# Air quality glossary: every term this site uses > Source: https://breathecheck.com/glossary/ > What PM2.5, NowCast, a thermal inversion and a MERV rating actually mean — the terms behind every reading on this site, defined once. # Glossary Every term this site uses in a reading, a verdict or a chip, defined once. The pollutant chips in our articles link straight to the entry. ## The scale - **Air Quality Index (AQI) #**: EPA's 0–500 scale for how polluted the air is right now. It's built from whichever tracked pollutant scores worst that hour, grouped into six color-coded bands. - **EPA AQI breakpoints #**: The pollutant-concentration cutoffs, in micrograms per cubic meter, that map to each AQI band. AirNow publishes the full table and revises it periodically. - **NowCast #**: AirNow's weighted-average method for a displayed AQI, weighting recent hours more heavily. Part of why a sudden smoke event takes hours to fully register. - **Code orange / Code red #**: Everyday shorthand for Unhealthy for Sensitive Groups (orange, 101–150) and Unhealthy (red, 151–200). Not an EPA term — but what local news and school alerts actually say. - **AQI colour bands #**: The six-color system — green, yellow, orange, red, purple, maroon — that EPA assigns to its six AQI ranges. Color signals risk before anyone reads the number. - **24-hour average #**: The averaging window the PM2.5 AQI is defined on. It is why a sharp afternoon plume can leave the published number looking calm: a clean morning is averaged into it. - **Micrograms per cubic meter (µg/m³) #**: The concentration unit behind a PM reading. The AQI is a translation of it — the breakpoint table maps ranges of µg/m³ onto the 0–500 scale. ## What is in the air - **PM2.5 PM2.5 #**: Particulate matter 2.5 micrometers or smaller — small enough to reach deep into the lungs. The pollutant tied to symptoms like "my clothes smell like campfire." - **PM10 PM10 #**: Coarser particulate matter up to 10 micrometers, mostly dust — what a haboob carries, distinct from wildfire smoke's finer PM2.5. - **O3 Ground-level ozone #**: A gas formed when sunlight reacts with exhaust, peaking on hot, clear afternoons. Invisible and odorless, unlike smoke or dust. - **Smoke aloft #**: Wildfire smoke sitting high in the atmosphere rather than at ground level. It can make the sky look hazy while the ground-level AQI stays low. - **Haboob #**: An Arabic-derived term for the wall-like dust storms common around Phoenix, carrying wind-lofted PM10, not combustion smoke. - **Brown cloud #**: Denver's name for its visible haze — vehicle and industrial nitrogen dioxide and black carbon, trapped by a thermal inversion. - **NO2 Nitrogen dioxide (NO2) #**: A traffic and combustion gas, sharpest near busy roads and in cold stagnant air. One of the pollutants the AQI is calculated from. - **SO2 Sulfur dioxide (SO2) #**: A gas from burning sulfur-bearing fuel and from volcanic vents — the pollutant behind Hawaii's vog. Also one of the AQI pollutants. ## Who it affects - **Sensitive groups #**: EPA's umbrella term for people at higher risk from a given AQI level — includes children by default, no diagnosis required. ## Why it builds up - **Thermal inversion #**: A layer of warm air atop cooler ground-level air, trapping pollutants that would otherwise disperse — the mechanism behind Denver's brown cloud. - **Subsidence inversion #**: Sinking air warms as it descends and forms a warm lid aloft, trapping cooler air and everything in it underneath. Unlike an overnight radiation inversion it can hold for days. - **Marine layer #**: Cool moist air sitting over coastal water beneath a temperature inversion. It caps the air below it and pushes inland overnight, which is why a coastal metro and a valley twenty miles inland read differently. - **North American Monsoon #**: The summer surge of moisture into the Southwest, roughly mid-June to the end of September. It brings afternoon cloud and thunderstorms — and the outflow winds that raise dust. - **Tule fog #**: Dense ground fog that forms in California's Central Valley in winter, under the same stagnant conditions that let particulates accumulate. - **Ozone season #**: The warm months when sunlight and heat drive ozone formation, and the window most state agencies issue ozone forecasts in. Dates vary by state. ## What helps - **HEPA filter #**: A filter rated to capture 99.97% of particles 0.3 microns in size — the standard behind most purifiers sold against wildfire smoke. - **MERV rating #**: A 1–16 scale for how well an HVAC filter captures particles. MERV 13 is most recommended for PM2.5, though not every furnace handles the added resistance. - **N95 / P100 #**: NIOSH particulate-filtration ratings: N95 blocks 95% of particles at the hardest-to-filter size, P100 blocks 99.97%. Either needs a tight facial seal to filter as rated. - **Corsi-Rosenthal box #**: A box fan taped to four furnace filters, arranged as a cube. It moves far more air than its parts cost, and it is the reason a purifier recommendation here is a category rather than a product. ## Where the numbers come from - **AirNow #**: EPA's national reporting service for air quality from regulatory monitors. It is where the official reading on every city page comes from, and it publishes the AQI breakpoint table the bands are built on. - **PurpleAir #**: A network of low-cost outdoor sensors owned by the people who install them. Far denser than the regulatory network, and it reads high in humidity — we correct it with the published US EPA formula for PA-II units before showing a median. - **Open-Meteo #**: An open weather and atmosphere API. Every UV figure on this site is modelled from it, not measured by an instrument, and every page that shows one says so. - **Regulatory monitor #**: An instrument sited and maintained to federal standards, whose readings can be used for compliance. One usually covers a whole metro, which is why the side of a city farthest from it is the least well described. ## Rules and advisories - **Nonattainment #**: An EPA designation for an area that does not meet a national ambient air quality standard for a pollutant. It carries planning obligations, and areas can be reclassified to a more severe category when a deadline passes. - **Ozone Action Day #**: A voluntary-action advisory issued by a state or local agency when ozone is forecast to reach an unhealthy level. It asks for changes in behaviour; it is not a regulation. - **Prescribed fire #**: Burning set deliberately under a plan to reduce the fuel that would otherwise feed a wildfire. It produces smoke on purpose, on days chosen for how the smoke will disperse. How the six bands are built → · How we measure →