The Furnace Just Kicked On. Here’s What the First Heat of the Season Puts in the Air You Breathe

Part 2 of our Healthy Lung Month series. The windows close, the heat comes on, and the air inside your home changes overnight. What that burning smell means, the gas you can’t smell at all, and a first-cold-night checklist for your lungs.

Key TAKEAWAYS

• The house closes up. When windows stay shut, less outdoor air comes in to dilute what’s produced inside, and the EPA says inadequate ventilation can increase indoor pollutant levels.1

• A burning smell for an hour or two is usually dust. A smell that lingers, smells metallic, or smells like rotten eggs is not.2

• Carbon monoxide has no smell at all. It kills more than 400 Americans a year and sends more than 100,000 to the emergency room.3

• Book the inspection. CDC, CPSC, and EPA all recommend a yearly professional check of furnaces and other fuel-burning appliances.4

• Clean ducts, cleaner start. Every time the system runs, it moves air through your ductwork. Industry guidance calls for a professional cleaning every 3 to 5 years, and more often with pets or allergies.5

• Wood smoke is fine-particle pollution. If you burn, burn dry wood, under 20% moisture, in a well-maintained appliance.6

Why the first cold night matters for your lungs

Last week, we looked at lung cancer in people who never smoked and how much of what we breathe happens indoors. This week is about the moment that changes the air inside most homes every fall: the first time the heat comes on.

Americans spend about 90% of their time indoors, where some pollutants already run 2 to 5 times higher than outdoors.7 Heating season tilts things further. Outdoor air enters a home through open windows, small leaks, and mechanical ventilation. When those are limited, the air exchange rate drops and pollutant levels can rise.8 At the same time, many homes start running fuel-burning appliances like furnaces, boilers, fireplaces, and space heaters every day. The EPA notes that unvented or malfunctioning appliances can release “higher and sometimes dangerous levels of pollutants indoors.”9

None of this means you should be afraid of your furnace. It means the first cold week is a good time for a quick lung-health tune-up.

Why does my furnace smell like burning?

Almost everyone notices it: the heat kicks on and the house smells faintly scorched. In most cases, it’s dust that settled inside the unit over the summer burning off the first time it heats up. Some heating manufacturers say to let the unit run for an hour or two, after which “the smell should subside or disappear.”10

Here’s when a smell is a warning sign instead of a nuisance:11

• It lingers or gets stronger. Turn the system off and call an HVAC professional.

• It smells metallic or like burning plastic. This can point to an electrical problem or something that fell into a vent. Shut the unit off and call a pro.

• It smells like rotten eggs. Natural gas has no odor of its own, so utilities add a sulfur compound called mercaptan to make leaks noticeable. Leave right away. Don’t flip light switches, use your phone, or try to find the leak. Once you’re outside and safe, call your gas utility or 911.

Carbon monoxide: the danger you can’t smell

A burning smell gets your attention. Carbon monoxide (CO) doesn’t. It’s colorless and odorless, and it’s produced when fuel burns. According to the CDC, unintentional CO poisoning not linked to fires kills more than 400 Americans every year, sends more than 100,000 to the emergency department, and leads to more than 14,000 hospitalizations.12

The symptoms are easy to mistake for the flu: headache, dizziness, weakness, nausea, vomiting, chest pain, and confusion. People who are asleep can die before they ever have symptoms.13 If a CO alarm sounds, get everyone into fresh air right away and call 911.

Where to put a carbon monoxide detector

• Install CO alarms on every level of your home and outside sleeping areas.14

• Choose battery-operated or battery-backup models, and check the batteries when you change your clocks. Daylight saving time ends Sunday, November 1, 2026, so this is the month.15

• Replace detectors on the manufacturer’s schedule, typically every five years.16

Book the annual furnace inspection

The CDC, the Consumer Product Safety Commission, and the EPA agree: have a qualified technician check your furnace, boiler, water heater, and other fuel-burning appliances every year, and make sure they’re properly vented. Have chimneys inspected or cleaned annually too.17

An inspection isn’t only about carbon monoxide. Faulty or poorly vented combustion appliances are also a source of nitrogen dioxide (NO₂), a gas that irritates the eyes, nose, throat, and airways. Even lower levels may make airways more sensitive in some people with asthma, reduce lung function in people with COPD, and raise the risk of respiratory infections, especially in young children.18

Start the season with clean ducts

Your ductwork is the delivery system for every breath of heated air in your home. All summer, dust, pet hair, pollen, and debris settle inside it. When the furnace kicks on for the season, that same air path starts moving again, room by room, many times a day. The National Air Duct Cleaners Association (NADCA) notes that air in a typical home is pulled through the system and re-circulated five to seven times a day, and that contaminants build up in the ductwork over time.19 A professional cleaning removes that buildup at the source, so it isn’t sitting in the path of the air you breathe all winter.

How often should air ducts be cleaned?

Industry guidance, including NADCA’s, puts most homes on a schedule of every 3 to 5 years.20 Plan on more frequent cleaning, every 2 to 3 years, if you:21

• Have pets that shed

• Have family members with allergies or asthma

• Have a smoker in the home

• Live in a dusty area, or near busy roads or construction

• Recently remodeled, or had water damage

Signs your ducts need attention now

Don’t wait for the calendar if you see the warning signs. The EPA recommends cleaning when there is substantial visible mold growth inside hard-surface ducts, when ducts are infested with rodents or insects, or when they’re clogged with excessive dust and debris.22 NADCA’s rule of thumb is simple: “if your air ducts look dirty, they probably are.”23

When you book, choose a reputable, certified HVAC cleaning professional.24 Then put a fresh filter in the furnace before the first run of the season. For help choosing one, see our cold and flu season home air checklist.

