Traditional welding helmets have passive lenses – that is they have a fixed shade value. They are worn in the up position to prepare the welding material and then moved downwards immediately before work begins. While they are cost effective, repeatedly lifting and lowering the helmet is inefficient and if the helmet isn't lowered in time there is danger of arc flashes.
An auto-darkening lens does what the name suggests. In its inactive state it has a low shade which is relatively easy to see through. When sensors on the helmet sense an arc start, the lens darkens in a fraction of a second. The helmet stays in position before, during, and after the weld which has the potential to improve weld quality and ease the neck strain associated with snapping the helmet into place
With HSE now classifying all welding fume as a carcinogen, the emphasis on protecting operators with RPE has increased. The pointers above hold true for the head top, but what about the PAPR (powered air purying respirator). You should consider the weight of the PAPR unit and its ability to provide constant air flow, battery life in hours per day and recharge time, price of the filters and the protection level of the unit supplied