Abstract: A personal dosimeter for measuring the time-average concentration of a gaseous contaminant in the atmosphere in a manner substantially independent of atmospheric motion relative to the dosimeter is provided. The dosimeter comprises a detector substance capable of chemically or physically combining with the gaseous contaminant disposed tightly between two substantially flat members, at least one of which has a plurality of through-and-through channels. The members have corresponding, meshing shapes such that the channels provide the only communication between the atmosphere and the detector substance, allowing the gaseous contaminant to diffuse through the channels.