Really there is no safe level or distance for the non thermal effects. You should avoid the closer range higher intensity exposure anyway, to be sensible, and when you can. See Safe Levels and Safe use of wireless technology.
Yes. 5G covers a spectrum from 0.6Ghz to 6Ghz, and 24-90Ghz. Mobile Spectrums recently went up to 2.1Ghz, and are now expanding. WiFi uses 2.4 and 5Ghz, and soon 6Ghz.
Whilst frequencies have their own windows of effect the general trigger of adaptive stress is the nature of all radio frequency transmissions which is that they are polarised and pulse modulated.
In fact indoor WiFi and mobile devices expose you to as much if not more radiation than external masts (although city streets are now showing readings often over 6 V/m) A possibly safe level is only 0.05 V/m. See Safe Levels
Phones in the UK do not yet use mmWave 5G, but are marketed as ‘5G ready’. The new phones have 8 pairs of transmitting antennas in order to overcome 'body blocking' - when you get in the way ...
These New 5G networks might be operational in a year or two, unless moratoriums and public concerns calling for proof of safety and security are successful in delaying or stopping the networks.
There are many shielding devices available on the market, but the risk with putting them on the back of your phone is that they can encourage the phone to ‘try harder’. It is preferable to learn new habits and use Airplane Mode when in your pocket. There are too many studies on rectal cancers and infertility to gamble with this.
There are also 'harmonisers/balancers' that work in the non-physical information field to de-polarise the signal, whilst keeping the physical signal intact. There is debate about the direct evidence, though science may catch up ... Best to do your own research and try what feels right. We would still advocate physical habits to reduce exposure as well.
The safety concerns are not fundamentally about the frequencies used, it has been known for decades that microwave radiofrequency radiation (RFR) in general terms causes biological stress.
Primary research studies
There is extra concern about mmWave 5G since these frequencies can affect living organisms in different ways and come in conjunction with existing 3G/4G networks, which are needed to support the new mmWave 5G. This increases the overall levels of exposure.
mmWave 5G handsets will need to be more powerful and will have more transmitters in them, so until or unless there are official tests published to show that this doesn't cause harm then a precautionary approach would seem sensible.
Frequencies above natural UVA are classed as Ionising, which means that they can injure you when too intense. Ionising radiation has the ability to knock electrons off atoms. Non-ionising radiation however does not, and hence the dogma that it cannot do harm. This is disproven within the fields of electrobiology, magneto-biology, quantum electro biology, and the basic knowledge that we are electrical beings.
Wikipedia: the non-ionizing lower energies of the lower ultraviolet spectrum cannot ionize atoms, but can disrupt the inter-atomic bonds which form molecules, thereby breaking down molecules rather than atoms.
Our bodily electrical systems communicate at pico Volts (very small voltages), whereas EMR signals expose us to electrical fields up to 16 V/m in daily life. Background Wifi is often 1-4 V/m. This does have an effect.
Sunlight is naturally un-polarised and is not pulse modulated. Man-made EMR (electromagnetic radiation) however is de-polarised and pulse modulated, these qualities are not native or recognized as normal by living organisms and have been shown to cause a stress response. We have evolved in an environment where natural background EMR levels below the THz range (visible light) were practically zero.
For one example of how the effects of man-made pulsed EMR have been understood (and resolved) refer to this patent from 1991: Protection of living systems from adverse effects of electric, magnetic and electromagnetic fields: US patent 5544665A.