I am not a researcher on the subject, and the material I have heard is primarily in Swedish. <a href="http:&#x2F;&#x2F;www.filurum.se&#x2F;forskning&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.filurum.se&#x2F;forskning&#x2F;</a> list research papers if you are looking for more precise numbers rather than anecdotes, and the site list a few articles in English at the bottom.