What are HEPA and ULPA?

I have just become aware of a higher grade of air filtration than HEPA.  It's called ULPA.

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

Ultra Low Penetration Air (ULPA) = 99.999% of particles 0.12 micrometers or greater is removed by the air filter.  ULPA is better clearly.  Here's a diagram that demonstrates the relative sizes of particles in household dust.  I'm not sure the gain is worth the cost.  Devices for the home are just starting to become available, but I'm warning you they can be pricey.  For now, stick with HEPA filtration.

Your Allergy Dude