Quick answer
From what the research shows, burning a candle now and then isn't bad for most people. Chamber tests find a well-burning candle gives off small amounts of soot, nitrogen dioxide and fragrance compounds, mostly well under indoor-air guidelines. Fragrance matters more than wax, and a long wick or a draft makes more soot. People with asthma should burn fewer: a 2023 study found mild airway inflammation after five hours of candle smoke.
Most of the time, for most people, no. The studies on candle emissions are more reassuring than you might expect: in chamber tests, a candle burning properly gives off small amounts of soot, gases and fragrance compounds, and most of them land well under indoor-air guidelines.
A candle still adds something to the air, though, and what it adds depends far more on the fragrance, the flame and the room than on the wax. We make candles, so we've read these studies closely. And if you or someone you live with has asthma, one recent study is a good reason to burn fewer and open a window. Here's what's in candle smoke, what the studies found, and how to burn a candle so it adds as little as possible.
What's in Candle Smoke
A candle burns gas, not wax. The flame melts the wax, the wick draws it up, and the heat turns it into vapor, which is what burns (our piece on how candles work walks through it). When that burn is complete, most of what comes off is carbon dioxide and water vapor. No flame burns perfectly, though, so a few other things come with it.
| What comes off | Where it comes from | What raises it |
|---|---|---|
| Soot and fine particles | Carbon that doesn't finish burning in the yellow part of the flame | A long wick, a flickering flame, a draft |
| Ultrafine particles | Every flame, with a burst in the first half hour to hour after lighting | Lighting up, several candles at once |
| Nitrogen dioxide | Nitrogen from the air reacting in the heat of any flame | More flames, a small closed room |
| Formaldehyde, benzene and other VOCs | Fragrance evaporating from the melt pool and breaking down in the flame | The fragrance, and how much of it |
| PAHs | Incomplete burning, the same process that makes soot | A sooting flame, fragrance |
Lead used to be on this list. Some older wicks had metal cores that released it, and the US banned lead-cored wicks in 2003. Everything left on the list comes from the flame itself or from the fragrance, which is why how a candle is made and how it's burned both matter.
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What the Studies Found
The most thorough recent test came out of Germany in 2021. A team led by Tunga Salthammer at the Fraunhofer Institute burned scented and unscented candles made from four waxes, with five fragrance families, in 24 tests of seven hours each, and measured everything in the table above (the full study is published). The work was set up together with candle makers and fragrance houses.
Scaled to an ordinary room, most results came in well under indoor-air guidelines. The exceptions were short-term nitrogen dioxide, which went over Germany's strict short-term guide value (the World Health Organization's was crossed in two tests; long-term limits were met), and a few acrolein and benzo[a]pyrene readings measured against guide values so low they sit near what instruments can detect. The authors also point out that their tests burned four candles at once in a small chamber, which is more than most people burn.
An earlier chamber study from Milan found something that matters as much as any single number: emissions varied a lot from one scented candle to the next. A label can't tell you a candle's emission rate, so how you burn it carries a lot of the weight.
The study that gives us the most pause came from Aarhus University in Denmark in 2023. Thirty-six young adults with mild asthma each spent five hours in a room with candle smoke, a room with cooking fumes and a room with filtered air, without knowing which was which. The candle room ran at about 90 micrograms of fine particles per cubic meter, a deliberately heavy dose. Both candles and cooking caused mild inflammation in the small airways, and the researchers' advice was simple: turn the ventilation up when you cook or burn candles, especially if you have asthma.
Are Scented Candles Worse?
Mostly, yes, and the fragrance is why. In the 2021 tests, unscented candles gave off far less organic material than scented ones. The extra came from fragrance compounds evaporating off the melt pool and breaking down in the flame, and fresh and fruity fragrances produced more formaldehyde than floral, spicy or oriental ones. The wax mattered less than you'd expect: no wax came out consistently cleaner than the others (more on that in Are Soy Candles Safe?).
So if you want a scented candle, the fragrance is the thing worth asking a brand about, more than the wax. Here's ours: phthalate-free fragrance oils, at 8% on Renew and Uplift and 4% on Unwind, with what we leave out on our ingredients page. Essential oils aren't automatically the cleaner pick either. They're blends of aromatic compounds too, and some are a real concern around pets (our guide to essential oils vs fragrance oils goes deeper).
Soot: The Part You Can See
Soot is the one most people notice: a black ring inside a container, a gray smudge on the wall above where a candle sits. It's unburned carbon. The yellow part of a flame is full of glowing carbon particles, and in a steady flame the hot outer edge burns nearly all of them off before they leave. When the flame flickers or the wick gets too long, some escape as smoke.
A smoking candle is also the clearest sign that something needs fixing, which makes soot the easiest part of all this to do something about. Our guide to clean burning candles covers what a clean flame looks like, and if you burn wood wicks, our wood wick guide covers the ones that won't behave.
Who Should Burn Fewer, or None
If you or someone you live with has asthma or another lung condition, the Aarhus study is a good reason to burn fewer candles, keep them to well-aired rooms, and skip them on days when breathing is already hard. The same goes for anyone a doctor has told to avoid smoke. A candle is a small flame, but it's still a flame.
