Arsenic turns up in dyes and pigments through the 1800s, particular in the color green. It was also used in mixtures to protect wood from insects and rot, and that went on well into the 1900s. So if you are using reclaimed wood from that time, there is a decent chance of Arsenic exposure. The fun thing about Arsenic is that it accumulates over time so cumulative exposure can lead to health problems in much the same way as Lead poisoning slowly adds up. If they used those old mill blades to saw down a bunch of railroad ties or treated wood they would have a bit knocking about, but I don't know if that would be enough to lead to a positive test. My bet would be that you cut and sanded some tainted wood. I would look into your dust collection system or check your respirator, and if those show positive you are going to need a thorough swabbing of your shop.
