Every fact on this site, with its evidence grade, its caveats, and a link to the source.
We start you on the strong ones: findings that have been replicated or come from one large, well-controlled study. Moderate and weak facts are here too, labeled for what they are. Use “Link to this fact” to share a single card.
Fragrance is everywhere / Self-reported survey
Among 718 non-smoking Saudi university students surveyed in 2020, 65.7% (472) reported using scented candles, and usage was far more common among women (74.9% of female respondents) than men (28.4% of male respondents, chi-square p=0.0001).
Moderate evidence: Convenience/snowball sample recruited via WhatsApp and Twitter, not a probability sample; may not generalize beyond young, social-media-active Saudi university students.
Source: Al Khathlan et al. 2023, BMC Public Health Link to this fact
Fragrance is everywhere / Other evidence
Researchers found multiple fragrance sources (cosmetics, air fresheners, cleaning/laundry products, fragrance-emitting devices, or candles) in every one of the 37 homes they surveyed for a chemical-intolerance study.
Moderate evidence.
Caveats
Sample is self-selected chemical-intolerance patients recruited from a specialty clinic, not a general-population sample; fragrance ubiquity may be overrepresented relative to the general population's home inventories.
Source: Rincón et al. 2024, Primary Health Care Research & Development Link to this fact
Green, natural and unscented / Measured in people or real products
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.
Caveats
Single lab, six products, no replication; wax type (paraffin vs. palm) did not track cleanly with formaldehyde output either, so this alone doesn't identify which factor (wax, wick, dye, or fragrance) actually controls it.
Source: Ahn et al. 2015, Journal of Hazardous Materials Link to this fact
Health effects / Measured in people or real products
Burning candles indoors measurably shifted a marker of heart-rhythm regulation (heart rate variability) toward a more 'rest and digest' pattern within about 30-90 minutes, in a chamber study using candle-smoke levels similar to real dinner-table use.
Moderate evidence: Single small (n=22) study, one lab; heart rate itself did not change; the effect (HF-HRV +22.3%, p=0.01 vs. clean air) is not yet independently replicated.
Source: Hagerman et al. 2014, Atmospheric Environment Link to this fact
Health effects / Measured in people or real products
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.
Caveats
Modeled estimate from measured chamber emission factors scaled with assumed apartment volume, air exchange rate, and deposition loss rate, not a direct measurement in an actual apartment; represents burning 5 candles at once, a higher-than-typical single-evening scenario.
Health effects / Measured in people or real products
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.
Caveats
Acrolein exceeded the US EPA reference concentration in all 8 experiments where it was quantifiable (compliance factor 1.3-6.3); benzo[a]pyrene exceeded the WHO guide value (factor 0.32-6.35) but not the more lenient US EPA reference concentration; nitrogen dioxide exceeded a precautionary German guide value but not the WHO/EU long-term value. Single chamber study; guide-value comparisons vary substantially by which agency's number is applied.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
Health effects / Measured in people or real products
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.
Caveats
Acetaldehyde stayed far below the ACGIH/OSHA guideline values in absolute terms (max 640 ppb vs. 25,000/200,000 ppb guidelines); this finding is about combustion massively amplifying this one compound's relative output, not about it reaching a regulatory threshold.
Source: Ahn et al. 2015, Journal of Hazardous Materials Link to this fact
Health effects / Measured in people or real products
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: Single lab, self-corrected; independent replication of the corrected emission factors has not been reported.
Source: Kim et al. 2016, Journal of Hazardous Materials Link to this fact
Health effects / Animal study
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.
Caveats
Single mouse study, N=8 per group, no independent replication; the candle-only lung effect is a within-study finding, but the same paper found no cardiac effect from candle fumes alone, so the finding is respiratory-specific in this model.
Source: Chandrasekaran 2021, Current Research in Toxicology Link to this fact
Health effects / Animal study
In mice, chronic social-disruption stress combined with scented candle fume exposure produced significantly worse cardiopulmonary inflammation and injury (BALF/lung/heart cytokines, NF-kB activity, serum LDH/CPK/CKMB, plasma BNP, heart HIF-1alpha, and lung histological injury) than either stress or candle fumes alone, though no group showed significant heart tissue histopathology.
Moderate evidence.
Caveats
Single lab, single mouse cohort (N=8/group), no independent replication; the stress model (repeated social defeat) and candle-fume dose are both occupational-intensity, so the additive effect size may not generalize to lower-intensity human exposures.
Source: Chandrasekaran 2021, Current Research in Toxicology Link to this fact
Health effects / Self-reported survey
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.
