The study is about VOCs. You’ll want a carbon filter, as even a HEPA filter won’t do much.
Also, don’t put a HEPA filler in your HVAC return — not only is it unhelpful overkill, it will have excessive static pressure drop, thus wasting power, increasing noise, reducing airflow, and potentially damaging the blower. Use an appropriate filter in the MERV 13-16 range.
Carbon filters in air purifiers are a pain. They do work, but due to their smaller volumes they tend to get full quite fast and require frequent replacements.
If you're considering one, make sure to get an air purifies where the carbon filter can be changed independently from the main filter.
Also, don’t put a HEPA filler in your HVAC return — not only is it unhelpful overkill, it will have excessive static pressure drop, thus wasting power, increasing noise, reducing airflow, and potentially damaging the blower. Use an appropriate filter in the MERV 13-16 range.