As you know, infections (such as colds, flu etc) can exacerbate or trigger Asthma.
Understand that the way many infections intitially increase to a tipping point where they then give rise to symptoms is by breeding in the nasal cavities.
I purchased an inexpensive device for jetting water upwards into my nasal cavities and have used it for about six weeks. I choose to fill it with a very weak solution of salt water, made tepid by heating in a jug for one minute in the microwave.
The Device I got from Amazon holds about a pint of water, and I spend about two to three minutes flushing half a pint of water up into each nostril daily.
I also suffer from Sinus, Nasal Polyps and Post Nasal Drip. One of the related issues I have is frequent loss of sense of smell.
It seems that the Nasal Flushing is helping alongside the Steroid Nasal Spray I use. Especially if using the Steroid Nasal Spray about 15 Minutes after Nasal Flushing as there is then less Nasal Gunk Blockage for the Steroid Nasal Spray to get through.
I suspect that Nasal Flushing is also useful for ridding my Nasal Passages of New Infection before the Infections gain a proper hold, thereby helping to stop some New Infections before they have bred to a level where they are able to make Asthma worse or trigger a severe attack.
There are many brands selling near identical (if not totally identical) Devices of the kind I've referred to, on Sites like Amazon. The one I got is called Waterpulse (or something like that) and cost about £10 including postage) but it probably does not matter which device you use - although I'd suggest one that can hold about a pint, as one or two squirts up the nose seems to have little effect, whereas half a pint up each nostril - for me at least - provides some benefit.
The Hospital I attended for Sinus and Nasal Polyp Advice recommended Nasal Flushing but it was my decision to do it with a weak salt solution, and to use a Device that holds a lot more water than the plastic syringe provided by the Hospital.
As always, get professional medical advice before trying out stuff like this. It seems to be helping me and often my sense of smell returns for a couple of days as an added benefit. Having a clear nose means I can breathe easier too which - in turn - means I get a better nights sleep because I'm not forced to breath through my mouth.
Recommend you consider if it might help you too.