Imac 24" Late 2009

Hi ALL,

I have a iMac Late 2009 24" iMac Intel Core-2 Du/2.66 MHz-640GB.

Sinds a view months I see like shadows from the upper left and right corners bending down to halfway the middle of my screen.

But now I notice also the same shadow coming from the upper site in the middle down.

Does anyone knows what this is?

Could it be that this is dirt behind the glass of the screen?

Could it be nicotine smoke behind the glass of the screen, my wife and I are both smokers.

IF, this is so ... how can I clean it??

Thanks in advance ...

Dimaxum