Chapter 22 Key Takeaways

Prioritize Before Spending

Radon testing ($15–$30) and CO detectors ($30–$50/unit) address life-safety hazards that many homeowners ignore while buying air purifiers. Fix the serious risks first. Filtration and air quality products come after: fix moisture problems, fix ventilation, then filter.

Indoor Air Pollutants Have Different Solutions

  • Particulates (dust, pollen, dander): filtration and source control
  • VOCs (formaldehyde, benzene): source control and ventilation — filtration does not help
  • Biological contaminants (mold, dust mites): humidity control is the primary tool
  • Combustion byproducts: proper venting and CO detectors; the HVAC system cannot filter these out
  • Radon: testing and active mitigation if needed

Higher MERV Is Not Always Better

MERV 8–11 is the appropriate range for most residential systems. MERV 13 or higher creates pressure drop that can freeze evaporator coils, overheat heat exchangers, and reduce airflow to conditioned spaces. Check your equipment manufacturer's recommendation. A 4-inch deep-media cabinet filter at MERV 11 provides better filtration than a 1-inch MERV 13 filter with far lower pressure drop.

HEPA Works for Particles, Not Gases

True HEPA (99.97% capture at 0.3 microns) is effective against dust, pollen, dander, mold spores, and many bacteria. It does nothing for VOCs, CO, radon, or odors. Portable HEPA purifiers work in the room where they're placed; no portable purifier meaningfully cleans whole-home air.

Ozone Generators Are Harmful — Do Not Use in Occupied Spaces

Ozone at effective concentrations is a respiratory irritant. It also reacts with household VOCs to form formaldehyde and other secondary pollutants. UV-C lights have limited effectiveness in residential airstreams due to short exposure time.

Humidity Target: 40–60% RH

Below 40%: respiratory irritation, wood damage, higher virus transmission risk. Above 60%: mold growth, dust mite proliferation, structural moisture problems. A digital hygrometer ($10–$20) tells you where you stand. Most cold-climate homes are too dry in winter (need a humidifier) and too humid in basements in summer (need a dehumidifier).

Humidifier Maintenance Is Non-Optional

A neglected humidifier becomes a mold and bacteria source. Replace water panels annually (bypass and fan-powered types). Clean the drain pan. Steam humidifiers need canister cleaning or replacement per manufacturer schedule. Set humidistat based on outdoor temperature — lower settings at lower outdoor temperatures prevent window condensation and wall cavity moisture.

CO Detectors: Non-Negotiable Requirements

  • One detector on every level of the home
  • Within 10 feet of each sleeping area
  • Replace every 5–7 years (electrochemical sensors degrade)
  • Test monthly
  • When an alarm sounds: get out immediately, call 911 from outside, do not re-enter until cleared by responders

Annual Combustion Appliance Inspection

All gas, oil, and propane equipment should be professionally inspected annually. The heat exchanger inspection is particularly critical — a cracked heat exchanger can allow CO to mix with supply air silently. This is the most serious HVAC safety issue in many homes.

The Diagnostic Before the Solution

Most indoor air quality problems are caused by identifiable sources: neglected filters, non-functional exhaust fans, mold from moisture accumulation, improperly vented appliances. Identify the cause before buying a solution. Source control and proper ventilation almost always outperform add-on purification products.