Safety advice for best EMF detector and body shielding for UHF RFID reader
From the Wikipedia dBm article, there is a table with some examples of devices with different power levels:


  33 dBm: 
  
  
  Maximum output from a UMTS/3G mobile phone (Power class 1 mobiles)
  Maximum output from a GSM850/900 mobile phone
  
  
  30 dBm:
  
  
  Typical RF leakage from a microwave oven
  DCS or GSM 1,800/1,900 MHz mobile phone. [...]
  
  
  27 dBm:
  
  
  Typical cellular phone transmission power 
  


If cell phones are safe at these power levels when kept in pockets, I would not be worried about having your 20-30 dBm transmitter sitting on a desk nearby.