Activated Carbon air filters
Because HEPA filters cannot remove gases and odors that are typically 0.3 microns and smaller, activated carbon air filters are used in many air purifiers. Activated carbon is charcoal that has been treated with oxygen to open millions of tiny pores between the carbon atoms.
Activated carbon filters then physically absorb gas molecules into these pores. Odors and gasses tend to be too small to be captured by anything but activated carbon. Keep in mind that as these pores get filled, eventually the filter will need to be changed as it drops in efficiency.