When you want to know where you've been using GPS, you basically have two options: active tracking and passive tracking.
Passive tracking is what most users of this site are doing: an electronic devices stores the positions internally and these positions are recovered at a later time.
Active tracking generally implies the transmission of the current position to some kind of server in "real time". And that is most of the time done using the GSM/GPRS network.
What you may not know is that the way you are using these active tracking devices may not be safe for your health.
The FCC for instance has defined maximum electromagnetic fields for the user and any emitting device must have some kind of approval. This approval indicates what the device can be used for.
Because getting approval may imply important testing costs, it is possible to approve a product based on 'modular approval'. That means that as long as the final device is using emitters that have themselves undergone a specific type of testing and have corresponding approval, that final device is approved itself.
So there are several GPRS trackers on the market that do not have an FCC ID of their own, but that must mention the FCC IDs of the used modules on their packing.
Several of these consumer devices use the SIM340DZ which is in itself a fine module that received the following approval: FCC Approval for SIM340DZ.
That approval says:
The antenna(s) used for this transmitter must be installed to provide a separation distance of at least 20 cm from all persons and must not be co-located or operating in conjunction with any other antenna or transmitter. Installers and end-users must be provided with installation instructions and transmitter operating conditions for satisfying RF exposure compliance.
The first condition is that any person (you for instance) should be at more than 20cm of the antenna. If the device that you are holding in your hand is smaller than a globe with a radius of 20cm, that is not guaranteed. Several tracking devices using the SIM340(DZ) are smaller than that and hence you might be exposed to more RF radiation than the FCC considers to be safe unless the end device has a specific approval.
The second condition is that this rule is only true if the module is not co-located with another antenna. There again, in the case of a tracker a second antenna (besides the GSM/GPRS antenna) is present. This second antenna might reinforce the emission of the original antenna which is why this second condition exists.
I've been surprised by the number of devices that use modular approval while in my humble opinion failing to respect the module's FCC conditions. Of course, it is not because the formal documentation is missing that the device is a danger, but there is just no proof of it.
Some devices are approved for belt-wear (1-2cm away from the body) or - better - for uncontrolled environments. Mobile phones have similar restrictions (if found article1 and article2 explaining belts for phones).
When talking to the representatives of the manufacturers they claim that there is no problem with their devices - the FCC are in order. I agree, but the way the device is to be used or documented is not in line with the FCC approvals.
Luckilly, there are some GPRS Trackers that have the right FCC approval(s), but that does not put the owners of the other GPRS Tracking Devices in safety.
So make sure that you are using a safe device or use it in a safe way. A distance of 1-2 cm for a module is quickly respected because of the device's casing, but it is another ball game for 20cm.
If you're a manufacturer and your device is safe in this respect, leave a note - I am interested!