I think one Orbi covers 2000ft. There is package of 2 units (Main Router + Satellite) thus you may see a 4000ft coverage
These figures are usually under ideal conditions (e.g in lab) or according to US residential (wooden house). When it comes to Singapore, this coverage will drop significantly due to concrete walls, other wifi signals interference etc...
Does this pose a problem to your health? WiFi equipment have FCC or CE certifications that prevent excess strong power emission or they have the same 'strong signal' as other wifi routers you are using. Lastly, if it is sold via normal shops in Singapore, it must have IDA approval too.
Also Netgear is a US company, I do trust them have strict regulations on the power emission
Hope this answer your questions.