This article wants a short answer to these questions: 

1) Is it safe to be exposed to wireless signal radiations?

2) Is my WIFI router safe at home?

3) Is electromagnetic signals are safe?

or similar questions...

I want to directly go to the point, people ask all the time: is my WIFI modem/router cause cancer? is wireless signal safe?

To be honest, as a person who works in this environment and spent many years under wireless signals, I asked this question every day!

I am summarizing my findings from [1], 

Right after 2000, higher frequency cheap devices introduced to the market, for those who are not familiar with the case, the higher the frequency of signal goes, repetition of signal increases!

see below:

the IEEE International Committee on Electromagnetic Safety states that for frequencies from 3 kHz to 300 GHz: “a review of the extensive literature on radio-frequency (RF) biological effects, consisting of well over 1,300 primary peer-reviewed publications published as early as 1950, reveals no adverse health effects that are not thermally related”[1]

So just a thermal heating problem?!

somewhere else they say: 

“no reproducible low-level (nonthermal) the effect that would occur even under extreme environmental exposures…harmful effects are and will be due to excessive absorption of energy, resulting in heating that can result in a detrimentally elevated temperature,”

So the issue is power!

Signal power can be expressed in Watts. Radiation is the power per square centimeter or Watt/cmxcm.

for example, airport security scanners (1microWatt/cmxcm) reported being harmless.

but what about higher frequencies such as mmwave?!

what is mmwave concept? 

Millimeter waves, also known as extremely high frequency (EHF), is a band of radio frequencies that is well suited for high bandwidth networks e.g. 5G. Transmission on frequencies between 30 GHz and 300 GHz or wavelength of 1cm to 0.1mm. 

Question: wait, Why ultraviolet, X-ray, and gamma radiation is bad but wireless mm-wave signals are not?

Long Answer: electromagnetic waves can be presented as "Photon" particle waves! each photon has a energy which can be calculated as,
E=hf, where h is Plank's constant and f is the frequency. 

and energy of mmwaves is about 0.1 to 1.32 meV but those harmful waves are about 12eV! 1200 times more! this energy will "Ionizing" the atoms which displacement of electrons cause eventually cancer or etc. 

Short Answer: wireless mmwave signals are not as strong as X-RAY, GAMMA or ultraviolet (the best-known source is SUN!!!) 

Conclusion: Heating is the major biological effect caused by the absorption of electromagnetic mmWave energy by tissues, cells, and biological fluids.

Regulation: Communication systems must comply with government exposure guidelines, such as [2] and [3], to prevent any adverse health effects related to these thermal effects.

100% safe? Nothing is 100% in this world! more studies should take place ofcourse but based on math and our knowledge the power is much lower than sunlight!!

My conclusion (Do not rely just to think about it! :) ) : Since FCC regulation won't allow devices to transmit from a specific level, and based on the researches it is safe, so no worries! but I think in the future everyone should have a spectrum search device to check if there is radiation in his home, work, or other environments.