Fireplaces and wood stoves: cozy, but it’s still smoke

A fire on the first cold night is one of the season’s pleasures. It’s also combustion inside your home. The EPA describes wood smoke as “a complex mixture of gases and fine particles” that includes toxic air pollutants such as benzene, formaldehyde, acrolein, and polycyclic aromatic hydrocarbons. The agency says “the biggest health threat from smoke is from fine particles,” which can worsen asthma, trigger asthma attacks, and contribute to heart attacks and strokes in people already at risk.25

If you burn, burn smarter. The EPA’s Burn Wise program recommends:26

• Burn only dry, split wood that has been seasoned for at least six months.

• Check it with a wood moisture meter. Wood burns best at under 20% moisture.

• Never burn household trash, cardboard, plastics, or painted or pressure-treated wood.

• Open the flue before you light a fire, and consider upgrading older stoves to an EPA-certified model with tight-fitting doors.

Gas stoves and space heaters

With the windows shut, cooking fumes have fewer places to go. Gas stoves and kerosene heaters are among the main indoor sources of nitrogen dioxide, and the EPA’s first recommendation is to vent those sources to the outdoors with an exhaust fan over the stove.27 If you’re replacing an unvented space heater, the EPA recommends choosing a vented model.28

Filtration can help with what venting misses. In a randomized Johns Hopkins study of Baltimore homes with gas stoves, Austin Air purifiers reduced nitrogen dioxide by 27% in the kitchen and 22% in the bedroom within one week.29

Electric space heaters don’t burn fuel, but they carry a different risk. The CPSC estimates portable heaters are involved in about 1,600 fires a year, causing roughly 70 deaths. Keep heaters at least three feet from drapes, furniture, and bedding; plug them directly into a wall outlet, not an extension cord or power strip; and turn them off before you go to sleep.30

Your first-cold-night checklist

Before the heat comes on

• Put a fresh filter in the furnace.

• Test every carbon monoxide alarm, and add one on any level that doesn’t have one.

• Schedule the yearly inspection for your furnace, boiler, water heater, and chimney.

The first time it runs

• Expect a faint dusty smell for an hour or two. Open a window briefly if it bothers you.

• Lingering, metallic, or plastic smell: shut it off and call a pro. Rotten eggs: get out, then call.

All season long

• Run the range hood, vented outdoors, every time you cook on a gas stove.

• Burn only dry, seasoned wood, and keep the flue open while the fire burns.

• Give space heaters three feet of clearance and turn them off at bedtime.

• Filter the air in the rooms where you spend the most time, especially the bedroom.

Where Austin Air fits in

Heating season brings two kinds of pollution indoors: fine particles from things like wood smoke and cooking, and gases from combustion. HEPA filters capture particles. For gases, the EPA notes that activated carbon filters can be effective “provided that there is a large amount of material used in the filter.”31

That’s why the HealthMate is built around up to 15 pounds of activated carbon and mineral-based filtration, paired with 60 square feet of True Medical Grade HEPA certified to capture 99% of airborne particles down to 0.1 microns.32 For the bedroom, where you spend about a third of your life, the Bedroom Machine adds a 5th stage of military-grade carbon cloth.33 Both are Built in Buffalo with solid steel housings and filters designed to last up to five years in normal residential use. Explore our air purifiers.

Frequently asked questions

Why does my furnace smell like burning when I first turn it on?

Usually it’s dust that collected inside the unit over the off-season burning off. The smell should fade after the furnace runs for an hour or two. If it lingers, gets worse, or smells metallic, turn the furnace off and call an HVAC professional.34

Is it normal to smell gas near my furnace?

No. A rotten-egg smell can mean a natural gas leak. Leave right away without using light switches or phones, and call your gas utility or 911 once you’re outside.35

Where should I put a carbon monoxide detector?

The CPSC recommends CO alarms on every level of the home and outside sleeping areas. The CDC recommends battery-operated or battery-backup detectors and replacing them about every five years.36

How often should a furnace be inspected?

Once a year. The CDC, CPSC, and EPA all recommend an annual inspection of furnaces and other fuel-burning appliances by a qualified technician.37

How often should I have my air ducts cleaned?

Most homes should have their ducts professionally cleaned every 3 to 5 years, according to industry guidance including NADCA’s, and every 2 to 3 years with pets, allergies, or a smoker in the home. Clean them right away if there is visible mold, pests, or heavy dust and debris.38

Is wood smoke bad for your lungs?

It can be. The EPA says fine particles are the biggest health threat from wood smoke, and they can worsen asthma and trigger asthma attacks. Burning only dry, seasoned wood in a well-maintained appliance reduces smoke.39

How can I improve indoor air quality in winter?

Start with the sources: maintain and vent fuel-burning appliances, run a vented range hood when cooking, and burn wood cleanly or not at all. Then filter. The EPA notes that filtration supplements, but doesn’t replace, source control and ventilation.40

The bottom line

The first cold night is a good reminder that the air inside your home changes with the seasons. A few minutes now, testing alarms, booking the inspection, swapping the filter, and deciding how you’ll handle smoke and cooking fumes, protects your lungs all winter. Next week, in the final part of our Healthy Lung Month series, we’ll follow a single particle on its journey into the lungs, and show why size matters so much.

That’s the Impact of Austin.