iPhone 5s Dust under camera cover

Hi!

I bought my iPhone 5s in October 2014 in Slovenia. In about 2-3 months after purchase I started to notice a lot of white dust under my main camera cover. At first the dust didn't have noticable affect on the images I took but when the dust almost completely covered the camera the images got worse - grainy, really bright photos and when using flash almost completly white. My front camera was making better pictures at the time. My friends at work or family members at home have iPhones too and no one seems to have this problem, so I expected the dust is not due to the enviroment I live/work in. I took the phone to the reseller and asked for a replacement, because clearly this was not my fault and the dust should not have come under the camera cover in the first place. Official service rejected my replacement request, saying that the dust was caused by my usage and not a factory fault. They instead cleaned the camera of the dust. I was pretty ****** at their decision. If usage was the case, everyone else I know (who owns an iPhone) would have the same problems. But at least after service my camera was clean and my photos were great again. Now about 3 months after service the dust is again covering my main camera. The fault my phone has was clearly not repaired and I don't want to have my phone repaired every 3 months.. what will I do when my warranty expires?

Before I file any complaints against our official service I want to get a second, third or fourth opinion on this matter. Should my iPhone be replaced when they first got it? What do you think is the cause of this dust?

Attached are images of the camera dust, and the quality difference in photos. One is taken with the main (back) camera and one is taken with the front camera (pictures were taken the same day and time).

Thank you for your answers.