It is an immune system overreaction causing inflammation causing then bronchial constriction and of more ramified aereal ducts reducing total air vol per breath

It is an immune system overreaction causing inflammation causing then bronchial constriction and of more ramified aereal ducts reducing total air vol per breath. Inmune causes are routinely tested to be of allergic origin for external allergens and a test panel is common to be done measuring reaction to agents such as dust mitels, pollen, animal fur. Often one cannot pin a single cause. Bad air quality (smog) or cold can be triggrers, and contradictorially unusual physical effort demands can trigger it reducing O2 availability just when more is what the situation demands. It's a self-immune problem in the end, not that well understood.