The Canairy Air Quality Guide

Clear, sourced answers about air quality, pollutants, and protecting your health — from wildfire smoke and HEPA filters to exercising on bad-air days.

• Air Pollution and Brain Health — The link between dirty air and the brain used to feel speculative. It does not anymore. The Lancet Commission added air pollution to its list of modifiable dementia risk factors in 2020, and more recent studies show measurable effects on memory, mood, and Alzheimer's pathology.
• Air Pollution and Cancer Risk — In 2013, the International Agency for Research on Cancer (IARC), the cancer arm of the World Health Organization, officially classified outdoor air pollution and particulate matter as Group 1 carcinogens. That puts air pollution in the same category as tobacco smoke and asbestos in terms of evidence linking it to cancer in humans.
• Air Pollution and Children's Health — The World Health Organization estimates that 93 percent of children under 15 worldwide (about 1.8 billion kids) breathe air dirty enough to threaten their health and development. In 2016, polluted air contributed to roughly 600,000 child deaths from lower respiratory infections.
• Air Pollution and Diabetes — Air pollution is not the first thing most people think of when they think about diabetes risk. It should be on the list. The Lancet Planetary Health estimates that PM2.5 exposure contributes to about 20 percent of new type 2 diabetes cases globally, making it a meaningful risk factor alongside diet, weight, and inactivity.
• Air Pollution and Heart Disease — Most people think of air pollution as a lung problem. The bigger killer is heart disease. The American Heart Association has formally recognized fine particle pollution (PM2.5) as a cause of cardiovascular illness and death, and the WHO estimates that air pollution contributes to roughly a quarter of all deaths from heart disease and stroke worldwide.
• Air Pollution and Mental Health — The link between dirty air and mental health used to sit at the edge of the research. In the last few years, it has moved closer to the center. Large studies across the U.S., Europe, and Asia now consistently associate long-term air pollution exposure with higher rates of depression and anxiety, even after controlling for income, education, and other risk factors.
• Air Pollution and Older Adults — Older adults are one of the groups most affected by poor air quality. Lung tissue loses elasticity with age, the immune system weakens, and many people over 65 have one or more chronic conditions (heart disease, COPD, diabetes) that pollution makes worse. The EPA lists adults over 65 as a sensitive group on every Air Quality Index level.
• Air Pollution During Pregnancy — Pregnancy is one of the windows when air pollution does the most damage. The pollutants a pregnant person breathes can affect placental function, fetal growth, and birth timing. The effects can show up at birth and continue into childhood.
• Air Quality in Gyms and Fitness Centers — You go to the gym to be healthier. During a hard workout you breathe roughly 10 times more air per minute than at rest, which means whatever is in the gym air gets ten times the dose into your lungs. That makes indoor air quality at gyms a real, often overlooked, factor.
• Air Quality in Schools — Kids spend roughly 1,000 hours a year in classrooms. Their lungs are still developing, they breathe more air per pound of body weight than adults, and they cannot control the building they sit in. Classroom air quality matters more than most parents and administrators realize, and it sits at the intersection of HVAC engineering, building age, location, and budget.
• Air Quality While Traveling — Travel scrambles the air quality habits you built at home. New rooms, new outdoor air, planes, ride-shares, and zero control over the HVAC. Here's the short list of things that actually matter.
• Ammonia — Ammonia (chemical formula NH3) is a colorless gas with a sharp pungent smell that most people detect at around 5 parts per million (ppm) and find clearly irritating above about 25 ppm. It is highly soluble in water (the household cleaner sold as "ammonia" is a dilute solution of NH3 in water). Anhydrous ammonia (the pure liquefied gas) is used widely in agriculture and refrigeration.
• Apartment vs Single-Family Air Quality — The kind of home you live in changes what you breathe. Apartments and single-family houses each have characteristic problems, and the right fixes are different.
• Asbestos Fibers — Asbestos is the name for six naturally occurring fibrous silicate minerals: chrysotile (white asbestos, by far the most common in commercial use), amosite, crocidolite, tremolite, anthophyllite, and actinolite. The fibers are strong, heat-resistant, and chemically stable, which made them useful in thousands of building products. When fibers become airborne and are inhaled, they can lodge in the lungs for decades.
• Asthma and Air Quality — About 25 million Americans have asthma, including roughly 5 million children. For most of them, air quality is one of the biggest day-to-day variables in how their lungs feel. A bad-air day can mean wheezing, missed work or school, an ER visit, or worse.
