Difference between HEPA and ULPA air filters

There is a higher grade of air filtration than HEPA - it is called ULPA.

High Efficiency Particulate Air (HEPA) = 99.97% of particles 0.3 micrometers or greater are removed by the air filter.

Ultra Low Penetration Air (ULPA) = 99.999% of particles 0.12 micrometers or greater are removed by the air filter.

References:

What are HEPA and ULPA? Dr. Neil Kao Allergy and Asthma Website.
How HEPA Filters Work http://goo.gl/qOLF
Image source: Amazon, used for illustrative purposes only - NOT a suggestion to purchase any products.

HEPA Air Cleaners Not Very Effective For Decreasing Visits and Asthma Symptoms in Children Exposed to Tobacco Smoke http://goo.gl/fhAmh