Headaches are their own question. If candles reliably give you one, the usual suspects are the wax, the fragrance and how the candle is burned, and our Headache Test walks through how to find which. Pets breathe the same air, so if you have cats or dogs, check how a candle is scented. For ours: Scented with phthalate-free fragrance oils, not essential oils. Veterinary poison centers list tea tree, eucalyptus, peppermint, and citrus among the essential oils toxic to cats and dogs.
How to Burn a Candle So It Adds Less to the Air
Most of what you can control comes down to the flame and the room. None of this makes a candle smoke-free, since a flame always adds something to the air, but these habits keep that something small. Several come straight from the National Candle Association's burning guide, and our guide to burning a candle properly has the rest, including the first burn.
- 1Trim the wick to a quarter inchBefore every burn, wood wicks and cotton alike. A long wick is the most common reason a candle smokes.
- 2Keep it out of draftsAway from fans, vents and windows that blow across the flame. You want fresh air in the room, not wind on the flame.
- 3Burn fewer at onceEvery flame adds its own share. One candle on a table is a different room from a dozen.
- 4Give the room some airCrack a window on the far side of the room or run the fan, and air the room out after you put the candle out.
- 5Keep burns to about four hoursThen let the candle cool for a couple of hours before you light it again.
- 6Fix a smoking flame right awayPut it out, let it cool, trim the wick and relight it. Smoke is the candle telling you something's off.
How We Think About Our Own Candles
We use 100% soy wax, natural wood wicks and phthalate-free fragrance oils, with wick clips welded in place instead of glued. Those choices are about what goes into a candle.
What comes off a flame is a separate question, and the answer holds for any candle, ours included: some soot, some nitrogen dioxide, some fragrance in the air, and less of all three when it's burned well. The habits above apply to The Sero Candle just like any other.
The Short Version
A candle now and then isn't bad for most people. Burning one gives off small amounts of soot, nitrogen dioxide and fragrance compounds, and in chamber tests most of that lands well under indoor-air guidelines. Fragrance matters more than wax, and the flame matters more than either: trim the wick, keep it out of drafts, burn fewer at once and give the room some air. If anyone at home has asthma, burn less and air the room more.
Sources
- Salthammer, Gu, Wientzek, Harrington and Thomann, "Measurement and evaluation of gaseous and particulate emissions from burning scented and unscented candles," Environment International, 2021
- Derudi et al., "Emissions of air pollutants from scented candles burning in a test chamber," Atmospheric Environment, 2012
- Laursen et al., "Airway and systemic biomarkers of health effects after short-term exposure to indoor ultrafine particles from cooking and candles," Particle and Fibre Toxicology, 2023
- Aarhus University, "Burning candles and fumes from cooking is harmful for people with mild asthma," 2023
- National Candle Association, "Your Foolproof Guide to Burning a Candle Correctly"
- US Consumer Product Safety Commission, "CPSC Bans Candles With Lead-Cored Wicks," 2003
Common Questions About Candles and Your Health
Are scented candles bad for you?
Scented candles give off more than unscented ones, mostly because fragrance compounds evaporate from the melt pool and break down in the flame. In a 2021 study of 24 candle burns, most results still came in under indoor-air guidelines once scaled to an ordinary room. Burn them with a trimmed wick, out of drafts, in a room with some fresh air, and burn fewer if anyone at home has asthma.
Is it bad to breathe in candle smoke?
Visible smoke means a candle isn't burning cleanly. It's mostly soot, carbon that escapes the flame when the wick is too long or a draft pulls the flame around. Put the candle out, let it cool, trim the wick to a quarter inch and relight it somewhere still. A steady flame makes far less smoke, though no flame adds nothing to the air.
Are candles bad for your lungs?
The clearest evidence is for people with asthma. In a 2023 Aarhus University study, young adults with mild asthma showed mild inflammation in the small airways after five hours in a room with candle smoke, and cooking fumes did much the same. Heavy, long exposure in a closed room is where effects show up, so for anyone, fewer flames and fresh air do the most.
Do candles release toxins?
A burning candle gives off small amounts of compounds you wouldn't choose to breathe, including formaldehyde, benzene, nitrogen dioxide and traces of PAHs, mostly made in the flame rather than added as ingredients. In chamber tests most stayed well under indoor-air guidelines for an ordinary room. Lead-cored wicks, once the real hazard, have been banned in the US since 2003.
What is the black soot from candles?
It's carbon that didn't finish burning. The yellow part of a flame is full of glowing carbon particles, and a steady flame burns nearly all of them off at its hot outer edge. A long wick or a draft lets some escape as smoke, which leaves black rings on containers and gray smudges on walls. A quarter-inch wick and a spot out of moving air are the fix.
Is it safe to burn a candle every day?
Plenty of people do. What matters is how each burn goes: keep it to about four hours, trim the wick before you light it, and give the room some fresh air, especially in winter when the windows stay shut. If someone at home has asthma or another lung condition, burn less often and keep candles to a well-aired room.
