PM2.5 — particulate matter under 2.5 microns — is the single pollutant with the strongest evidence linking it to cardiovascular and respiratory harm, and it is also the one an air purifier addresses best. That combination is why it deserves its own page rather than a line on a spec sheet.
The particles are small enough to reach the alveoli and pass into the bloodstream, which is what distinguishes them from coarser PM10 that the nose and upper airway largely handle. In much of North India the winter daily average runs many times the WHO guideline, and indoor levels track outdoor levels closely in an ordinary flat.
A true H13 HEPA filter removes at least 99.95% of particles at the most-penetrating size, so per-pass efficiency is not the constraint. The constraint is throughput: how many times per hour the unit can process the room's entire air volume against a continuous inflow. That is CADR, and it is the number to rank on.
One further recommendation specific to this page: buy a unit with a numeric PM2.5 readout rather than a coloured ring. It is the difference between trusting a claim and verifying it, and it will teach you more about your own home in a week — cooking spikes, the effect of an open window, the difference between 6 a.m. and 2 p.m. — than any article can.