PUREFLOW® 2013 Cadillac Escalade Cabin Air Filter with HEPA and Antibacterial Technology, PC9957HX1st Stage: For drivers and passengers who suffer from severe allergies, PUREFLOW HEPA (High Efficiency Particulate Air) offers 99. 97% efficiency, capturing microscopic pollutants including pollen and other allergens (may result in a minor restriction to airflow). 2nd & 3rd Stage: Activated charcoal adsorbs harmful gases like carbon monoxide and sulfur dioxide. This layer is combined with baking soda to absorb bad odors. 4th Stage: Advanced