Just because you're covering your mouth and nose with a mask of some sort doesn't necessarily mean you have all the protection you need. Some respirators are designed to filter out particulates, while others will also protect you from harmful fumes. No single respirator will protect you from all potential threats, though.
When evaluating a job, the materials and tools you're using will usually provide information about the right respirator to buy. Check all the reference documentation before you start a project, and add the right protection to your shopping list.