From University of Minnesota Equine Extension Program:
Poor air quality can be harmful not only to us but to our horses, too. Smoke from wildfires can irritate a horse’s respiratory system and cause fatigue, coughing, difficulty breathing, etc.
You can monitor local air quality at www.airnow.gov. During periods of poor air quality, you can protect your horse’s health by:
Limiting or avoiding exercise.
Providing free access to fresh water.
Controlling dust such as watering dry lots and soaking or feeding dust-free hay.
Allowing time for recovery: airway damage from wildfire smoke takes 4 to 6 weeks to heal after air quality returns to normal.
Consult your vet if you notice signs of smoke irritation.
For more detailed information, view the article from UC Davis Weill School of Veterinary Medicine:
https://ceh.vetmed.ucdavis.edu/health-topics/wildfire-smoke-and-horses