IAQ on a budget: best impact per dollar, in order

Most of the IAQ improvement available to a typical household sits in the first $0 to $200 of intervention. Beyond that, the curve flattens and each additional dollar buys progressively less air-quality benefit (a $1,500 HRV is wonderful but the marginal improvement over a $60 box-fan filter is small in absolute terms). The ladder below is ordered by impact per dollar, drawing on the same AHAM CADR and ASHRAE MERV performance data the dashboard uses for its recommendations.

Climb the ladder in order; each rung buys less air-quality benefit per dollar than the one before it.

1. $0. Walk-off mat at every entry door (use one you already own). Shoes off inside (catches PM₁₀, allergens, lead dust, pesticide residue at the threshold). Damp-dust hard surfaces weekly instead of feather-dusting (re-suspends nothing). Open windows on opposite sides of the house for ten minutes a day, weather permitting, for cross-ventilation. None of this costs money and together it handles a substantial fraction of PM₁₀, settled VOCs, and CO₂.
2. $15-30. A MERV-13 furnace filter. Even a single MERV-13 installed in place of the default MERV-8 cuts indoor PM_2.5 meaningfully when the HVAC fan runs.
3. $60-100. A Corsi-Rosenthal box-fan filter: $25 box fan, four MERV-13 filters, duct tape. Total CADR is in the range of a $300-500 commercial HEPA, the parts are widely available, and you can build it in 15 minutes. This is the single highest impact-per-dollar intervention available for PM_2.5.
4. $150-300. Replace a recirculating range hood with a vented one if your kitchen has gas appliances (used or refurbished is fine; vented to outdoors is the requirement). A used dehumidifier ($50 thrifted, $200 new) for the basement if humidity is the problem. A budget HEPA purifier sized per CADR for the bedroom.

Diminishing returns past $300. The cheap things have been done. Further dollars buy fractional improvements: a smart thermostat with humidity control ($150), a CO₂ meter beyond what the dashboard already provides ($100-200), an HRV for whole-house ventilation ($1,500-3,000 installed). All real, all worth doing eventually; none of them deliver the impact-per-dollar of the first three rungs. What does not work cheap: an HRV done partially (don't), DIY UV-C disinfection (skip), or houseplants as filtration (see plants as air purifiers). For envelope work see sealing and air tightness.