A scented candle is perfume set on fire in a room you have to breathe in. Here’s what researchers have measured coming off the flame, what happened when one study’s numbers looked bad for candles, and why the cleanest candle is the one you don’t own.
Evidence key Strong Moderate Weak How we grade
In the industry’s own study, the scent made the smoke worse
Burning a scented candle releases far more organic combustion by-products than burning an unscented candle of the same wax: total VOC emission rates reached up to 14,388 micrograms per candle per hour for scented candles versus 70-204 for the four unscented candles tested (24 fuel/fragrance combinations, chamber study). Moderate evidence. Salthammer 2021 Industry-funded
The fragrance added to a candle, not the wax it is made from, is the main driver of extra combustion emissions: unscented candles made from four different waxes (palm, paraffin, soy, stearin) formed the lowest-emission cluster in the study's statistical analysis, while scented versions of the same waxes released far more VOCs, PAHs, and formaldehyde. Moderate evidence. Salthammer 2021 Industry-funded
Benzo[a]pyrene, a combustion-linked carcinogen, was measurable in 5 of 24 candle-burning experiments (up to 12 nanograms per candle per hour) and only in scented candles; none of the 4 unscented candles produced a detectable amount. Moderate evidence. Salthammer 2021 Industry-funded
Keep in mind who paid for those measurements. The candle recipes tested in this study, the criteria for making them, and the study's funding all came from an advisory committee of candle and fragrance industry groups: the European Candle Association, National Candle Association, Latin American Candle Manufacturers Association, and the fragrance houses Arylessence, Belmay, Firmenich, Givaudan, International Flavors & Fragrances, Symrise, and Takasago. Strong evidence. Salthammer 2021 Industry-funded
What’s in the smoke
When candle emissions were converted to realistic household exposure (a 30 m3 room, candle burning 4 hours a day, 4 days a week), most measured pollutants (carbon monoxide, carbon dioxide, formaldehyde, PM2.5, benzene, most VOCs) stayed below international indoor air guide values, but nitrogen dioxide, acrolein, and benzo[a]pyrene exceeded some (not all) of the guide values tested, depending on which health agency's reference value was used. Moderate evidence. Salthammer 2021 Industry-funded
Acetaldehyde, a probable human carcinogen, rose far more than its boiling point alone would predict once candles were lit — one strawberry candle's on/off concentration ratio for acetaldehyde was 3,688-fold, the largest ratio of any of the 34 compounds tracked in the study. Moderate evidence. Ahn et al. 2015
Some of it would never show up on a label. GC-MS of the scented candle's emissions detected methylene chloride (2.71% of peak area, a carcinogen with two known toxic-metabolite pathways) and the phthalate DEHP (0.1%), neither of which would appear on a candle's fragrance or wax label. Moderate evidence. Mohamed et al. 2025
Formaldehyde, and the sentence that got deleted
One Korean lab measured formaldehyde in air pulled straight past the flame. In a lab chamber test, three of six candles — a strawberry candle, a clean-cotton candle, and a plain (unscented) reference candle — produced formaldehyde levels above both the ACGIH occupational guideline (325 ppb) and OSHA's legal limit (730 ppb) while lit, up to 2,098 ppb for the strawberry candle. Moderate evidence. Ahn et al. 2015
Then the candle industry stepped in. The 2016 published critique that prompted Kim et al. to revise their scented-candle formaldehyde paper was authored by two National Candle Association staff members, not independent scientists. Moderate evidence. Kim et al. 2016 After a separate 'exception letter' sent in March 2015 by the National Candle Association, the original study's authors deleted a sentence reporting that formaldehyde in three scented-candle types (925, 1022, and 2098 ppb) exceeded workplace TLV (325 ppb) and PEL (730 ppb) limits, and softened a paper highlight from 'exposure levels of some species are high enough to exceed guideline values' to 'can approach or exceed certain guideline values.' Moderate evidence. Kim et al. 2016
The correction went the other way. Kim et al. 2016 found their own original 2015 emission factors for carbonyl compounds from scented candles (formaldehyde and related aldehydes) had been computationally underestimated by exactly a factor of 250, due to a missed sample-dilution correction; the revised formaldehyde emission factor for the highest-emitting candle (SB-on) rose from 383 ng/g to 95.7 ug/g. Moderate evidence. Kim et al. 2016
