Nitrogen dioxide (NO₂) is a toxic, reddish-brown gas known for its strong, acrid odor and significant role in air pollution. In cybersecurity contexts, NO₂ is not directly relevant, but it is notable in scientific and industrial environments. It absorbs visible light and is used in photoacoustic spectroscopy, where absorbed light energy is converted into sound waves. This photoacoustic effect can be exploited in physical security and sensor systems, potentially intersecting with cybersecurity when monitoring for hazardous gases in secure facilities.