Caveats
Cross-sectional self-report; exposure and symptoms measured simultaneously so recall bias and reverse causation cannot be excluded, and no chemical/particulate exposure was measured alongside symptoms.
Source: Al Khathlan et al. 2023, BMC Public Health Link to this fact
Health effects / Self-reported survey
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: Self-reported, non-clinically-confirmed symptoms in a non-representative convenience sample; no comparison group of non-candle-users was asked the same respiratory questions.
Source: Al Khathlan et al. 2023, BMC Public Health Link to this fact
Health effects / Other evidence
Polycyclic aromatic hydrocarbons identified as carcinogens, including naphthalene, anthracene, and pyrene, have been found in candle fumes from wax, aroma substances or combustion dyes.
Moderate evidence: Directionally consistent with our own chamber PAH data (Salthammer 2021, Andersen 2021), though the editorial's own citation (Orecchio 2011) is not itself in this evidence base.
Source: Nazir et al. 2024, Annals of Medicine & Surgery Link to this fact
Health effects / Animal study
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.
Caveats
Single lab, one scented candle product (brand/composition undisclosed), n=6 rats/group; values for cytokines and oxidative-stress markers are approximate, read from bar charts rather than printed as numbers in the text.
Source: Mohamed et al. 2025, Frontiers in Public Health Link to this fact
Health effects / Test-chamber measurement
Scented candle fumes failed to reduce chronic-stress-induced serum corticosterone and instead increased inflammatory biomarkers, proinflammatory cytokine production, and NF-kB activation in the lungs and heart, causing cardiopulmonary injury.
Moderate evidence.
Caveats
Single-lab mouse study at an occupational-intensity chamber dose (already our own grading for this primary at C328); not a demonstration that candle scent itself fails to provide subjective psychological relief in humans.
Source: Singh 2023, Environmental Science & Technology Link to this fact
Hidden ingredients / Review of other studies
Fragrance turns up in products people don't think of as scented: household cleaners and detergents, tampons and panty liners, medicines, medical devices, toys, and candles.
Moderate evidence: Qualitative list of exposure sources.
Source: Sukakul 2024, Acta Dermato-Venereologica Industry-funded Link to this fact
Hidden ingredients / Review of other studies
Unlike most retail candles labeled only "fragrance," the candles in this study had their full wax and fragrance formulations precisely known and reported, because they were purpose-built for the research with fragrance industry cooperation.
Moderate evidence.
Caveats
This transparency applies only to these custom research candles, not to any candle sold at retail; the study does not show that any specific commercial candle's contents are disclosed.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
Hidden ingredients / Measured in people or real products
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.
Caveats
Most of these ester identities were quantified by a predictive (ECN) method rather than authentic calibration standards, so individual concentrations carry more uncertainty than the 24 directly-calibrated target compounds; no specific health hazard was established here for most of the named esters.
Source: Ahn et al. 2015, Journal of Hazardous Materials Link to this fact
Hidden ingredients / Animal study
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.
Caveats
Detected in headspace/combustion emissions, not necessarily added as ingredients; percentages are of total VOC peak area, not of candle mass, and the specific candle's ingredient list is not published.
Source: Mohamed et al. 2025, Frontiers in Public Health Link to this fact
Indoor air / Measured in people or real products
The UK home with the heaviest, most frequent use of cleaning and fragranced products (9 different products, some used more than 10 times in a week, plus a scented candle) had a 5-day average D-limonene concentration of up to 1,439 ug/m3, around 80 times the study's median home and possibly the highest domestic value reported in the literature at the time.
Moderate evidence: Single home out of 25 driving the extreme; 5-day average, not a short-term peak, which the authors state was likely higher.
Source: Wang 2017, Environmental Science: Processes & Impacts Link to this fact
Indoor air / Measured in people or real products
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.
Caveats
Comparison is for a single candle type (candle 1) with one steady-burn control experiment versus two replicate stressed-burn experiments; the 60-1700x range reflects the size of the effect across the measurement, not a single precise multiplier.
Indoor air / Measured in people or real products
Candle wax and wick choice makes up to a 27-fold difference in soot, fine-particle, and particle-bound PAH emissions between candle types, but essentially no difference in NOx, formaldehyde, or gas-phase PAH emissions, in a controlled chamber study of five unscented pillar candles.
Moderate evidence.
Caveats
Five candle types, two replicate experiments each, single chamber, and single research group; not verified across other candle shapes (e.g. tapered, container candles) or other labs.
