A HEPA filter is specifically designed and certified to capture particles down to 0.3 micron in size at a an efficiency of at least 99.97%. An ULPA filter is a variant of a HEPA except that in is certified to a higher efficiency at what is defined as the most penetrating particle size (MPPS). An ULPA example could have an efficiency of 99.9995% at MPPS. These air filters are tested and certified at the factory to ensure they meet efficiency, airflow and resistance requirements. The requirements are defined by the Institutes of Science and Technology (IEST) in their Recommended Practices for various types of HEPA filters. These filters are used in the most critical of applications so users should always require that a Certificate of Compliance accompany each filter to ensure that the filter meets published standards of cognizant authorities.