There has been on-going debate and allegations in the media that cell phones, among other mobile devices, transmit radiation that cause cancer. Considering the number of cell phone users, there has been an increase in concerns about brain cancer among other health hazards that may be induced by cell phone usage. The issue was first brought to the public in 1993, when a Florida man appeared on a popular TV talk show claiming that his wife's brian cancer had been caused by radiofrequency (RF) radiation from her cell phone.  The resulting lawsuit was dismissed in 1995 because of the lack of scientific evidence and medical support for the claim. However, the issue was brought to light to the public, and people are concerned about it more than ever.
One of the current issues with research is that there are no good way to measure the effect of FF-radiation. First, it is impossible to conduct an experiment with human subjects like with mouse. Second, Existing exposure limits by the IEEE/ANSI standard and the thermal hazard keeps the population exposures to RF-radiation relatively low. As a result, it is unlikely to find a group within the population that has exposurues to high doses. Currently, even occupational exposures are either below the limits or intermittently slightly higher than the limit. Third, although the current method of measuring RF-radiation levels are sophisticated, there are no such sophisticated method to monitor individual exporusres or examine the exposures retrospectively. Despite the limitations, there are still some analysis that can be done from existing epidemiological studies. 
Studies for radar lab workers show that there was no evidence of increased risk with higher exposures of RF-radiation.  Furthermore, studies on U.S. Navy personnel who were likely to have encountered RF radiation in their occupations were done. The follow-up was roughly 20 years. The analysis identified a subset of the personnel who were high exposure groups even within the exposed occupation. The study found that the mortality ratio were indeed higher for cancer in general and for cancers of the lymphatic and hematopoietic system and respiratory tract. However, the only increase that was statistically significant was the increase in respiratory tract cancer. No exposure-respond trends were apparent either by their occupation or the identified hazard amount. This study was unable to provide a link between exposure and cancer. [2,3] A survey of various epidemiological studies of RF-radiation, including U.S. Air Force, Norwegian electrical workers, etc, all reveal no evidence that exposure to RF-radiation a the cell phone level would increase the likelihood of cancer or mortality. 
On the other hand, there has been studies on a large-scale specifically on cell phone exposure. A nationwide cohort study in Denmark reveals that there was no evidence for higher rates of cancer from exposure to cell phones. Although there was a decreased risk estimate for total cancers among men, it was due to the difference in social class: cell phones were expensive before 1992 and their users are also less likely to smoke cigarettes. The study also showed that heavy users of cell phones also find it more difficult to smoke while talking on the phone. Furthermore, among the younger generation with limited budget, those who choose cell phones are less likely purchase cigarrettes. Interestingly, the statistical correlation between the use of cell phones and the decrease in lung cancer was higher than that of the user of cell phone and increase in other cancers. 
Despite the widely-spread myth that cell phone radiation may increase the likelihood of cancer, most studies suggest that there is not a strong enough evidence to show the correlation between the two. That said, most studies are limited by the subjects they measure and observe. Furthermore, the absense of such evidence does not indicate that cell phone usage has no harm to the human health --it just means that it has not been shown yet. The use of cell phones are also a fairly recent phenomenon. In other words, it is possible that in 30 years after tracing people who have been using cell phones for 70 years that there is a direct correlation between cell phone usage to cancer. In summary, the cautious approach would be to try to limit the amount of time cell phone is close to the human body, such as putting it in a bag or a purse and using headphones instead of holding the cell phone directly next to the ear.
© Kenny Kao. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. E. Moulder et al, "Cell Phones and Cancer: What is the Evidence for a Connection?" Radiat. Res. 151, 513 (1999).
 F. C. Garland et al "Non-Hodgkin's Lymphomas in U.S. Navy Personnel" Arch. Environ. Health 43 425 (1988).
 F. C. Garland et al "Incidence of Leukemia in Occupations With Potential Electromagnetic Field Exposure in the United States Navy Personnel," Am. J. Epidemiol. 132 293 (1990).
 C. Johansen et al, "Cellular Telephones and Cancer - a Nationwide Cohort Study in Denmark," J. Natl. Cancer Inst. 93 203 (2001).