Why is my white shirt turning yellow?

This problem arises when white and pastel fabrics begin to yellow. When this happens, a little investigative work typically reveals a manufacturer defect in the optical or fluorescent whitening agent applied to the fabric. When this agent begins to break down as the result of exposure to light, atmospheric gases, or dry cleaning or washing solutions, yellowing results. The problem cannot be corrected and can only be prevented by the manufacturer using stable brighteners.