The meters usually use the ZigBee, which is encrypted, low bit rate, low power (supposed to operate from a single use Li battery for years even with rather cheap radio design) protocol working in the license-free 2.4GHz band. And for that band, the legal maximum limit for any license free use is 100mW ERP (the real maximum transmitter power is adjusted according to the gain/losses of the used antenna), but in most cases the signal quality management commands for way less (to save power, as well as radio band space). That is way less than the mobile phones or so. The thing is, the low bit-rate (around 1kbps) allows to reach rather great distances (way longer than the broadband WiFi specification expect with the same 100mW ERP limit).
So even 50 meters, each transmitting each 10 minutes, won't cause any stronger RF energy background than the radio broadcasting and mainly the GSM/3G/4G/... networks.

Some meters are using the cellular phone network. Over here water meters are mostly smart meters ans there is a reception unit installed in some places. The electricity meters are mostly mechanical, but i never seen any special reading equipment used for the few smart units (they are too far inbetween to justify the effort to install it anyway) so i'd guess that they use the cell phone network

The cell phone network is too much energy hungry for the water meters or similar devices. It does offer way greater bandwidth, but that is quite useless with the applications like smart meters.
The ZigBee (and similar protocols; there are few more) low bitrate allows range in km with even the very limited power (it has modes for way below 1kbps, just to get the communication distance). Plus they do not need to communicate more often than one short packet a month or so...
And even when there is a GSM relay box somewhere, it won't transmit more than that once a month either, so way less than your pocket phone...

Smart meters that communicate over the cell phone network do exist. Its a matter of where they are used. From the perspective of the installing company they are convenient, as they dont have to invest in the installation of any relay units

The communication frequency is what it was configured to be. In fact it does not have to be any more frequent than the visits from the meter man with the mechanical meter, but in reality some exeutive may order it to be as frequently as every few minutes, with who knows what considerations in mind...