbers will not appear on the outside of cell phone packages, only inside along with a brochure explaining radiation safety guidelines. To check a phone before buying, consumers must look it up on the Internet.
''Unless these numbers are explained and put into the proper context, they will be regarded as gradients of safety, which they are not,'' contends Norm Sandler of Motorola, a leading cell phone equipment maker.
Amid this confusion, a new cottage industry is marketing products claiming to limit radiation absorption. They range from foam wafers stuck onto the phone's earpiece to an antenna-attached gadget that looks like a ceramic ladybug.
Experts warn against the hype.
''For a device which actually claims to reduce radiation, they should look for the data that it in fact does that. But still be aware of the fact that they're taking a precaution that may not be necessary,'' says Dr. David Feigal, chief of the FDA branch overseeing cell phone safety.
If precautions are not necessary, why did the FDA just call for more safety studies?
''We're not encouraging research because we know of a problem, but because it's important to continue making sure there's not a problem,'' said Feigal, who uses a cell phone himself.
Reassuringly, newer digital phones already emit less radiation then older analog models. Still, one expert on radiofrequency energy says choosing lower-emitting phones makes sense for now.
''I think people need to make decisions with respect to their own sense of comfort given the uncertainty right now as to whether cell phones are safe or not,'' said Dr. Patrick Breysse of Johns Hopkins School of Public Health, who keeps a phone in his car only for emergencies.
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Radiofrequency waves are non-ionizing radiation, far less worrisome than other types of radiation types such as what suntanners absorb. On an energy-frequency scale, cell phone RF waves fall somewhere between microwave ovens and television signals.
But any radiation-emitting device used by millions raises health questions that can take years to settle.
''One of the biggest concerns we have ... is that some of the same (energy) frequencies in cell phones have been used in microwave ovens,'' explained Kenneth Nichols, head of the FCC lab where Chan studies radiation absorption.
Microwaves damage tissue by burning it. Cell phones' lower-powered RF waves are not thought to cause enough heat to hurt. Frequent callers who feel heat are actually feeling the phone's battery. The question is whether low-energy RF waves can cause any significant non-thermal biological effects.
How much RF energy is absorbed by a caller's head is a measurement known as the ''standard absorption rate'' or SAR. The FCC limits the SAR to no higher than a level of 1.6 - the energy, measured in watts per kilogram, that one gram of tissue absorbs from a cell phone.
Companies say if they comply with that standard, their phones are safe. But, yielding to increasing public concern, major wireless companies plan to disclose each a phone's SAR with models now beginning to hit store shelves.
Phone packages will bear an ID number enabling consumers to look up radiation absorption levels on the FCC's Web site before buying. Or after they buy the phone, they can find the SAR and an explanation of what it means inside the package.
The effort is meant to reassure consumers ''that these phones must meet strict federal guidelines,'' said Jo-Anne R. Basile of the Cellular Telecommunications and Internet Association, the leading trade group.
But to industry's chagrin, some Web sites - including www.domode.com - have begun ranking certain phones' radiation levels. ''Looking at raw numbers can be misleading,'' Basile complains.
To determine SARs, cell phones undergo rigorous testing using equipment like Chan's in the FCC's concrete-block laboratory in this Washington suburb.
A large hollow model of the head holds a solution, made mostly of water and sugar, that mimics the electrical properties of brain and other tissue. Chan dips a large robotic arm carrying a radiation probe into the model while another robotic arm holds a phone at the model's ''ear.'' The probe records the spot inside the head where the highest RF absorption occurs.
The level can differ according to how people hold the phone or if the antenna is built-in by the ear or pulls out. Some models leak RF through the phone's dial-pad into the person's cheek.
But that 1.6 setting is somewhat arbitrary, says physiologist Ross Adey of Loma Linda University School of Medicine, an expert on RF energy.
''There's not a precipice that means below 1.6 is safe and above you're going to get a brain tumor,'' Adey said.
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Independent analysts say giving the public more information about radiation levels probably will not dampen booming consumer appetite for wireless phones.
But some consumers do worry. A Maryland brain tumor patient recently filed an $800 million lawsuit against cell phone makers. And doctors at the National Cancer Institute say brain tumor patients frequently ask if cell phones were to blame. More radiation hits skin around the ear and cheek than penetrates the skull to the brain, yet there is no panic about skin cancer.
''When patients with brain tumors ask, I tell them that every neurosurgeon that I know regularly uses a cell phone,'' says Dr. Henry Brem, Johns Hopkins University's neurosurgery chief.
Six of every 100,000 Americans gets a brain tumor each year, whether they used a phone or not. Brain tumor incidence has risen since the 1970s. But starting in 1990, brain tumor rates inched down again, about 1.4 percent a year, until 1997, the latest date available.
Cell phone use did not begin skyrocketing until around 1996, so the statistics are not definitive. Critics question whether using a cell phone for many years might eventually increase someone's risk.
To tell, most safety studies have been performed on tissue exposed to RF energy in laboratory dishes, or performed on animals. Most are reassuring; a few raised questions. The FDA is focusing on test-tube research that discovered RF-exposed human blood cells developed genetic damage, called micronuclei. That type of damage can be one of several steps needed for a cell to turn cancerous, explained FDA scientist Russell Owen.
Now the FDA has ordered follow-up studies to see if the finding is a real concern to people or a test-tube fluke: It took a radiation level, or SAR, about 10 times higher than people typically absorb to cause damage in the initial study.
But reliable answers require huge human studies. French scientists are comparing 1,500 brain tumor patients with 1,500 healthy people to see if cell phone use made any difference; results are due in about three years.
A smaller but eagerly awaited NCI study of U.S. brain-tumor patients has been accepted by a major medical journal but will not be published until at least January. NCI is mum until then about whether it will ease Americans' fears.
Until then, the FDA says worried consumers have an easy answer: Just use one of those headset devices that let you talk while keeping the phone held away from your body.
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On the Net:
Food and Drug Administration's Center for Devices and Radiological Health: http://www.fda.gov/cdrh/radhealth.html
National Cancer Institute site: http://www.cancer.gov
Federal Communications Commission site on radiation levels: http://www.fcc.gov/oet/fccid/
Cellular Telecommunications Industry Association: http://www.wow-com.com