Using a mass-balance model of a realistic room scenario (one person present 8 h, two candles lit for 4 h, 20 cubic-meter room, 0.5 air changes per hour), Kim et al. 2016 calculate an 8-hour time-weighted-average formaldehyde concentration exceeding 100 ppb in the worst case, more than 6 times NIOSH's 16 ppb 8-hour recommended exposure limit. Moderate evidence. Kim et al. 2016
Soot and fine particles
Scaled to a realistic 90-square-meter apartment burning 5 candles for 3.5 hours, the highest-soot candle type produced an estimated fine-particle (PM2.5) concentration of 70 micrograms per cubic meter, above the WHO 24-hour air quality guideline of 25 micrograms per cubic meter, while the estimated cancer-risk marker (benzo[a]pyrene equivalent) stayed at, not above, the WHO guideline in both the highest- and lowest-emitting candle scenarios tested. Moderate evidence. Andersen 2021
A flickering (flame-disturbed) candle can emit up to 1,700 times more soot than the same candle burning steadily and undisturbed, based on chamber measurements of one candle type under both conditions. Moderate evidence. Andersen 2021
Ultrafine particle release from a burning candle is highest in the first 30-60 minutes after lighting, on average 3 to 5 times higher than during steady burning, so ventilating a room in the first hour after lighting candles removes more particles than ventilating later in a burn. Moderate evidence. Salthammer 2021 Industry-funded
In a small three-home pilot, the smoke didn’t stay by the candle. Finer particles (PM2.5, PM1) from a lit scented candle did not simply decay with distance: their concentration kept rising over the hour measured at 3 m and 6 m from the candle, while PM10 peaked and declined fastest at the candle itself, consistent with warm particle-laden air traveling along the ceiling into adjacent spaces (the Coanda effect). Weak evidence. Yun et al. 2025
One candle gave off more unlit than lit
One kiwi-melon scented candle emitted more total measured VOC while sitting unlit (12,742 ppbC) than while it was actually burning (2,766 ppbC) — more than 4 times as much — and the unlit figure was comparable to a published emission rate for a carpet cleaner. Moderate evidence. Ahn et al. 2015
Keeping a scented candle unlit doesn't necessarily stop it from adding VOCs to a room's air — one product in this study off-gassed heavily even unlit, so if you want to avoid a candle's chemical emissions, removing the product from the room (not just leaving it unlit) is what this data suggests actually helps. Weak evidence. Ahn et al. 2015
Before candles were lit, the largest class of chemicals detected across the products was esters (30 of 34 dominant compounds identified), which manufacturers disclose on the label only as 'fragrance' or 'aroma oil.' Moderate evidence. Ahn et al. 2015
Wax melts still pollute your air
No flame and no smoke, but still a burst of particles. Scented wax melts, even though they involve no flame or smoke, released enough fragrance terpenes to react with ordinary indoor ozone and trigger bursts of new nanoparticle formation in a full-scale test house, reaching concentrations above 1 million particles per cubic centimeter. Moderate evidence. Patra et al. 2025
The estimated respiratory tract deposited dose of these new nanoparticles from scented wax melt use (median 2.9 x 10^10 particles per minute) was comparable to the dose rates reported for burning a scented candle, running a gas stove, or diesel and natural gas engine exhaust, and about 1,000 times higher than the same test house's background air. Moderate evidence. Patra et al. 2025
In the same test house and wax warmer, an unscented wax melt produced no significant terpene release and no measurable nanoparticle formation, showing that the fragrance chemicals — not the melted wax itself — are what drives the particle-forming reaction. Moderate evidence. Patra et al. 2025
No toxicological testing has been done on the specific nanoparticles formed during scented wax melt use; the authors explicitly call for future studies on their toxicological properties before drawing health conclusions. Moderate evidence. Patra et al. 2025
Unscented candles still pollute
