Embodiments described herein provide for terahertz tags and methods of making and using them. A tag may include a terahertz reflective material; and a saturated hygroscopic material positioned on the terahertz reflective material. A tag may include a terahertz reflective material; and an anhydrous hygroscopic material positioned on the terahertz reflective material. A humidity sensor may include a terahertz reflective material; and an anhydrous hygroscopic material positioned on the terahertz reflective material. A temperature sensor may include a terahertz reflective material; an anhydrous hygroscopic material positioned on the terahertz reflective material; and a polymer overlay having thermally controlled water permeability disposed on the anhydrous hygroscopic material. Some embodiments relate to a tag identification device configured to transmit an incident signal toward the tag, and to receive a reply signal from the tag in response to the incident signal.