It's the wrong metric. The question isn't how much stuff it lets through, but how much stuff it removes.
A filter with 99% efficacy at a flow rate of 30cfm will remove the same amount of stuff as a filter with 50% efficacy at 60cfm.
A 99% sealed HEPA system isn't cleaning the air that's not passing through it right now, which is most of the air in the room. It reduces the pollution by some nominal rate. After it's been running for a while, the air is a lot cleaner.
The major problem with box fan with filter are noise, aesthetics, and similar. Box fans aren't designed for for high loads, and just don't work great in that context. It will clean the air just fine, but a proper filter will do so much more efficiently and pleasantly.
In terms of effect:
I have allergies. I feel a lot better post-COVID, with HEPA filters, face masks, and similar precautions everywhere. I can't pin that down to any root cause, though. Particulate pollution? Allergens? HEPA filters at home? Masking outside. Dunno.
A filter with 99% efficacy at a flow rate of 30cfm will remove the same amount of stuff as a filter with 50% efficacy at 60cfm.
A 99% sealed HEPA system isn't cleaning the air that's not passing through it right now, which is most of the air in the room. It reduces the pollution by some nominal rate. After it's been running for a while, the air is a lot cleaner.
The major problem with box fan with filter are noise, aesthetics, and similar. Box fans aren't designed for for high loads, and just don't work great in that context. It will clean the air just fine, but a proper filter will do so much more efficiently and pleasantly.
In terms of effect:
I have allergies. I feel a lot better post-COVID, with HEPA filters, face masks, and similar precautions everywhere. I can't pin that down to any root cause, though. Particulate pollution? Allergens? HEPA filters at home? Masking outside. Dunno.