Get yourself a Blueair Classic Air filter and don't look back.

Which model you get depends on how big your room is and they outperform most HEPA filters (some much more expensive) by a factor of 100%. It is the best thing on the market.

While it is not strictly HEPA as it uses charge to collect particles from the air, it is tested to a much higher standard than HEPA and has the most independent certifications; Its the only major brand to verify 100% of models with AHAM for performance, Energy Star, and safe ozone.

The new models have Wifi & clever features like air quality monitoring and phone control, but you can pick up an older model on ebay without the bells and whistles for a bargain. Just chuck some new filters in it and you're set.