One of the most elusive to treat the symptoms are cough. We usually count on at least twenty different types of cough.
Cough can have many underlying causes.
It may be that coughing during exertion, it could be asthma.
The cough may come up with an infection or an allergic response.
Some coughs at a certain type of climate, some when they eat a certain type of food. Etc.
It is important to have as much information as possible
One will worsen by soaking hay for some horses.
The idea is to solve the problem. That's when it starts to get complicated.
A horse that just coughs during humid weather is probably not bothered by dust. It's probably harder with haylage or soaked hay. I've been with horses who have been ideal weather forecasters.