You never know when set notation is going to pop up. Usually, you'll see it when you learn about solving inequalities, because for some reason saying x < 3 isn't good enough, so instead they'll want you to phrase the answer as the solution set is { x | x is a real number and x < 3 }.
If the skin really looks like the color of ash, a deeper more persistent gray or gray-blue color, this could potentially be due to a side effect of certain medications, with or without interaction from sunlight.
Tooth decay is often identified as the cause of graying teeth, and Cigna advises that dealing with the problem promptly can prevent painful symptoms and infection. As teeth age, the combination of staining from foods and drinks and thinning enamel can also create a gray hue on the teeth, according to More magazine.