The only effective frequencies are 10Hz and below and above 50Hz are harmful.
One book on PEMF makes this argument based on their belief that the resonant frequency of the earth is 10Hz.
Because disease breaks down into two basic categories, so does the frequency range in order to deal with them most effectively. There are two types of PEMF sessions; frequency ranges that decimate pathogens (disease arising from outside the body) and frequency ranges that signal our cells to accomplish tasks they would naturally do but help them do it better or faster (disease arising from within the body).
So, when choosing a PEMF system, the first and foremost consideration is the frequency range the system is capable of producing. Since there are no pathogen decimating frequencies below 50hz, it's important to choose a machine that has a much higher range if this is the purpose you are getting a machine. Even if you currently only need the cell signaling frequencies (generally below 50hz), you should consider the use you might need it for in the future, should you acquire a pathogen you need to deal with. Plus there are general wellness (non-pathogen) frequencies well above 50hz as well.
In response to those machine makers who say any frequency above 50hz is dangerous and potentially harmful to the body, we have this to say...
If that is true, then why did the FDA approve PEMF for bone healing at 76.6hz, and where are the properly done studies and/or research papers that back this claim of harmful frequencies up? So the real question is, do they make machines that only go up to 50hz because above that is harmful, or do they make the claim because their machines will not produce frequencies above 50hz?