Home Air Quality Risk Estimator

Indoor concentration is modeled as outdoor air entering through the building envelope plus what is generated inside, minus what ventilation and filtration remove. The infiltration factor - the fraction of outdoor fine particles that survives passage indoors - typically falls between 0.3 and 0.8, lower in tight recent construction and higher in leaky older housing. Indoor sources are frequently the larger term of the two: cooking on a gas burner can drive PM2.5 above 100 micro-g/m3 for tens of minutes, and occupants, candles, cleaning products and off-gassing furnishings contribute continuously in the background. Two reference sets anchor interpretation. The World Health Organization's 2021 global air quality guidelines set 5 micro-g/m3 for annual mean PM2.5 and 15 micro-g/m3 for a 24-hour mean. The US EPA maps concentration onto the 0 to 500 Air Quality Index; note that EPA lowered the annual PM2.5 standard to 9.0 micro-g/m3 in February 2024 and revised the AQI breakpoints in May 2024, so older tools anchored at 12.0 micro-g/m3 for AQI 50 read optimistically. Carbon dioxide functions as a ventilation proxy rather than a toxin at household levels: outdoor air now sits near 420 ppm, below 800 ppm indoors indicates good air exchange, and readings above roughly 1,400 ppm indicate undersized ventilation for the occupant load. This estimate ignores radon, formaldehyde and combustion NO2, each of which needs its own measurement, and a calibrated monitor should always override a model.

The idea that occupancy loads indoor air came from Max von Pettenkofer, the Munich hygienist who in the 1850s proposed carbon dioxide as an index of air freshness and set roughly 1,000 ppm as the limit of acceptability, a threshold still embedded in ventilation guidance more than 160 years later. Mechanical filtration advanced sharply during the Second World War, when the US Manhattan Project developed high-efficiency particulate filter media to capture radioactive aerosols; declassification in the 1950s carried HEPA filtration into hospitals, semiconductor cleanrooms and eventually consumer air purifiers. Public reporting of outdoor air followed the US Clean Air Act of 1970 and the creation of the Environmental Protection Agency the same year. EPA introduced the Pollutant Standards Index in 1976 to give the public a single number comparable across cities; it was revised and renamed the Air Quality Index in 1999, when fine particulate matter below 2.5 micrometers was added as a reported pollutant following adoption of the PM2.5 standard in 1997. Indoor air became a distinct policy concern after the energy crises of the 1970s pushed builders to tighten envelopes, which produced the sick building syndrome literature of the 1980s. ASHRAE Standard 62 and its residential offshoot 62.2, first published in 2003, established the minimum whole-house ventilation rates that modern residential codes now reference.