First of all, let me talk about the surface treatment of baking tray. According to your description, "the gray of metallic iron has spread around from the center of the fire, and the color is not as bright as before." If the original color of your baking tray is silvery white, the surface treatment should be aluminized; If the original color of the baking tray is dark gray-black, it is necessary to spray Teflon or spray high-temperature powder. Neither of these treatments is fireproof, but if it is enamel, there is definitely no problem. Some suggestions: although the enamel baking tray is small and expensive, it is the best choice for cleaning or baking food, and it is only a cheaper part chosen by domestic manufacturers to save costs.

Secondly, from the safety point of view, the general surface treatment should be safe and can directly contact with food, because the first purpose of baking tray is to catch slag (so many domestic factories also call baking tray a slag-catching tray) and oil (so it will catch fire), and the second purpose is to directly load food for barbecue, such as cakes, toast, rolls, etc.; If the surface comes into contact with food and the coating falls off, the dish can only be scrapped. If yours is the other way around, there is no problem if you don't touch food, and you can continue to use it. But if the coating falls off, it will rust quickly. Try to apply some non-toxic paste antirust agent, and it is best to buy an enamel baking tray of the same size.

Finally, the gas released by the fire (mainly oil fume) has no effect on the oven and food and will not be poisoned. If it blows impurities (coating dust), it should not be used.