Manifests as: unprinted areas of the document or image turning yellow
Primarily caused by: heat, light, pollutants, or poor quality storage and framing materials

Yellowing is the discoloration of a print’s paper, making it appear more yellow and less bright, usually with an apparent loss of contrast.  Yellowing is most noticeable in non-printed or light areas of the print; however yellowing in an image area can also cause the image to shift in hue (e.g. yellowing can make a blue sky appear greenish). 

Yellowing is a natural aging phenomenon and most papers naturally discolor over time.  The rate of change decreases with decreases in the temperature of the storage environment, which is why cool or cold temperature storage is recommended for chemically unstable papers such as those high in lignin. Depending on their composition, some papers will yellow faster than others.  Yellowing of most digital prints by heat is usually very slow, unless recycled or groundwood containing papers were used. Exposure to light, pollutants, and poor quality storage and framing materials, however, can dramatically accelerate the damage, resulting in a significant yellowing, loss of image contrast, and an overall reduction in the quality of the print.

Yellowing - Original
Original Image

Yellowing - Yellowed Image
Yellowed Image

Documents are primarily text, therefore yellowing of the paper in a document can be considerably less objectionable than with images.  Yellowing of documents does not usually impede readability, but does affect the aesthetic quality of the object. However, severe yellowing can also be a leading indicator of other types of deterioration such as paper embrittlement.

The image below shows yellowing of a text-based document.

Yellowing - Original
Original Document

Yellowing - Yellowed Document
Yellowed Document

Another cause of yellowing is deterioration of the optical brightening agents (OBAs) sometimes added to paper during manufacture.  OBAs are dyes that absorb UV energy and reemit the energy in the blue portion of visible spectrum. This makes the paper appear brighter and bluer than it would otherwise. During long-term exposure to light and pollutants, the optical brightening agents break down.  When this occurs, the increased brightness and blueness is lost and the paper appears yellower.