Which of the following is a cause of split ends in hair?

Split ends occur when the protective outer layer of the hair cuticle becomes damaged, leading to the fraying of the hair strand. Excessive blow drying is a significant contributing factor to this damage. It can strip the hair of its natural moisture, causing it to become dry and brittle. The intense heat from blow drying can weaken the hair structure, which often results in split ends.

In contrast, the other options tend to have neutral or positive effects on hair health. Humidity can affect the appearance and manageability of hair, but it does not inherently cause split ends. Using conditioner generally helps to moisturize and protect the hair, reducing the likelihood of split ends. Regular trims are actually recommended to maintain hair health, as they remove damaged ends and can prevent the progression of splits up the hair shaft.