• Baby and Nursery Air Quality — Babies breathe faster than adults and weigh less, so per pound of body weight, they get a bigger dose of whatever is in the air. Their airways and lungs are still developing through age 6 or so. The nursery is one of the easier places to get right.
• Backyard Fires, Grilling, and Bonfires — Open-air burning feels like outdoor activity, but it dumps PM2.5, CO, and PAHs right where you and your neighbors are sitting. A casual fire pit can outpollute a busy intersection within 10 feet.
• Bedroom and Sleep Air Quality — You spend roughly a third of your life in your bedroom, mostly with the door closed and the windows shut. That makes it the room where small air quality problems get the most chance to bother you.
• Best Time of Day to Exercise Outside — Outdoor exercise doubles or triples your breathing rate, which means you pull in more of whatever is in the air. Picking the right hour can cut your dose substantially without changing your training.
• Black Carbon — Black carbon (BC) is the sooty particle formed when fossil fuels, wood, and other carbon fuels burn incompletely. It is the strongly light-absorbing component of fine particulate matter and almost always falls within the PM2.5 size range, with a large fraction in the ultrafine range (under 0.1 micrometers). Black carbon is what makes diesel exhaust black and wood smoke gray.
• Building a Corsi-Rosenthal Box Fan Air Cleaner — For about $65 in parts and 15 minutes of duct-taping, you can build an air cleaner that moves more clean air than a $400 HEPA unit. The Corsi-Rosenthal Box (CR Box) was invented in 2020 by Richard Corsi and Jim Rosenthal during the pandemic, and has held up in independent testing as one of the best dollar-per-CFM air cleaners you can make.
• Carbon Monoxide — Carbon monoxide (CO) is a colorless, odorless, tasteless gas formed when carbon-based fuels burn incompletely. It is the same molecule whether it comes from a car engine, a charcoal grill, or a malfunctioning furnace.
• Choosing a Mask for Smoke and Pollution — When the air gets bad, the right mask cuts your exposure to fine particles by 90% or more. The wrong one (or one worn loose) does almost nothing. Here's how to pick and wear one that actually works.
• Climate Change and Air Quality — Climate change and air pollution share a lot of chemistry. Burning fossil fuels emits both CO2 and the precursors of PM2.5 and ozone. Warming, in turn, changes how those pollutants form, move, and persist. Researchers now describe the feedback between the two as the "climate penalty" on air quality.
• Coastal vs Inland Air Quality — Coastal air gets called "sea air" and assumed to be clean. Sometimes it is. Often it isn't, especially near ports and on certain weather patterns. The wind, salt, ship traffic, and recirculation patterns make coastal air quality its own special case.
• Commuting and Route Choice — Your commute is often the largest pollution exposure of your day, even though it only takes 30 to 60 minutes. Highway traffic concentrates ultrafine particles, black carbon, and CO into a narrow corridor right where you are.
• Cooking Timing and Ventilation — Cooking is the single biggest source of indoor PM2.5 in most homes. The good news: you can cut your exposure 80% or more with timing and habits you already control, no equipment upgrade needed.
• Creating a Clean Air Room at Home — When the AQI hits red or purple, you don't need to filter your whole house. You need one room where the air is dramatically cleaner than outside, where the family can sleep, work, and ride out the smoke event. EPA calls this a "clean room," and setting one up takes an hour and around $100 to $300.
• Diesel Exhaust — Diesel exhaust is the mix of gases and soot released when diesel fuel burns in trucks, buses, ships, locomotives, construction equipment, generators, and some passenger cars. It contains over 40 substances that EPA and the California Air Resources Board (CARB) classify as toxic air contaminants, including formaldehyde, benzene, 1,3-butadiene, and a long list of polycyclic aromatic hydrocarbons. The visible black soot is diesel particulate matter (DPM), most of which is in the PM2.5 size range and a large fraction is even smaller (ultrafine, under 0.1 micrometers).
• Environmental Justice and Air Pollution — The environmental justice movement traces its modern origin to a 1982 protest in Warren County, North Carolina, where mostly Black residents blocked trucks dumping PCB-contaminated soil. Forty years later, environmental justice research has accumulated overwhelming evidence that air pollution in the United States is not distributed evenly, and that the unevenness tracks race more closely than income.
