Methods, systems, and apparatuses, including computer programs encoded on computer-readable media, for monitoring volatile chemicals. A system includes an radio-frequency identification (RFID) tag composed of a patterned metal. The patterned metal is configured to absorb a volatile chemical. The RFID tag includes a non-volatile memory configured to store identification data. The RFID tag also includes a receiver that receives a signal at a frequency in a frequency range. The frequency is based upon an amount of the volatile chemical absorbed in the patterned metal. A transmitter of the RFID tag transmits the identification data in response to receiving the signal. The strength of the transmitted identification data is based upon an amount of the absorbed volatile chemical.