Air Quality Guides
Plain-English explainers about the air around you, written with reference to the U.S. EPA, the World Health Organization, and the U.S. CDC. Pick a topic.
Understanding AQI
The 0-to-500 number you see in every air-quality app, decoded. EPA breakpoints, what each color means, and how the index is calculated from raw measurements.
Read guide →AQI and Your Health
What "Unhealthy for Sensitive Groups" actually means, who counts as a sensitive group, and what to do at each AQI level — from a casual walk to canceling outdoor exercise.
Read guide →Common Air Pollutants
PM2.5, PM10, ozone, NO₂, SO₂, CO. Where each one comes from, what it does to the body, and which seasons and weather conditions tend to make it worse.
Read guide →Wildfire Smoke
What's actually in wildfire smoke, why it can drive AQI to Hazardous hundreds of miles from any fire, and the practical mitigations that work — N95s, MERV-13 filters, DIY box-fan cleaners.
Read guide →Indoor Air Quality
You spend ~90% of your time indoors. What that air is really like, why outdoor AQI doesn't tell the full story, and how to actually clean indoor air without buying snake oil.
Read guide →How AirNow Data Works
The path from a regulatory monitor on a building roof to the AQI on your phone. Federal Reference Methods, NowCast vs. raw hourly data, why the number can lag, and what gets flagged for QA.
Read guide →AQI and Exercise
How outdoor air quality affects athletic performance and recovery, when to move workouts indoors, what masks help (and don't), and the AQI thresholds different athletic governing bodies use.
Read guide →AQI and Outbreaks
What happens when a smoke event lands during a flu, RSV, or COVID wave. The biology of why bad AQI and respiratory illness compound each other, who's at higher risk, and how to combine the two data feeds for joint decisions.
Read guide →Reading CDC + AQI alerts together
The CDC, state, and county health departments each publish respiratory-illness data on different timetables. What each feed actually measures, the lag and resolution each has, and how to combine them with AirNow for one coherent household decision.
Read guide →Florida Air Quality
A Florida-specific guide: long summer ozone, Saharan dust intrusions in June and July, sugar-cane and prescribed burns, Karenia brevis red-tide aerosols. What to watch for, when, and resources for Florida families.
Read guide →California Wildfire Smoke
Regional smoke patterns for the Bay Area, LA, Central Valley, Sierra foothills, and North Coast. State agencies, the fire-season calendar, PSPS implications, and the household kit California residents should have ready.
Read guide →Pacific Northwest Air Quality
The two-season pattern that defines air in Washington, Oregon, and southern BC: wildfire smoke from July through September, and winter wood-smoke inversions in Seattle, Portland, and the valley basins from November through February.
Read guide →Mountain West Air Quality
How elevation, valley inversions, wildfire smoke, and windblown dust shape air quality across Colorado, Utah, Idaho, Wyoming, Montana, Nevada, and New Mexico. Salt Lake's PM2.5 inversions, Denver's brown cloud, and why thin air complicates exertion at altitude.
Read guide →Texas Air Quality
Houston ozone, petrochemical corridor emissions, Saharan dust in summer, agricultural and prescribed burns, and grass-fire smoke. What air quality looks like across Houston, DFW, San Antonio, Austin, and the border cities.
Read guide →U.S.–Mexico Border Air Quality
Cross-border PM2.5 from brick kilns, agricultural burns, and informal industry along the Texas, New Mexico, Arizona, and California border. Why monitor coverage is sparser south of the line, and how to interpret the binational data you can get.
Read guide →PM2.5 Explained
The most consequential pollutant in the modern AQI. What fine particulate matter is, where it comes from, what acute and chronic exposure does to the body, and the standards (EPA, WHO, EU) that try to limit it.
Read guide →Ground-Level Ozone
Why ozone spikes on hot summer afternoons, how it forms from NOₓ + VOCs + sunlight, why an N95 doesn't help against it, and the diurnal shift that makes a 6 AM run a different exposure than a 5 PM run.
Read guide →NO₂ and Gas Stoves
What the research actually says about gas-stove combustion byproducts, the 2022 attribution analysis that put childhood asthma at 12.7%, and practical mitigations from range hoods to induction cooktops.
Read guide →VOCs at Home
Volatile organic compounds — paints, cleaners, new furniture, formaldehyde, benzene. Which ones the EPA flags as health risks, why indoor concentrations are typically 2–5× outdoor, and what actually reduces them.
Read guide →Carbon Monoxide Indoor Risk
Outdoor CO has plummeted since 1980; indoor CO from generators, malfunctioning appliances, and grills still kills 400+ Americans every year. Sources, symptoms by exposure level, alarm placement, and emergency response.
Read guide →Masks and AQI
What N95, KN95, and P100 respirators actually do against PM2.5, when each makes sense, why cloth and surgical masks don't help with fine particles, and how fit can change effective protection by 90 percentage points.
Read guide →Air Purifier Sizing
How to size an air purifier for a real room: what CADR actually measures, the air-changes-per-hour the EPA recommends, the back-of-envelope math, and a quick-reference sizing table by room.
Read guide →DIY Corsi-Rosenthal Box
How to build the box-fan-plus-four-MERV-13-filters air cleaner that outperforms most retail purifiers at a fraction of the cost. EPA-validated; step-by-step build instructions and a parts list.
