HVAC Indoor Air Quality Guide — Pollutants, ASHRAE 62.2 Ventilation, MERV/HEPA Filtration, Humidity & Radon Control

Source control — eliminate or reduce pollutant emissions at the source rather than trying to filter them out after release. The IAQ hierarchy per EPA: (1) Source control (most effective; eliminates the pollutant), (2) Ventilation (dilutes pollutants with outdoor air), (3) Filtration (removes pollutants after release). Examples of source control: switch from gas to electric cooking (eliminates NO₂ + ultrafine combustion particles); store paints, solvents, and cleaning products outside the conditioned envelope (eliminates VOC emissions); seal radon entry points (eliminates radon ingress); fix moisture sources (eliminates mold growth substrate); use low-VOC building materials (eliminates formaldehyde and other emissions). Source control reduces pollutant load by 70-95% in typical residential; ventilation + filtration handle the remaining 5-30% that can't be eliminated at source. Trying to filter your way to clean air without addressing sources is fighting a losing battle.

Probably not, if you have a properly-sized HVAC system with MERV 13+ filtration and ASHRAE 62.2-compliant mechanical ventilation. The HVAC system moves and filters far more air than any portable purifier — typical 2,000 sq ft home circulates 800-1,200 CFM through the central filter vs 200-400 CFM through a typical portable unit. Portable air cleaners are useful for: (a) supplementing HVAC filtration in a specific room (bedroom, home office); (b) handling acute episodes (wildfire smoke, contractor work releasing dust); (c) homes where HVAC filter housing can't accommodate MERV 13+ without static pressure problems; (d) homes with allergic or immunocompromised occupants needing extra capacity in high-occupancy rooms. Look for AHAM CADR rating (Clean Air Delivery Rate) appropriate for the room size; HEPA-class filtration; ozone-free designs. Avoid ionizers and ozone generators — both produce byproducts (ozone, ultrafine particles) that worsen IAQ rather than improving it. EPA explicitly warns against ozone generators sold as air purifiers.

MERV 13 is the post-2020 baseline for households where IAQ matters (households with children, allergies, asthma, immune sensitivity, pets, or smoking near the home). MERV 13 captures 90%+ of particles 0.3-1.0 μm — the size range that includes most viral and bacterial aerosols, fine smoke particles, and ultrafine combustion products. CDC explicitly recommends MERV 13 minimum for respiratory illness prevention in occupied spaces. ASHRAE Standard 52.2 specifies the test methodology. The catch: MERV 13 in a 1-inch filter slot adds 0.10-0.15 in.w.c. of pressure drop at typical residential airflow — beyond what some systems can handle without losing significant CFM. Solution: install a 4-5 inch deep-pleated filter housing (significantly more filter area = lower face velocity = lower pressure drop for the same MERV). A 4-inch MERV 13 typically has the same pressure drop as a 1-inch MERV 8. For homes without IAQ-sensitive occupants and standard residential exposure, MERV 8 is the practical minimum and adequately protects HVAC equipment.

Required by IRC 2021 Section M1505 in most US jurisdictions for new residential construction. Required by ENERGY STAR Single-Family New Homes program. Required by Passive House certification. The technical argument: tight modern construction (≤3 ACH50 per IECC R402.4.1.2) reduces natural infiltration to roughly 0.15-0.25 ACH natural — well below the air-quality threshold for typical residential occupancy. ASHRAE 62.2 specifies the minimum mechanical ventilation rate to maintain acceptable IAQ in tight construction: typically 7.5 CFM per occupant + 3 CFM per 100 ft² of conditioned floor area (total continuous ventilation). For a 2,000 sq ft home with 4 occupants: 7.5×4 + 3×20 = 90 CFM continuous outdoor air. This is delivered via bathroom exhaust fans running continuously, a separate exhaust-only ventilation fan, or a balanced ERV/HRV system. Without ASHRAE 62.2 mechanical ventilation in a tight home, indoor CO₂ regularly exceeds 1,500-2,000 ppm during occupied hours, indoor VOCs accumulate from construction materials and household products, and humidity control becomes problematic.

