Most of the major brands make a resparator that can fit under the welding hood. Removable Cartridges are available for a variety fumes/particulate
The most important part (and your not gonna like this) is that to choose the best and be assured that you are truly protected, you will need to be
fit-tested with your hood on going through the motions you will be going through while welding and inspecting.
Every hood and every face are different, having a good resparator seal can be tricky for welders, especially when working out of position.
For me this one works best.. Fits under a standard Huntsman and the air pickup is *behind* the welders back, which is just that much farther away from the plume.
http://www.websoft-solutions.net/North_Safety_respirator_half_face_respirator_p/bp1201.htm