They'll let some dust particles through and fill the air when the stove or air conditioner is on, but hey, they're not described as effective for everything, no matter the size . They do their job very well, but if you want dust-free, ultra-clean air, you need a more stringent (and more expensive) filter. In the winter they last about a month (it says "up to 90 days"), but that's to be expected since I have an inefficient stove that works a lot and a large house with very high ceilings. One filter a month is of course acceptable. In spring and autumn they may last 2-3 months. Summer is again about a month because the air conditioner works a lot.
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