Yes — EPA recommends every home be tested for radon, regardless of age, location, or construction type. Radon (a naturally-occurring radioactive gas) is the second-leading cause of lung cancer in the US after smoking per EPA and CDC estimates. Radon enters homes through foundation cracks, crawlspaces, and well water. Levels vary widely by home — neighbors can have very different readings. EPA action level: 4.0 pCi/L; recommended action above 2.0 pCi/L; no "safe" level (lower is better). Testing: short-term (2-90 days, $15-50 DIY kit from hardware store) for initial screening; long-term (90+ days, $30-100) for accurate baseline. Continuous monitors ($150-300) provide ongoing measurement and are increasingly recommended for high-radon-zone homes. Mitigation if elevated: typically sub-slab depressurization (vent under the slab to outside via a fan); cost $1,000-3,000 installed; reduces radon 50-95%. EPA Indoor Environments Division publishes Radon Map and current protocols at epa.gov/radon.

Two likely causes. (1) Oversized AC: equipment that's too large for the load short-cycles, satisfying thermostat (sensible cooling) before pulling enough moisture (latent cooling). Symptom: 72°F indoor but 65-75% RH. Fix: properly-sized variable-capacity AC that modulates to part load and runs longer at lower capacity (longer run time = more moisture removal per unit cooling). Use our load calculator to verify sizing. (2) Inadequate latent capacity: the AC's Sensible Heat Ratio (SHR) is higher than the home's load SHR — the equipment is sensible-heavy and can't produce enough latent removal. Fix: add a dedicated whole-home dehumidifier (~$1,500-3,000 installed, 60-130 pints/day capacity); ASHRAE recommends keeping indoor RH 30-60% with 40-50% optimal for comfort and IAQ. Indoor RH above 60% promotes mold growth and dust mite reproduction; below 30% causes static discharge, dry skin, and respiratory irritation.

Wildfire smoke contains PM2.5 (fine particulate matter under 2.5 μm) at concentrations that frequently exceed EPA's 24-hour standard (35 μg/m³) by 5-50× during active fires. PM2.5 penetrates deep into lungs and bloodstream; documented health effects include increased respiratory illness, cardiovascular events, and premature mortality even from short-term exposure. EPA AirNow.gov provides real-time outdoor AQI for any US location. Indoor protection strategy: (1) Run HVAC continuously on the "fan" setting (forces air through the filter) with MERV 13+ filter — the central system is the most effective indoor air cleaner. (2) Close all windows; minimize door opening; create a "clean room" (typically the bedroom) with a portable HEPA air cleaner sized for the room (AHAM CADR ≥ 2/3 of room area in ft²). (3) Avoid using gas stoves, fireplaces, candles, or other indoor combustion that adds particulate to already-high indoor levels. (4) Wear N95 mask outside; significantly reduces inhaled PM2.5. (5) Monitor indoor AQI with portable monitor (PurpleAir, IQAir, Awair); typical wildfire indoor AQI 75-150 even with mitigation vs outdoor 200-500+.

Generally no — typical residential HVAC blowers cannot tolerate the pressure drop of true HEPA filtration (typically 0.5-0.8 in.w.c. at filter face vs 0.1-0.2 for MERV 8 and 0.2-0.3 for MERV 13). True HEPA in residential central HVAC requires either a dedicated HEPA bypass filter (parallel path that filters a portion of supply air) or a much larger filter housing than typical residential design accommodates. For most residential, MERV 13 in a 4-5 inch deep-pleated filter gives 90%+ of the IAQ benefit of HEPA at a fraction of the pressure drop penalty and equipment compatibility issues. For specific situations needing HEPA (immunocompromised occupant, severe allergy, lab cleanroom): use portable HEPA air cleaners in occupied rooms instead of trying to retrofit central HEPA. CDC and ASHRAE both note that MERV 13 captures sufficient viral/bacterial aerosol for typical residential IAQ goals; HEPA is overkill for most household exposures.