Choosing an unscented candle over a scented one made from the same wax measurably lowers the release of combustion pollutants (VOCs, PAHs, formaldehyde), based on this chamber study, though unscented candles still produce carbon dioxide, carbon monoxide, nitrogen oxides, and ultrafine particles from ordinary combustion. Moderate evidence. Salthammer 2021 Industry-funded
The unscented 'plain' paraffin candle used as this study's own combustion reference produced the third-highest formaldehyde level of the six candles tested when lit (925 ppb), higher than two of the five scented candles. Moderate evidence. Ahn et al. 2015
What it does to lungs in the lab
These are rodent studies with long daily exposures, not measurements in people. They’re still not what you want to read about something sold as relaxing. In rats, burning a scented candle 1-6 hours/day for 8 weeks in a sealed room raised serum TNF-alpha to about 115-220 pg/mL and IL-6 to about 120-215 pg/mL (both approximate, read from Fig. 3), versus about 30 and 50 pg/mL in fresh-air controls, and caused progressively worse lung damage on histopathology, reaching severe injury (fibrosis, necrobiotic changes, dense inflammatory infiltration) after as little as 3 hours/day. Moderate evidence. Mohamed et al. 2025
An "unscented" candle was not toxicologically inert: its headspace was 51.65% toluene by GC-MS peak area, and burning it in rats for 8 weeks still caused duration-dependent serum inflammation and lung injury compared with fresh air, though consistently less severe than the scented candle at every exposure duration tested. Moderate evidence. Mohamed et al. 2025
Scented candle fumes alone, at an occupational-intensity chamber dose, significantly increased inflammatory cytokines (TNF-alpha, IL-6, IL-1beta) and NF-kB activity in the lungs of mice, and caused measurable lung histological injury, without significantly raising heart inflammatory markers, heart NF-kB, cardiac injury markers (LDH, CPK, CKMB), plasma BNP, or heart HIF-1alpha above unexposed controls. Moderate evidence. Chandrasekaran 2021
What people who burn them report
Among 472 Saudi university students who used scented candles, 24.8% reported at least one adverse health symptom in the past 12 months, most commonly headache (15.2%), shortness of breath (8.9%) and cough (7.8%); skin irritation was rare (1.3%). Moderate evidence. Al Khathlan et al. 2023
Among the same scented-candle users, respiratory-specific complaints in the past 12 months included waking with chest tightness (36.0%), waking from a coughing attack (33.1%), nasal allergy/hay fever (25.0%), a diagnosed respiratory illness (16.3%), a shortness-of-breath attack on waking (17.2%), wheezing (14.6%) and an asthma attack (9.5%). Moderate evidence. Al Khathlan et al. 2023
No safety proof required, says one review
No law requires manufacturers of scented candles to prove the safety of their products before releasing them onto the market in the US or EU, per the authors. Weak evidence. Adamowicz 2019
What to do instead
Blow it out and get it out of the house. Dermatologists already tell fragrance-allergic patients to. For fragrance-allergic people, dermatologists recommend removing diffusers, scented candles, room deodorizers, incense, and wax melts, switching household cleaners, wipes, and laundry products to fragrance-free, and bringing your own shampoo to the salon and your own oil to massages. Moderate evidence. Rodriguez 2024
In a small study of three homes, clearing out scented products and other chemical sources cut measured pollutants. In a small case series, removing mothballs and reducing scented cleaning/personal-care products, candles, and stored chemicals cut targeted indoor-air VOC categories by roughly a quarter to over 95% (e.g., naphthalene from mothballs down 96%, p-dichlorobenzene from moth crystals down 56-80%, odorants/fragrances down 26-80%) across three homes, measured by GC/MS air sampling before and after a structured home intervention. Moderate evidence. Rincón et al. 2024
- Skip candles, wax melts, and incense, especially around anyone who’s told you scent makes them sick.
- For a room that smells stale, open a window and find the source of the odor instead of covering it up.
- Stop giving scented candles as gifts, and say so if people keep giving them to you.
- Ask restaurants, spas, hotels, and offices to put the candles out. A printable sign or a letter can do the asking for you.
More on what scent does to indoor air: air fresheners and scented cleaning.
Every claim above shows its evidence grade and names its source. See all the candle facts and the claims we won’t make. Information, not medical advice.