Welding is a risky hobby that calls for the right safety gear, such as a helmet. It is not advised to weld without a helmet because doing so could seriously injure your face and eyes.
Your eyes are shielded by a welding helmet from flying sparks and debris, as well as from the damaging UV rays released by the welding arc. You run the risk of burns, eye injuries, and even blindness if you don't wear a helmet. Furthermore, skin cancer is a risk that increases with exposure to UV radiation.
Overall, welding without a helmet is never acceptable. When welding, put your safety first and make sure you're wearing the right gear.