If you are looking for a mask or two, look into the 3M N95 or P95 particulate respirator/dust masks. They're very compact and disposable, and affordable. The more advanced cartridge-based respirators are very inexpensive as well, but they take up a lot more bulk. A quick GTFO pack should be as compact and light as possible, IMO. The smaller dust masks like the 95 series are ideal for most uses, to include biological protection from many airborne pathogens like TB, norovirus and the common cold.