And an even dirtier longer used filter would be even worse. It means nothing really. A filter would never have less or equal to background because a filter collects dust particles and other radioactive nuclei carried by the atmosphere that finally settle in the filter. The more you use it without cleaning the higher the concentration of radioactive material it collects. Its absolutely logical, especially after an event like Japan's but even without due to prior events for decades.
Try this for a real scientific analysis without the conspiracy bs.
http://www.nuc.berkeley.edu/UCBAirSampling
If you tested the vacuum cleaner filter or the trash it collects in the bag you would also detect higher levels than the background! Hello! Anybody home? It doesnt mean that the atmosphere out there is 5 times more radioactive than it was 1 year ago today. It just means that whatever the air has the filter collects additively over time until its concentration becomes large enough to be seen as a potent signal.
The only objective analysis possible using filters would require to have tested one used for the same amount of time in the same location 5 years ago or 2 years ago and compare the 2. Even then a larger measurement on the second filter would still not imply a disaster, only hint of a larger concentration of isotopes in the atmosphere the second time around. To make any argument one simply needs to collect air samples, water samples, milk, soil etc and see what we have there and compare with past values years ago. Until one does that, filters will be unable to tell you anything unless you have recorded properly how they were used and can compare with past ones. They simply reflect with their high readings the fact that radioactive isotopes in the air are found in a filter at much higher concentration (density). Its not just radiation, if you checked other chemicals they would be higher too.
Think of it like that. You have a water filter that clears your water to drink and after a year you open it and it has deposits of salts in it. Well of course those deposits are far denser than they would be in say 1 liter of water. They are the accumulated result of thousands of liters over time. The more you use it the higher the measurements too. It wont mean your water recently is bad , it means that the filter has accumulated material for a long time. Now bring me a filter that has double the concentration after 1 year of usage than it had say 2 years ago (again used for a year) and it will mean something about the relative increase in isotopes from past. They still can however be not a concern in general. Many elements are found after last spring that are many times over their historical values locally. But why is that a big deal if say something is 1 part in a billion and now its say 5 but its total contribution to your body is still less over a year than 10 min of flight. Thats why you measure absolute contribution in samples rather than ratios of past concentrations of particular rare isotopes. That last one can be many times larger than 1 without the absolute contribution to total background radiation to even increase it by 0.01%.