I have been told a mask is essential for visiting Santiagito in Guatemala.
Volcanic gases can be very very dangerous.
You should not only make sure you have an adequate gas mask but you must not go into gaseous areas alone. If it is a new area to you then you really have to go with people with experience on that particular volcano. The gas mask I use came from an industrial supply company here in Honolulu. You will have to make sure you know what types of gases you are likely to encounter and get they type of filter appropriate for those gases. Other volcanoes produce other gases so find out first.