Lots of good advice here, especially the simple paper bag.
BTW some individuals are sensitive to the byproducts of muscle exertion, namely sodium lactate. Thus, after exertion such as running, biking, stairs, sex, kickboxing, an attack will ensue that day or the next. It helps to rehydrate completely after intense physical activity.
Also, mitral valve prolapse can bring on anxiety attacks. This can be temporary or ongoing.
(PhD in anxiety disorders...)hg