I followed the issue quite closely here in Italy, and I can assure that most of the limits have been set for political reasons. The reasoning is more or less this: "ICNIRP identifies some thresholds, and sets limits that are 50 times below (for population)? Well, we can do better, we can drop it by other 3 dBs!". This at a national level. Then comes the local authority and says: "national limits? We can do better! Shove another 3 dBs!". There you go, without any real connection with the studies.

The rationale for this lowering is simple: "brief" studies can only apply to short-term effects (heating). But what if there are more subtle, long-term effects? Here come lower thresholds, but note that this issue can be addressed only with statistical studies over reasonable timespans. So the "I can do better" lowering race can be brought to whatever level you like: it's something that we don't know much about, so a lower level makes a better politician.

One interesting thing we had to reflect upon is that science will never, ever be able to say that radiations, at any level, aren't harmful. As a matter of fact, it cannot say this for anything we are aware of: we're only allowed to demonstrate that something can be harmful, not that it cannot be. So, it makes me laugh when they ask me "do EM fields harm?" - the only truthful answer being "we can never be sure they don't harm, evidence so far shows this and that, but it could be discovered something different in the future". Which sounds much like "we don't have a clue, just avoid them". So you have massive campaigns against "antennas" on the buildings, where a big chunk of the very same people is willing to put an EM emitter 3cm from the brain. Go figure!

Last, but not least, there is the issue of the non-disclosure of results, for political or economical reasons. Many research centers are privately funded, and are less likely to spread results that could harm "the industry". This is really sad, and eventually backfires to "the industry" itself.