I have used many types od respirators over the years, and a full face Supplied Air Respirator is the way to go. It hurts a little when buying it, but the pain goes away when you start wearing it
In response to your question about the half-face unit. Long wearing of these can cause a headache for several reasons:
One is that smelling any chemical for hours will cause a headache. The rubber smells. Maybe not bad, but it has a volatile content that is breathed in. Six hours may be enough to give you "painters headache" ( from breathing the volatiles of paint).
The second reason is that you have to breathe harder when wearing a respirator. It isn't a lot harder, but it may slowly change your blood chemistry after many continuous hours. Additionally, you are rebreathing a much higher percentage of expelled breath than normal. This is higher in CO2, than a room filled with fresh air.Changes in oxygen and increased CO2 levels level will cause a headache.
Filters need to be changed often. You may not think they are blocked, but as they are used, they become less able to pass air through them as well as the charcoal becomes saturated with contaminants and no longer filter gasses. The industry standards vary, but in high dust situations, they are changed every 40 hours. I think every 100 hours is a good idea. Most knifemakers do one of two things, the change them yearly, or never. If you get three hours shop time with the respirator on a week, you should change it yearly. If you get six hours in a day, you need to track your use and change the filters at least monthly.
Third is the weight and position issue. The half face mask may make you lower your head more to see over the mask, thus causing positional headache. The added weight will acerbate this.
Last, and most likely in your case, muscle tension can cause a real bad headache. The strap being tight across your head and neck, as well as the inability to move your jaw for hours can lead to a tension headache.
One other possibility is that you are sensitive to mold and mildew. Most people are to some degree. This is a primordial inherited trait to keep us alive and out of places where you will get sick. The most common form of reaction to this trigger is a headache.
The mask is filled with moisture from your breath and sweat, dead skin cells, some of the bratwurst you had for lunch, maybe a bit of beer, and grinding dust on the outside. This can get a bit musty at the least, and moldy at the worst. That mustiness can trigger the histamine reaction that makes us want to leave an area with the trigger. Normally, you leave a musty room pretty soon after entering. If the musty room is strapped to your face for six hours, it could quite likely cause a headache. A respirator should be washed well with soapy hot water weekly, and dried fully on open air before being put away. It is best not to store it in a sealed bag.
The cumulative effect of all these may combine to cause your headache, so dealing with as many of them as possible is a good idea.