Read guide →The 2024 PM2.5 NAAQS Revision
The EPA tightened the annual PM2.5 standard from 12 to 9 µg/m³ in February 2024 — the first major change in over a decade. What changed, why, and what it means for the AQI breakpoints you see today.
Read guide →AirNow vs PurpleAir
Why two air-quality apps can show different numbers for the same location. EPA regulatory monitors vs. citizen sensors, the EPA correction for smoke, and which to trust in which situation.
Read guide →AQI Around the World
The U.S. AQI, the European AQI, the UK DAQI, Canada's AQHI, India's CPCB index, and the WHO's guideline values — what each measures, why they disagree, and how to read air quality when traveling.
Read guide →The UK DAQI
Britain's Daily Air Quality Index, decoded: the 1–10 scale, the four bands (Low to Very High), Defra's per-pollutant breakpoints, and how it lines up with the U.S. AQI.
Read guide →The European AQI
The European Environment Agency's six-category index, from Good to Extremely poor. The WHO-aligned band revision, the five-pollutant breakpoints, and why a U.S. "Good" day can read "Fair" in Europe.
Read guide →Canada's AQHI
Canada's Air Quality Health Index works differently: a 1–10+ scale built from a health-risk formula that blends ozone, NO₂, and PM2.5 rather than reporting the single worst pollutant.
Read guide →India's National AQI
India's CPCB index shares the 0–500 range with the U.S. AQI but spaces its six categories very differently. The eight-pollutant breakpoints, and why an Indian AQI of 150 is worse air than a U.S. AQI of 150.
Read guide →Singapore's PSI
Singapore's Pollutant Standards Index: the 0–500 scale, the five bands, the six-pollutant breakpoints, and why the 1-hour PM2.5 reading matters more than the headline PSI during haze season.
Read guide →South Korea's CAI
South Korea's Comprehensive Air-quality Index: the 0–500 scale, the four grades, and the six-pollutant breakpoints — several reported in parts per million rather than µg/m³.
Read guide →Australia's AQI
Australia builds its AQI on a different idea — each pollutant as a percentage of its national standard, where 100 means "at the standard." The NEPM standards, the category bands, and why states differ.
Read guide →China's AQI
China's 0–500 index (HJ 633-2012): the six categories, the PM2.5/PM10 breakpoints, the six pollutants, and why a Chinese AQI of 100 is dirtier air than a US AQI of 100.
Read guide →Japan's Air Quality Standards
Japan has no single 0–500 AQI. How its per-pollutant Environmental Quality Standards work instead, plus the photochemical-oxidant warning and the PM2.5 provisional advisory.
Read guide →Hong Kong's AQHI
Hong Kong's 1–10+ health-risk index: the five bands, the added-health-risk formula over four pollutants, roadside vs general stations, and why it differs from Canada's same-named AQHI.
Read guide →Southeast Asia Haze
Why peatland fires in Indonesia blanket Singapore, Malaysia, and the region in smoke each dry season, the record episodes, and why the 1-hour PM2.5 reading matters more than the headline PSI.
Read guide →Asian Dust (Hwangsa)
The spring "yellow dust" that sweeps from the Gobi across Korea and Japan — where it comes from, why it carries industrial pollution, and how to read the CAI during an event.
Read guide →Delhi Smog & Stubble Burning
Why Delhi's air collapses every autumn: crop-residue burning in Punjab and Haryana, Diwali, winter inversions, the policy response, and how to read India's AQI when it goes "Severe."
Read guide →Saharan Dust
How desert plumes reach Europe (calima, "blood rain") and cross the Atlantic via the Saharan Air Layer, the record 2022 episode, the health effects, and how it shows up in the European AQI.
Read guide →London's ULEZ & NO₂
Why nitrogen dioxide is London's defining pollutant, how the Ultra Low Emission Zone works and expanded city-wide, the evidence on its impact, and how NO₂ shows up in the DAQI.
Read guide →Air Quality in the UK
The DAQI, the dominant pollutants (NO₂, PM2.5, summer ozone), the seasonal pattern, London, and the agencies that monitor Britain's air.
Read guide →Air Quality in India
The National AQI, why PM2.5 dominates, the Indo-Gangetic Plain crisis, the autumn stubble-burning season, and the major cities to watch.
Read guide →Air Quality in Canada
The health-based AQHI, the excellent baseline, wildfire smoke as the dominant modern hazard, and the agencies behind the index.
Read guide →Air Quality in Australia
The percentage-of-standard AQI, the clean baseline, bushfire smoke, state-by-state reporting, and the cities to watch.
Read guide →Air Quality in South Korea
The CAI, why fine dust dominates, the spring Asian-dust season, transboundary transport, and how Seoul's air behaves.
Read guide →Air Quality in France
The European AQI, urban NO₂, winter particulate episodes, summer ozone in the south, Saharan dust, and Paris's pollution peaks.
Read guide →Air Quality in Germany
The European AQI, traffic NO₂, winter particulate, the Umweltzonen low-emission zones, and how Berlin's air behaves.
Read guide →Air Quality in Spain
The European AQI, urban NO₂ in Madrid and Barcelona, summer ozone, and the Saharan "calima" dust that Spain is most exposed to.
Read guide →Air Quality in Italy
The European AQI, the heavily polluted Po Valley, winter particulate and traffic bans, urban NO₂, and Milan and Rome.
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