Is it okay to solder without a mask?

For occasional hobby soldering in a well-ventilated room — an open window, a desk fan moving air away from your face, or working near an open door — most people don't wear a dedicated respirator mask, and it isn't strictly mandatory for light, infrequent use. Rosin flux gives off a visible smoke when it burns, and that smoke is a genuine respiratory irritant; the risk scales with how much soldering you do and how enclosed the space is, not with any single joint.

Where it becomes worth taking seriously is frequent or prolonged soldering in a small, poorly ventilated room — rosin-based fume exposure has recognised links to occupational asthma in people who solder for a living without extraction. If you solder regularly, a small benchtop fume extractor pulling smoke away from your breathing zone is a better long-term solution than a mask, since it also protects your eyes and clears the air in the room rather than just filtering what you personally breathe.

See do I need a fume extractor with a soldering station for the fuller picture, and should you wear gloves when soldering for the rest of basic soldering PPE.