we bleed the lines for safety reasons in case of an uncontrolled fire as acetylene is a very volatile gas.Mixtures with air containing between 3% and 82% acetylene are explosive on ignition. The minimum ignition temperature is 335 °C. It burns at about 3300 degrees C in oxygen. Goggles should be minimum shade 5. Certain types of UV radiation can produce an injury to the surface and mucous membrane (conjunctiva) of the eye called "arc eye," "welders' eye" or "arc flash." These names are common names for "conjunctivitis" - an inflammation of the mucous membrane of the front of the eye. The symptoms include: pain - ranging from a mild feeling of pressure in the eyes to intense pain in severe instances, tearing and reddening of the eye and membranes around the eye, sensation of "sand in the eye" or abnormal sensitivity to light, and inability to look at light sources (photophobia). The amount of time required to cause these effects depends on several factors such as the intensity of the radiation, the distance from the welding arc, the angle at which the radiation enters the eye, and type of eye protection that the welder or bystander is using. However, exposure to just a few seconds of intense UV light can cause arc eye. These symptoms may not be felt until several hours after exposure. GMAW - gas metal arc welding Normally use shade 10 for electric arc welding Protects us mainly from UV.