While it is generally believed that white or gray hair caused by the natural aging process cannot be reversed back to the original hair color, there is some evidence that white hairs caused by other factors may be able to regain their original color in certain circumstances. Some research suggests that white hairs resulting from vitamin deficiencies, exposure to pollutants, physical or emotional stress, or other external influences can sometimes return to their natural color if the underlying cause is addressed. For example, studies have shown that providing a diet deficient in certain nutrients like copper can turn the fur of black animals white, and then restoring those nutrients can reverse the color change. Similarly, reducing high levels of stress has been linked to some graying hairs regaining their original pigmentation. However, the mechanisms behind this phenomenon are not fully understood, and the extent to which it can occur naturally is still unclear. Ultimately, while white hair from aging is permanent, white hairs caused by other factors may have the potential to darken again in some cases, though more research is needed to confirm the specifics.