I have a very small shop (200 sq. ft.). The first year in it, I used a detector and opened both doors and used a small box fan. This past summer I installed a small variable speed controlled gable fan. It has made a world of difference to shop temps while forging, or the summertime sun. A detector is a necessity, just like my fire extinguisher, forging or not.
Some years ago (in a different shop), I was coating a boat with a product that required "good ventilation". 10 minutes into the project I was literally crawling to the door, and the next thing I knew was someone slapping my face and dumping cold water on me. My wife had found me with just my head sticking out the door, dragged me outside and brought me around. I thought I had good ventilation.