I too have asthma, though not as bad as yours as I no longer use my inhalers. Though I find that there hasn't been much of a problem with it, though i too have quite a mucous problem that does plague me from time to time, so if you find any way of improving that I'd like to know

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I just know what my triggers are, stress is one of them, so I take the diving relatively slowly in terms of progression. I try not to over step my limits.
So if you keep to one rule of freediving, it would have to be know oneself! From there you can progress with relative safety. If your feeling tired or wound up then put the target back a bit.