In what cases is Ethical Committee approval not necessary for a study?
The exact regulations differ per country, but prospective studies with human subjects almost always need approval.

Since you tagged your question with "Netherlands", the relevant law is the WMO = "Wet medisch-wetenschappelijk onderzoek met mensen". Any research that falls under this law requires approval.


  Research falls under the WMO if the following criteria are met:
  
  
  It concerns medical/scientific research and
  Participants are subject to procedures or are required to follow rules of behaviour
  


There is a lot of information on the website of the Central Committee on Research Involving Human Subjects (CCMO).

You can usually also apply for a waiver from the IRB (at least in the Netherlands). You submit a request with a description of your intended research and the IRB will review your plans to determine whether your research does fall under the WMO and needs approval, or whether this requirement can be waived.

From the description of your plans, it seems likely that you need approval. Not having approval would not only mean that journals won't accept your paper, it would also mean that you'd be doing something illegal.