Air quality at home: particles, ozone and cleaner wood smoke

Hollow & Headwater is an independent journal, not a government office. This page explains air quality in general terms; for official air data, alerts or local burning rules, go to your state or local air agency.

On a still January morning in a mountain valley, you can often see the air before you breathe it. A flat, blue-gray layer hangs at about rooftop height. Smoke from a dozen chimneys rises a little, bends and stops, as if it had hit a ceiling. Up on the ridge the sky is clean and sharp. By lunch the sun has warmed the valley floor and the layer is gone.

That layer is a decent introduction to household air quality. Most of it is invisible, all of it is shaped by weather, and a surprising share of it is made at home. This guide covers the pollutants that matter most to households, why wood smoke deserves special attention, how the air gets measured, and what keeps the air inside a house healthy.

The two pollutants behind most bad days

The Air Quality Index on weather apps is built from five common outdoor pollutants the U.S. EPA tracks under the Clean Air Act: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide. In most American towns, most of the time, the first two decide whether a day is green, yellow or worse. For a band-by-band reading of the colors, see our guide to what the AQI means on a smoky day.

Fine particles

PM2.5 is shorthand for particles 2.5 micrometers across or smaller, far thinner than a human hair. They slip past the nose and throat and settle deep in the lungs, and some may reach the bloodstream. Long-term exposure is linked to heart and lung disease, and short spikes can set off asthma attacks.

Almost anything that burns makes them: wood stoves, wildfires, diesel engines, power plants, a pan of onions on high heat. Some also form in the air itself as gases react and condense. Coarser PM10 particles, like road dust and soil blowing off dry fields, mostly lodge higher in the airways.

Ozone

Ground-level ozone isn't emitted directly. It forms when nitrogen oxides from engines and power plants react with volatile organic compounds from fuels, solvents and even trees, with sunlight driving the chemistry. That's why it peaks on hot, bright, still afternoons, fades after dark, and is mostly a warm-season problem. High in the stratosphere the same molecule screens out ultraviolet light. At breathing height it inflames airways.

Ozone also travels while it forms, so a rural valley downwind of a city can see higher ozone than the city itself.

Wood smoke, close to home

Wildfire smoke gets the headlines, but in many small towns the worst particle days come in winter, from home heating. Wood smoke is mostly fine particles, plus carbon monoxide and a long list of organic compounds, some of them toxic, such as benzene and formaldehyde. It smells cozy. Your lungs don't care.

Geography makes it worse. On clear, calm winter nights the ground loses heat fast, and the air touching it cools and grows dense. Cold air drains downhill and pools in hollows and valley floors under a lid of warmer air, a temperature inversion. Smoke released into that pool can't rise through the lid, so it spreads sideways and builds up until the sun breaks the inversion, sometimes not for several days. Appalachian hollows, the valleys of western Montana and Idaho, interior Alaska and foothill towns in the Cascades all know this pattern. In a narrow hollow, one smoky chimney can foul the air for every household in it.

Some counties and air districts, especially in the West, call no-burn days during stagnant weather; if you heat with wood, find out whether yours does.

Burn-wise habits for stoves and fireplaces

The U.S. EPA runs a public education campaign on cleaner wood burning called Burn Wise, and its core advice matches what experienced wood burners already know. Smoke is unburned fuel. A fire that smokes heavily is wasting wood and polluting at the same time, so cleaner burning and cheaper heating usually go together.

A checklist for the heating season:

• Burn only dry, seasoned wood. Split it, stack it off the ground with the top covered and the sides open to the wind, and give it time: often six months to a year, longer for dense hardwoods like oak. EPA's guidance is a moisture content of 20 percent or less, which an inexpensive moisture meter can check on a freshly split face. Dry wood sounds hollow when two pieces are knocked together; wet wood hisses and steams.
• Start hot, then load small. Use plenty of kindling, open the air control fully until the fire is established, and add a few pieces at a time.
• Don't damp it down to smolder overnight. Starving a fire of air makes the most smoke and the most creosote.
• Never burn trash, cardboard, painted or treated lumber, or plastics. They release toxic compounds and can damage the stove and chimney.
• Check the chimney top. Thick gray or yellowish smoke means a poor burn. The goal is a thin, nearly clear plume; white wisps that vanish quickly on a cold morning are often just water vapor.
• Have the chimney swept and inspected regularly, typically at least once a year for a stove in steady use.
• Consider the appliance. Newer EPA-certified wood stoves burn much more cleanly than old uncertified ones, and an open fireplace sends most of its heat straight up the flue.

If you're weighing whether to keep burning at all, our piece on heat pumps in cold mountain valleys looks at the alternatives in hill country.

How the air gets measured

The official numbers come from regulatory monitors run by state, local and tribal air agencies. Their instruments meet federal reference or equivalent methods. They're expensive, so most sit near population centers, and a rural county may have none.

Readings flow into AirNow, the national system the EPA runs with those agencies, which converts concentrations into AQI values. Each pollutant gets its own AQI number, and the one you see is the highest. For current conditions, AirNow uses a method called the NowCast that weights the most recent hours most heavily, so the number responds within a few hours when smoke rolls in.

Low-cost consumer sensors have filled in the map. They count particles by how they scatter a laser beam and react quickly, but tend to read high in humid air and need correction. AirNow and its Fire and Smoke Map show corrected sensor readings alongside official monitors. Our guide to public maps and data tools explains how to find the stations nearest you.

Common household air pollutants at a glance

Indoor air basics

Most Americans spend most of their time indoors, where some pollutants run higher than outside. It's also where you have the most control, using three moves in order.

Control the source. This does more than any gadget. Keep combustion appliances tuned and vented, fix the stove that puffs smoke when you open the door, store paint and gasoline outside the house, and never idle a car in an attached garage. Put a carbon monoxide alarm on every level and near sleeping areas, since CO has no smell.

Ventilate. Run a range hood that exhausts outdoors whenever you cook, especially on a gas stove, and run bathroom fans during and after showers. Open windows when the outdoor air is clean; on bad days, keep them shut and lean on filtration.

Filter and manage moisture. A portable air cleaner with a true HEPA filter, sized for the room by its clean air delivery rate (CADR), cuts fine particles where you sleep. Skip devices that generate ozone on purpose; the EPA has long cautioned against them. Keep indoor humidity roughly between 30 and 50 percent to discourage mold and dust mites.

One indoor pollutant ignores all of this: radon, a radioactive gas from the ground that only a test will reveal. Our guide to radon testing and fixes covers the numbers and the options.

FAQ

Is a fireplace better or worse than a wood stove for air quality?

Usually worse. An open fireplace has little control over its air supply, so combustion is incomplete and smoky. A well-run certified stove or fireplace insert burns far more cleanly on the same wood.

Why does my town get smoky only on certain winter nights?

Look at the sky and the wind. Clear, calm nights produce the strongest inversions and trap smoke near the ground. Cloudy or breezy nights mix the air and carry smoke away.

What's the single most useful change a wood-burning household can make?

Burn dry wood. Seasoned, split, covered wood fixes more smoke problems than anything else, and it puts more heat into the house from every armload you carry in through the snow.