For people operating on tight budgets, reusing cooking oil is less a matter of preference than a way of cutting costs. But food scientists and toxicologists warn that repeatedly heating cooking oil causes chemical changes that can potentially increase health risks with prolonged exposure. Experts caution, however, that no cooking oil should be considered immune to breakdown when it is repeatedly exposed to high temperatures. Dieticians have also cautioned that the health qualities of a cooking oil cannot be considered separately from how it is used. As cooking oil becomes more expensive, the question is no longer just how many times oil should be reused.