Permafrost_ATPL makes a very good point, even a cursory glance at this study throws up a lot of potential problems.
Public health panorama is an OK journal but is quite low-ranking. It's not one where I'd expect to see ground breaking research - you'd send that somewhere better. If you see ground breaking research in a journal like this then it should set alarm bells ringing. It's also worth pointing out that pretty much the same research (at least, it contains the same studies without any changes) has been published before. Maybe I miss something, but I don't see anything new in this 'new' paper.
The studies also seem to be expecting a result and then going out to prove it. As Permafrost_ATPL says, there's absolutely no control studies done, no monitoring of other groups. Avoiding that bias is one of the first things you are taught as a scientist. The entire paper is based around positive selection bias, as a researcher it really makes me cringe to see stuff like that.
I've said it before and I'll say it again: This type of pseudo-scientific study undermines the actual evidence for health problems associated with cabin air. It's easy to dismiss other evidence when you also see this type of rubbish being published.
What we need is proper science, proper evidence and an in-depth study to understand both the prevalence of fume events and the effect that these have upon crew. What we have is a bunch of people who appear to come up with the conclusion first and then seek evidence to justify it. This is the worst kind of bad science.