I would be leery of buying most surplus masks and filters, as they are just that surplus, either because they are no longer servicable or out of date in most cases. I would avoid anything that I could not get a FRESH, newly manufactured filter for. Finally, I would not count on interchanability of filters. In my experience, that is not the case.<br>gasmasks.com is a good site for new stuff. If your concern is solely bio hazard, you can get a mask with a HEPA/particle filter from someplace like Labsafety. <br>As a member of a WMD team sponsored by the USPHS, we use commercial, off the shelf stuff for NBC protection.<br><br>Jeffery S. Anderson, M.D.<br><br>