The filter elements in many world war 2 gas masks contain asbestos. They may be a nice thing to own as a curiosity but please do not wear one. The chances of inhaling asbestos fibre could be very high, the filter element is likely to be in very poor internal condition. Those used for demonstrations such as in living museums have usually been modified for safe use. Do not attempt to dismantle the filter to carry out such a modification yourself!!
You have been warned.