With cell phones now as ubiquitous as their land-line counterparts, few people worry about the potential health risks associated with flipping open a handset or standing near a cell phone tower.

But among researchers who study the effects of electromagnetic emissions from cell phones and towers, the prevailing wisdom is that it's too early to conclude that they are harmless to humans.

"At the moment, there are too few properly controlled scientific studies to draw any strong conclusions," said Elaine Fox, a professor in the psychology department at the University of Essex who is studying whether the electromagnetic fields emitted from cell phone base stations have a direct effect on human health.

Fox's project is one of several that received funding last month from the United Kingdom's Cell Telecommunications and Health Research program, a group formed in 2001 to report on whether exposure to cell phone radiation adversely affects people's health. The group formed following the publication of a report that failed to find evidence of health risks, but noted that research to date was not expansive enough to conclude that no dangers exist.

Topics of past research include the potential link between brain tumors and cell phone use, effects of cell phone radiation on blood pressure, and the possible link between cancer incidence in early childhood and proximity to cell phone base stations.

As part of the current studies, researchers are examining the scientific basis of "electrical sensitivity," a collection of symptoms, such as headaches and fatigue, that some people believe is caused by exposure to wireless phones and cell phone towers.

Fox's research, an extension of a volunteer study that begun in January, will investigate whether some people are particularly sensitive to cell phone electromagnetic fields. In the first stage of the project, Fox collected survey results from 4,000 participants, about 6 percent of whom indicated some degree of symptoms, such as headaches or burning skin, that they attributed to electromagnetic fields.

The second part of the project, launched in November, involves testing people who identify themselves as hypersensitive to electromagnetic fields alongside people who are not, to establish whether cell phone base stations really are affecting health and well-being.

Another research group, based at King's College in London, is testing 120 people, half of whom consider themselves hypersensitive to cell phone emissions. James Rubin, a research fellow at King's College who is overseeing the project, is hoping to submit findings for publication by the end of next year, provided he finds enough volunteers.

"People who report being hypersensitive to mobiles are often understandably cautious about taking part in a study which involves exposure to a cell phone signal," he said. The study will examine whether cell phone signals cause such symptoms as headaches, nausea, dizziness and fatigue, and whether they affect the levels of certain hormones that are important in regulating metabolism.

Since cell phone adoption reached critical mass in the mid-1990s, research into the effects of long-term cell phone usage has also become more feasible. However, representatives of the cell phone industry say they have yet to see any findings that should give cell users reason for alarm.

"There is no conclusive evidence that wireless phones contribute to health risks, and the same goes for cell phone towers," said Erin McGee, a spokeswoman for the Cellular Telecommunications Industry Association.

The U.S. Food and Drug Administration takes a similar position on health risks caused by cell phone base stations.

"Measurements made near cellular and PCS base-station antennas mounted on towers have confirmed that ground-level exposures are typically thousands of times less than the exposure limits adopted by the (Federal Communications Commission)," the agency states on its website.

The FDA also maintains that there's no scientific evidence to link any health problems to cell phone use. On the other hand, the agency says there is no proof that they are absolutely safe.

The World Health Organization, meanwhile, expects to complete health-risk assessments in 2007 under its International EMF Project, which examines effects of exposure to electric and magnetic fields up to 300 GHz in frequency, which includes cellular-phone emissions.

Libby Kelly, executive director of the Council on Wireless Technology Impacts, an activist group favoring greater regulation of electromagnetic emissions, believes health agencies are understating the risks posed by wireless phones and towers.

She cited a study released in October by Sweden's Karolinska Institute, which found that 10 or more years of cell phone use increases the risk of acoustic neuroma -- a benign tumor on the auditory nerve. However, the study was conducted on analog cell phones that had been in use for more than a decade, and researchers said they could not say whether results would be similar after long-term use of digital phones.