When is wearing a mask most beneficial to a healthy respiratory system? when the day is clear with good air quality when cleaning the kitchen when it is raining when sweeping a dusty garage

Ihave a really high metabolism and only weigh 70 pounds (13 almost 14). people always ask me two questions: do you ever eat and how do you eat so much and stay so skinny. i am worried that a high metabolism wont me forever. what are some safe steps i can take to maintain being fit. (also, i already work out i am a dacer so i get cardio a lot)