If they are on paper, and haven't been under glass, you're looking at two things - the paper will have faded and so look 'dirty' where it has been exposed to light and air, and the paper will also have absorbed chemicals such as soot particles, nicotine and other pollutants from the air its been exposed to.
Fading can't be reversed. Removing the dirt can be attempted, but there's a danger you might also remove the pigment - the paint or print colour.
All told, unless you are completely attached to these paintings I'd save your efforts and buy some nice new art.