Why is my horse coughing?

My 8-year-old QH gelding has never really coughed before, but today he started it. He had the first spell when I was tacking him up, and then again when we were warming up. He seemed miserable so I quit riding him. He is in a huge field all day, and there is not a whole lot of dust. It has been extremely hot here lately but its never bothered him before. I wanted to here about different things before I called a vet and spent a lot of money if it could be something simple. Thanks.