The amount of exposure is difficult to determine. Exposure to forces produced by external sources is generally constant, but the structure of a building determines how much of the final field strength can be found inside the building. EMFs produced inside a home can widely vary depending upon the wiring system and appliance usage. These variables cloud efforts to determine the relative importance of local power lines as opposed to the use of appliances or to internal house wiring. In most cases, the findings are difficult to trust because of the difficulty in controlling the amount of exposure and other variables, including socioeconomic status, nutrition, lifestyles, genetics, occupation, and exposure to other cancer-causing factors.
Many study designs lack validity, which causes further doubt about their value.
Given these limitations and shortcomings, the research nevertheless does suggest the need for further investigation of the association between EMFs and some cancers. These include childhood brain cancer and female breast cancer. Occupational concerns related to EMFs include adult brain cancer, adult leukemia, and male breast cancer. The evidence for adult central nervous system cancers, male breast cancer, and leukemia is also very weak. Many respected scientists critique the studies as being flawed.