• Exercising Outdoors on Bad Air Days — When you exercise, you pull in 10 to 20 times more air than at rest, mostly through your mouth (skipping the nose's filtering). On a polluted day, that turns a workout into a high-dose exposure event. The rule of thumb is simple: scale intensity to AQI, and move indoors past certain thresholds.
• Formaldehyde — Formaldehyde (chemical formula HCHO, also written CH2O) is a colorless gas with a sharp pungent odor most people can smell at around 0.5 to 1 part per million (ppm). It is a volatile organic compound (VOC) and one of the most common indoor air contaminants. The body itself produces small amounts as a normal metabolic byproduct, but inhaling formaldehyde from outside sources adds to that load and irritates tissue at much lower concentrations than most VOCs.
• Global Burden of Disease and Air Pollution — How do you count deaths from something nobody dies of directly? Air pollution doesn't appear on death certificates. People die of heart attacks, strokes, lung cancer, pneumonia, COPD. The Global Burden of Disease (GBD) study, run by the Institute for Health Metrics and Evaluation at the University of Washington, has spent two decades building the statistical machinery to estimate how many of those deaths are pushed forward in time by dirty air.
• Ground-Level Ozone — Ozone (O3) is a gas made of three oxygen atoms. The ozone layer 10 to 30 miles up in the stratosphere is the "good" ozone that blocks UV radiation. At ground level, the same molecule is a pollutant and the main ingredient in summertime smog.
• HEPA Air Purifiers: What to Look For — A good HEPA purifier can drop indoor PM2.5 by 50% to 80% within an hour. A bad one is an expensive nightlight. Three numbers and one filter type tell you most of what you need to know.
• High Altitude Air Quality — Mountain air comes with assumptions: thin, crisp, clean. The first part is true. The other two are more situational. Higher elevations change how pollution forms, where it sits, and how it hits your body.
• Holiday Cooking and Indoor Air — Holidays mean a closed house full of people, an oven running for hours, multiple stovetop burners going, candles lit, and a fireplace maybe. Indoor PM2.5 on Thanksgiving routinely beats the worst outdoor cities in the world.
• How Air Pollution Affects the Lungs — Your lungs filter about 11,000 liters of air every day. When that air carries fine particles, ozone, and toxic gases, the lungs are the first organ to absorb the damage. Some particles stay in lung tissue. Others cross into the bloodstream and reach the rest of the body.
• How to Read the Air Quality Index — The AQI is the number you see in every weather app and on every air quality monitor. Knowing what it means takes about two minutes and changes the decisions you make on bad air days.
• HVAC Filters and MERV Ratings — The cheapest air quality upgrade in most homes is the filter that's already in your furnace. Swap it for a better one, change it on schedule, and you can drop indoor PM2.5 by 30% to 60% on smoke days without buying any new equipment.
• Indoor Air Quality Research — Americans spend about 90 percent of their time indoors, according to the EPA's National Human Activity Pattern Survey. Indoor air pollution concentrations are often two to five times higher than outdoor levels, and during certain activities (cooking, cleaning, smoke events with windows closed) can spike much higher. Indoor air quality (IAQ) research is older than outdoor air pollution epidemiology in some ways, but the modern field has been reshaped by tighter buildings, gas appliances, wildfire smoke infiltration, and the COVID-era rediscovery of ventilation.
• Lead in Air — Lead is a soft heavy metal (chemical symbol Pb) that is highly toxic to humans, especially children. In the air, lead appears as fine particles or attached to dust. It does not break down in the environment. Once airborne lead settles, it accumulates in soil, dust, and water, where it can be re-suspended or ingested later.
• Living Near a Highway or Major Road — About 45 million people in the United States live, work, or attend school within 300 feet (roughly 100 meters) of a major road, highway, or railroad, according to EPA. That proximity changes the air they breathe in measurable ways.
• Living Near a Small Airport — Living near a small general aviation (GA) airport can expose your household to airborne lead — a risk most people have never heard of. Piston-engine planes still burn leaded fuel ("100LL," or 100 low-lead avgas), and that lead settles around the airport and into nearby homes, soil, and children. Big commercial airports get most of the attention, but this particular risk belongs to the small fields.
• Living Near Agriculture — Rural and exurban addresses are often pictured as "clean air" by default. The reality is more mixed. Farming activity, from tractors and tillage to manure lagoons and pesticide spraying, produces measurable pollution that drifts into nearby homes and towns.
