HEPA filters are the most important part of any air purifier. There are several classes of this type of filter and it is true that the higher the number in the name, the less particulate will pass through the filter. Take a look at a comparison of the performance of each class
| Class | Efficiency (in%) | How many particles out of 10,000 penetrates |
| EPA 10 | > 85 | 1 500 |
| EPA 11 | > 95 | 500 |
| EPA 12 | > 99.5 | 50 |
| HEPA 13 | > 99.95 | 5 |
| HEPA 14 | > 99.995 | 0.5 |
| ULPA 15 | > 99.9995 | 0.05 |
For simplicity, these filters are collectively referred to as HEPA type filters, technically there are three subcategories:
EPA - Efficiency particulate air (Efficient microparticle capture)
HEPA - High-efficiency particulate air (highly efficient microparticle capture)
ULPA - Ultra low penetration air (ultra-low particle penetration)
