Carbon monoxide by averaging window
Bad air at home
You cannot see most indoor pollutants, but you can look for measurable signals and compare them against published limits. This guide walks through what to observe, what numbers matter, and how to pick a ventilation approach that fits your building.
Carbon monoxide by averaging window
0.50 to 1.47 air changes per hour
90% of time spent indoors
3.2 million premature deaths per year
Source control, filtration, ventilation
What is commonly found indoors
Watch for measurable signals — odor persistence, condensation, CO readings — and compare them against known limits before buying any device.
Start with what you can observe without instruments. Stale odors that do not clear after ten minutes of an open window, persistent window condensation in winter, and visible mold at wall-floor junctions all point to moisture or ventilation problems that affect air quality.
Carbon monoxide is the pollutant with the clearest published numbers. The WHO recommends that acute exposure not exceed 10 mg/m³ over 8 hours, 35 mg/m³ over one hour, and 100 mg/m³ over 15 minutes. A working CO alarm in the sleeping area is the cheapest way to know whether you are near those limits.
Because you spend roughly 90% of your time indoors, even moderate pollutant levels accumulate. The practical question is not whether your air is perfect, but whether any single pollutant stays inside its exposure window.
CO limits in mg/m³, winter air change rates in ACH, and the 90% indoor time share are the three figures that frame every home air decision.
Exposure limits are always tied to an averaging window. Reading a single peak without knowing whether it lasted 15 minutes or 8 hours tells you almost nothing.
Ventilation rates carry their own units. Volumetric flow, airflow per person (CFM per person), airflow per floor area (CFM per square foot), and air changes per hour (ACH) all describe the same process from different angles. For a home, ACH is the most intuitive: it is how many times per hour the room's air volume is replaced.
In winter, air changes per hour may range from about 0.50 in a tightly air-sealed house to about 1.47 in a loosely air-sealed house. That range is your baseline: below 0.50 you are likely under-ventilated, near 1.47 you are losing a lot of heat.
Secondhand smoke, combustion products, radon, mold, carbon monoxide, VOCs, legionella, asbestos, CO₂, ozone and particulates cover the common indoor list.
The list of common indoor pollutants is long, but they do not all behave the same way. Combustion products and CO respond to ventilation within minutes; radon and asbestos need source control because dilution alone is not enough.
Molds and other allergens follow moisture. If you fix the water source, the biological pollutant usually follows within weeks.
Volatile organic compounds off-gas from materials and cleaning products. Here, source control — choosing lower-emitting products — beats filtration, because the emission is continuous.
Each figure carries its unit and averaging window; read them together, not in isolation.
The three primary methods are source control, filtration, and ventilation to dilute contaminants — in that order of priority.
Source control is the cheapest and most durable fix: remove the emitter rather than treat its output. Stop smoking indoors, service combustion appliances, fix leaks that feed mold.
Filtration captures what is already airborne. It does not add oxygen or remove CO₂, so it complements — never replaces — ventilation.
Ventilation is the intentional introduction of outdoor air into a space to dilute and displace indoor pollutants. It can be natural, mechanical, or mixed-mode, and each has a different winter ACH profile.
| What it measures | Value | Unit or window |
|---|---|---|
| Time spent indoors (US, EU) | 90% | average share of time |
| WHO CO limit, long window | 10 | mg/m³ over 8 hours |
| WHO CO limit, one hour | 35 | mg/m³ over 1 hour |
| WHO CO limit, short spike | 100 | mg/m³ over 15 minutes |
| Winter ACH, tight house | 0.50 | air changes per hour |
| Winter ACH, loose house | 1.47 | air changes per hour |
| Deaths attributed to indoor air pollution | 3.2 million | per year, WHO estimate |
Figures cited here trace back to WHO exposure recommendations and standard winter ACH ranges for residential buildings.