• Living Near Refineries or Heavy Industry — More than six million people in the United States live within three miles of an oil refinery. Many more live near steel mills, cement plants, chemical facilities, power plants, ports, and metal processors. These "fenceline" communities carry a heavier pollution load than the regional average, and the difference is often visible in health outcomes.
• Long-Term PM2.5 Mortality Studies — The science of PM2.5 and premature death rests on a handful of very large cohort studies that follow people for years, measure their long-term exposure to fine particles, and count who dies and why. After the Harvard Six Cities Study opened the field in 1993, the cohorts grew bigger and richer, and the central finding kept getting more confident: long-term exposure to fine particles shortens lives, with no clear safe level.
• Low-Cost Sensor Networks and Science — Until about 2015, air quality data in the United States came almost entirely from a network of around 1,000 federal reference monitors run by state and tribal agencies. They cost $30,000 to $100,000 each, need climate-controlled shelters, and are sparse: most U.S. counties have none. The rise of consumer-grade optical sensors (PurpleAir, Clarity, AirGradient, IQAir AirVisual, and others) has changed that picture, but the research community spent the better part of a decade figuring out how trustworthy the new data actually is.
• Mold Spores — Mold is a kind of fungus that grows on damp organic surfaces. It reproduces by releasing microscopic spores (usually 2 to 100 micrometers across) that float through indoor and outdoor air. Common indoor genera include Cladosporium, Aspergillus, Penicillium, Alternaria, and Stachybotrys (often called black mold). Some molds also release volatile organic compounds (called mVOCs) that produce the musty smell people associate with water damage, and a smaller number produce mycotoxins.
• Nitrogen Dioxide — Nitrogen dioxide (NO2) is a reddish-brown gas with a sharp, biting odor. It belongs to a group of gases called nitrogen oxides (NOx), which also includes nitric oxide (NO). The two interconvert in the atmosphere, but NO2 is the form most directly linked to health effects.
• Pets and Air Pollution — Pets breathe the same air you do, often closer to the ground where heavier particles settle, and they can't tell you when their chest hurts. Dogs, cats, birds, and small mammals all have more sensitive airways than humans, and birds are extremely vulnerable. Here's how to protect them on bad air days.
• PM10 Coarse Particulate Matter — PM10 is the category of inhalable particles with diameters of 10 micrometers or less. It includes PM2.5 (the fine fraction) plus larger "coarse" particles between 2.5 and 10 micrometers, sometimes called PM10-2.5. For comparison, a strand of human hair is about 70 micrometers across.
• PM2.5 Fine Particulate Matter — PM2.5 refers to fine particles in the air with a diameter of 2.5 micrometers or less. That is about 30 times thinner than a human hair. The particles are a mix of soot, sulfates, nitrates, organic chemicals, metals, and dust, often coated with other compounds that piggyback into the lungs.
• Pollen and Bioaerosols — Bioaerosols are airborne particles of biological origin. The category covers pollen grains, fungal (mold) spores, bacteria, viruses, dust mite fragments, cockroach allergens, and pet dander. Sizes range from about 0.02 micrometers for viruses to 100 micrometers for the largest pollens.
• Polycyclic Aromatic Hydrocarbons — Polycyclic aromatic hydrocarbons (PAHs) are a family of more than 100 chemicals made up of fused benzene rings. They form whenever organic material (wood, coal, oil, gas, tobacco, food) burns incompletely. EPA tracks 16 PAHs as priority pollutants, including naphthalene, fluorene, phenanthrene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, and the most studied, benzo[a]pyrene (B[a]P).
• Protecting Kids and Schools from Smoke — Kids breathe more air per pound of body weight than adults, have narrower airways, and their lungs are still developing through their late teens. Wildfire smoke and ozone hit them harder. The good news: a few cheap, consistent moves cut their exposure dramatically.
• Radon — Radon is a colorless, odorless, tasteless radioactive gas. It forms from the natural decay of uranium and radium in soil and rock. Outdoors, it disperses to harmless concentrations. Indoors, it can build up to levels that significantly increase lung cancer risk.
