Cryogenic circulation for indoor air pollution control

Hazardous outdoor air pollution has severely affected indoor air quality, threatening the health of billions of people. However, existing indoor air purification technologies are unsatisfactory due to some inherent limitations such as poor efficiency, limited target pollutants, the need to frequently replace filters or adsorbents, or the generation of harmful by-products. Here, we studied the effect and mechanism of cryogenic circulation for indoor air purification. Experimental results show that up to 99% of indoor PM_2.5 from ambient air was removed at -18 °C. The morphological measurements indicate that micrometer-sized particles are formed concomitantly with the reduction of nanometer- or submicron-sized particles, suggesting that condensational growth of fine particles is responsible for the removal. Applying the method to gaseous pollutant purification demonstrates that 98% of NO₂ is condensed and removed from the ambient air at -50 °C, implying that the method would be effective for multiple indoor pollutants with higher boiling points. Cryogenic condensation may provide a principle for continuous indoor air purification via modified air conditioners and humidifiers in cases where health benefits outweigh energy consumption concerns.