The webpage examines various air filter rating systems, focusing on their limitations and potential to mislead consumers. It critiques the Clean Air Delivery Rate (CADR), a standard developed by the Association of Home Appliance Manufacturers (AHAM) in the 1980s. CADR measures a purifier’s efficiency in removing three pollutants (dust, pollen, and tobacco smoke) during a 20-minute test in an 1,008-cubic-foot chamber. However, the test duration is criticized for failing to assess long-term performance, as many purifiers lose efficiency after one hour of use. For example, a leading hybrid model reportedly declines by 50% efficiency within eight weeks. CADR also does not evaluate effectiveness against ultrafine particles (smaller than 0.1 microns), which constitute over 90% of airborne particles and pose significant health risks. Additionally, CADR ignores gaseous pollutants, chemical removal, ozone filtration, and the potential for ionizing air purifiers to deposit harmful particles in the lungs. These omissions raise concerns about CADR’s reliability, prompting entities like IQAir to reject its use. Room size ratings, which depend on CADR data, are similarly criticized. AHAM calculates them by multiplying CADR by 1.55 (assuming 8-foot ceilings), but since CADR itself is flawed, room size estimates may be inaccurate. The webpage further explores HEPA (High Efficiency Particulate Air) filters, originally designed for industrial safety and defined as removing 99.97% of 0.3-micron particles. While HEPA is a trusted standard in medical and lab settings, household air purifiers lack mandatory testing for compliance. This has led to inconsistent marketing terms like “True HEPA,” “HEPA-type,” and “HEPA-like,” which often lack verification. Many so-called HEPA filters use synthetic fibers or electrostatic charging, reducing efficacy and raising health concerns. Even filters meeting true HEPA standards may overlook ultrafine particles, the most hazardous size range (0.1 microns and smaller), which can penetrate deeper into the respiratory system and bloodstream. The article also warns against ionizers and ozone-generating purifiers. While some use ionization to attract particles—potentially depositing them on surfaces or in lungs—ozone, a byproduct, can cause respiratory irritation and long-term health issues. The U.S. EPA and American Lung Association caution against these technologies due to their risks. Overall, the content emphasizes the inadequacy of existing rating systems in addressing real-world performance, ultrafine particles, and chemical pollutants, urging consumers to scrutinize claims and seek independently verified filtration technologies.