Will there still be harmful substances left after plastic is cooled at high temperature?

When I was cooking today, I accidentally put the lunch box next to the gas stove. It was close to the fire. The high temperature led to a piece of the box cover, which melted a little and deformed. It took about two minutes. I took it away immediately. Will there be harmful substances left after the cover cools down? Can I continue to use it? If the cover can't be used, can the glass box still be used?