Paper is made from types of wood. Wood mainly contains two polymers: cellulose and lignin. It is the lignin that causes paper to turn yellow. Lignin has a naturally dark color, which is what makes wood hard and stiff. Lignin undergoes oxidation when exposed to sunlight and air, and it breaks down into many phenolic acids that are yellow in color.

Nowadays, papers are made acid-free, and to achieve this, lignin is removed from the wood. This makes modern paper last longer, as compared to older types of paper made with lignin. An additional chemical process is required for removing the lignin from wood and this process is more expensive than making paper with lignin still in it.

This is the reason why newspapers turn yellow over time, as they are produced in bulk and more importance is placed on their economical cost of printing, rather than the length of time they remain in good condition.