How portable air cleaners work and why CADR matters
Clean Air Delivery Rate (CADR) quantifies how much "clean" air a portable air cleaner delivers and is reported separately for smoke, dust, and pollen in cubic feet per minute (cfm); CADR is AHAM's standard test metric shown on product labels.
AHAM provides simple sizing guidance: for routine allergy control use the "two‑thirds" rule (choose a unit whose smoke CADR is at least two‑thirds of the room's square footage), and for active wildfire smoke AHAM recommends a smoke CADR approximately equal to the room's square footage for best performance.
Example: a 200 ft² bedroom would use a smoke CADR ≥ ~133 cfm for routine control (2/3 rule) and about 200 cfm for an active smoke event.
Verify AHAM Verifide labeling or the manufacturer's CADR spec before purchase; ENERGY STAR labels indicate energy efficiency for qualified models.
Filter types and what they remove
True HEPA filters capture fine particles including PM2.5, which are the primary health concern in wildfire smoke; properly sized portable HEPA cleaners run continuously in occupied rooms can substantially reduce indoor PM2.5 during smoke events.
HEPA filters do not remove gases or many odor‑causing volatile organic compounds (VOCs) from smoke; activated‑carbon or other gas‑adsorbent stages are used to address gaseous components, but effectiveness depends on the carbon type, quantity, and the specific pollutant.
Operational best practices: run, place, and isolate
Run portable air cleaners continuously and on the highest fan setting you can tolerate while smoke is present to maximize air changes and particulate removal.
Place the unit in the room you occupy (for example, the sleeping area), keep doors closed to create a "clean room," and position the purifier away from walls and obstructions so airflow is not blocked—avoid tucking it tightly into a corner.
One portable unit cleans only its rated room area; for whole‑house protection use multiple appropriately sized units located in the highest‑use rooms rather than relying on a single unit.
DIY box‑fan + filter options and cautions
The EPA publishes tested DIY configurations using a 20" box fan and MERV‑13 furnace/AC filters, lists suggested components, and provides safety cautions including using UL/ETL‑listed fans and not leaving DIY units unattended.
Pros and cons: box‑fan + MERV‑13 setups are low cost and can move large airflow (potentially high effective CADR), but they are not AHAM‑tested, have variable performance, and require careful assembly and safe operation per EPA guidance.
When to upgrade your HVAC filtration and system compatibility
Public‑health guidance recommends using high‑efficiency HVAC filters (MERV‑13 or as high as the system can handle) and running the air‑handler fan continuously to capture fine particles from outdoor smoke when possible.
System‑compatibility caution: higher‑MERV or thicker filters increase pressure drop and can reduce airflow or stress older blower motors—ask an HVAC professional whether your furnace/air handler can safely handle a 1" MERV‑13 pleated filter or whether a deeper filter/media cabinet is required; if not compatible, use the highest recommended rating and supplement with portable cleaners.
Health evidence and regulatory cautions
The CDC BRACE technical report concludes portable HEPA filtration reduces indoor PM2.5 and personal exposures during wildfire smoke episodes and can lower biomarkers of exposure in some studies, while noting variability based on building tightness, unit sizing, and user behavior.
EPA advises against ozone‑generating "ionizer" or "ozone" air purifiers in occupied spaces; CARB certifies devices sold in California for low ozone emissions and maintains a list of certified devices—check CARB listings or choose non‑ozone‑producing units.
Cost and maintenance realities
Retail examples: an extra‑large portable HEPA (Honeywell HPA300) lists AHAM Verifide CADR values and recommended room coverage (example listing shows ~465 ft² coverage) and retail prices commonly fall in the roughly $150–$300 range depending on model and retailer.
HVAC filter cost example: MERV‑13 furnace/AC replacement filters are sold at consumer retail sites; prices vary by size, brand, and vendor.
Expect to replace filters per manufacturer guidance—prefilters and HEPA cartridges, activated carbon stages, and HVAC filters all have lifespans that depend on use during smoke events; check manufacturer instructions for replacement intervals and costs.
Short checklist for an active smoke event
Close windows and exterior doors and make one occupied room a "clean room."
Run a properly sized portable HEPA cleaner continuously on the highest tolerable fan speed in that room.
Use multiple portables for multiple occupied rooms rather than one unit for the whole house.
If using a DIY box‑fan + MERV‑13 unit, follow EPA safety cautions (listed fan models, do not leave unattended).
After the event, check and replace filters per manufacturer guidance.