• Reducing Indoor Pollution from Cooking — Cooking is the biggest indoor air pollution event in most kitchens, easily worse than a smoky outdoor day. A Berkeley Lab study found that 60% of US homes with gas stoves regularly exceed outdoor air quality limits for nitrogen dioxide when cooking. Searing on any stovetop (gas, electric, or induction) throws off PM2.5. The fix is mostly about pulling that air out the window before it spreads.
• Secondhand and Thirdhand Smoke — Secondhand smoke (SHS) is the smoke exhaled by a person smoking plus the smoke coming off the burning end of a cigarette, cigar, pipe, or hookah. It contains more than 7,000 chemicals, of which at least 70 are known carcinogens, including benzene, formaldehyde, 1,3-butadiene, polycyclic aromatic hydrocarbons, and tobacco-specific nitrosamines.
• Sulfur Dioxide — Sulfur dioxide (SO2) is a colorless gas with a sharp, choking odor, like the smell of a freshly struck match. It belongs to a family of gases called sulfur oxides (SOx). In the atmosphere, SO2 reacts with water vapor and other compounds to form sulfuric acid and sulfate particles, both of which contribute to acid rain and to PM2.5.
• The Harvard Six Cities Study — When the EPA tightened its fine-particle pollution standard in 1997, one study did most of the heavy lifting in the science behind it. The Harvard Six Cities Study, published in the New England Journal of Medicine in 1993, was the first large prospective study to link long-term exposure to fine particulate matter (PM2.5) with shortened lives in the United States.
• Traffic-Related Air Pollution Research — Roughly 45 million Americans live within 300 feet of a highway, major road, or rail line. Traffic-related air pollution (TRAP) is a distinct subfield because the mix near roadways differs from the regional average: more nitrogen dioxide, more ultrafine particles, more black carbon, more PAHs, more brake and tire wear. Forty years of research has built a strong case that living close to busy traffic is a measurable health risk on top of the regional pollution load.
• Urban, Suburban, and Rural Air Quality — The common assumption is that air gets cleaner the farther you go from the city. That's partly true and partly wrong. The mix of pollutants changes more than the total burden, and rural air can be just as harmful as urban air despite lower particle counts.
• Volatile Organic Compounds — Volatile organic compounds (VOCs) are carbon-based chemicals that evaporate into the air at room temperature. The category covers thousands of different molecules, from familiar ones like formaldehyde, benzene, toluene, and xylene to fragrance compounds like limonene and pinene.
• WHO Global Air Quality Guidelines 2021 — On September 22, 2021, the World Health Organization released its first update to the Global Air Quality Guidelines (AQGs) in 16 years. The headline change: the annual PM2.5 guideline was cut in half, from 10 to 5 micrograms per cubic meter. The WHO estimated that meeting the new level worldwide could prevent millions of premature deaths each year.
• Wildfire Smoke — Wildfire smoke is a mix of gases and fine particles released when vegetation, structures, and soil burn. The dominant pollutant by health risk is PM2.5: fine particles small enough to lodge deep in the lungs and cross into the bloodstream.
• Wildfire Smoke and Health Research — For most of the last 30 years, the science of fine-particle pollution was a science of tailpipes, smokestacks, and power plants. Then the American West started burning longer and harder, and in 2023 Canadian wildfire smoke turned New York City orange. Wildfire smoke is now one of the most active areas of air pollution research, and one of the most uncomfortable: even as regulated sources clean up, smoke from fires keeps erasing the gains.
• Wildfire-Prone Regions — Living in fire country has become a year-round commitment, not a summer worry. Fire seasons in the western US, parts of the South, and increasingly the Northeast and Midwest have stretched longer, run hotter, and put more homes in the path of flames and smoke. This article focuses on what changes when you live there, not on what wildfire smoke is (covered in the Pollutant Guide) or short-term smoke response (covered in the Protection section).
• Wood Smoke from Home Fireplaces and Stoves — Wood smoke is the mix of gases and fine particles released when wood burns in a fireplace, wood stove, pellet stove, or outdoor wood boiler. The dominant pollutant is PM2.5, particles smaller than 2.5 micrometers across, small enough to reach deep into the lungs and pass into the bloodstream. Wood smoke also carries carbon monoxide, nitrogen oxides, volatile organic compounds, polycyclic aromatic hydrocarbons, and benzene.
• Working from Home Air Quality Setup — If you work from home full time, you're spending 50 to 70 hours a week in a single room. The air in that room shapes your focus, mood, and the quality of decisions you make. Here's how to set it up.