When I first adopted Dexter I was using an inexpensive non-clumping litter ( I can't think of the name right now) that I could buy in the supermarket. After about a week, he began wheezing & having difficulty breathing. I called my vet & warned them that I was coming in with an emergency. After chest x-rays, blood work & an EKG...heart problems were ruled out & he was diagnosed & treated for asthma.
You are doing exactly what I did....a search for the cause.
I finally decided that his attack might have been caused by the dust from the litter he was using. I won't go into the details but I tried every brand
of litter the PetCo store near me carried. For one reason or another I had to return it. That is one good thing about a big pet store...they will take food or litter back if you can't use it...even if it has been opened. Finally, I settled on their store brand...PetGold Clumping Unsented. Dexter had gone almost almost 5 years without an attack.
PetGold is still dusty but it is odor free and not as dusty as the first one I used. I am not suggesting that you use PetGold. I am saying that you might need to go through several brands, if litter is the cause of the asthma, before you find one that works for you.
I would love to find a non-dusty, unscented litter that both my cats could use. Maybe you will get some answers here ... that we both can use.
You're a good cat Mama. I hope your furbaby never has to suffer an asthma attack again.