Masks need to be worn in the wastes to protect yourself from any potential contaminants and spores (if you are in proximity toxic jungles). If you are investigating any abandoned buildings or man made structures, a mask will protect you from dust, mold spores and airborne viruses from vermin or other creatures that may be lurking there. While it's not a design requirement for a scavenger, you can expect most nomads and scavengers to wear masks for safety and anonymity.