When I was in high school, the physical education teacher would make us run a lap around the baseball and football fields before starting class. Most of the time, I came in last. I could feel my throat closing up and I couldn't get enough breath to keep running comfortably. Some of the other students made fun of me because they thought I was completely out of shape.
I later looked up all the causes of inflammation in the airway, and discovered there were several different asthma types, including "exercise-induced asthma". When I was running in the open field, there were all sorts of weeds and grass clippings releasing pollen into the air. Once the airway inflammation started, I couldn't do anything to stop it.
If I ran laps indoor, I was fine. I told the instructor about my problems with asthma and hay fever, and he let me do my running warm-up inside the gym from then on. It's a scary feeling when you can't get enough breath and it feels like your throat is going to close up completely.