Indoor air / Measured in people or real products
In unscented candles, formaldehyde and other carbonyl emissions stayed flat and low (39-54 micrograms per hour) regardless of wax type, wick type, or whether the flame was steady or flickering, which the study's authors attribute to the absence of added fragrance or dye, since those additives (not the wax or wick) are understood to be the main source of a candle's aldehyde emissions.
Moderate evidence.
Caveats
The attribution to fragrance/dye is the authors' interpretation, citing prior literature (Derudi et al. 2014), not a within-study comparison against scented versions of the same candles; this study tested no scented candles.
Indoor air / Measured in people or real products
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.
Caveats
Single lab (Fraunhofer WKI), industry/candle-association and fragrance-house funded (European Candle Association, National Candle Association, Latin American Candle Manufacturers Association; advisory committee incl. Arylessence, Belmay, Firmenich, Givaudan, IFF, Symrise, Takasago); n=4 candles per combination, purpose-built research candles not verified as representative of specific retail products.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
Indoor air / Measured in people or real products
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: Small n (5/24 detections); detection near the analytical limit of quantification, which the authors themselves note adds uncertainty to the calculated exposure comparisons.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
Indoor air / Measured in people or real products
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.
Caveats
Based on a hierarchical cluster analysis of 24 experiments in one chamber study; individual scented fragrance families varied in how much they raised emissions (floral, oriental, and spice & edibles raised emissions less than fresh and fruit).
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
Indoor air / Measured in people or real products
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.
Caveats
Modeled dose rate from measured particle size distributions using a published respiratory deposition method, not a direct measurement of dose in a human airway; combustion-source comparison values come from separate studies with different instruments and settings.
Source: Patra et al. 2025, Environmental Science & Technology Letters Link to this fact
Indoor air / Measured in people or real products
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.
Caveats
These are concentrations from a small impinger/chamber system fed by 2 L/min of pure air pulled directly past the flame, not measured or modeled room air; single lab, six specific retail products, no replication.
Source: Ahn et al. 2015, Journal of Hazardous Materials Link to this fact
Indoor air / Measured in people or real products
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.
Caveats
TVOC is a summed, toluene-equivalent proxy, not a hazard-weighted measure; the carpet-cleaner comparison is to a different study's figures for a different product, not a side-by-side measurement.
Source: Ahn et al. 2015, Journal of Hazardous Materials Link to this fact
Indoor air / Measured in people or real products
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.
Caveats
This is a mass-balance model calculation built on chamber-derived emission factors, not a direct room measurement; the two-candle, 4-hour worst-case assumptions may not reflect typical single-candle household use.
Source: Kim et al. 2016, Journal of Hazardous Materials Link to this fact
Indoor air / Animal study
In a mouse chamber study modeling occupational candle exposure, burning scented candles for 4.5 hours a day, 5 days a week, produced measured chamber levels of 2157.5 ng/m3 PAHs, 181 micrograms per cubic meter VOCs, and 219,000 nanoparticles per cubic centimeter (median size 10.7 nanometers) during active burning.
Moderate evidence.
Caveats
This is an occupational-intensity exposure chamber (hours per day, days per week, for 8 weeks), not a measurement of typical home candle-burning exposure; single lab, single candle type (brand/composition undisclosed).
Source: Chandrasekaran 2021, Current Research in Toxicology Link to this fact
Indoor air / Animal study
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: One unscented candle product; wax/wick composition not disclosed; toluene here is an emission from the candle itself, not an added fragrance ingredient.
Source: Mohamed et al. 2025, Frontiers in Public Health Link to this fact
Indoor air / Other evidence
Burning a scented candle at low ventilation (0.5 air changes per hour) produced significantly higher total PAH and cancer-risk-weighted PAH emissions than at 1.0 or 2.0 air changes per hour, in a controlled chamber study.
Moderate evidence.
Caveats
Single lab, single scented-candle formulation (paraffin + 6% lavender oil), small chamber; total PAH itself was not monotonic with ACH (0.5 > 2.0 > 1.0), only the toxicity-weighted BaPeq fraction fell monotonically.
Indoor air / Other evidence
Relative humidity had a bell-shaped, non-monotonic effect on a scented candle's PAH emissions, peaking at 75% RH rather than at the lowest or highest humidity tested.
Moderate evidence.
Caveats
Single lab, single candle formulation; mechanism (flame-temperature/water-vapor effects) is inferred from indirect combustion indicators, not directly measured — authors call it a plausible hypothesis, not a demonstrated mechanism.
Indoor air / Other evidence
Low-molecular-weight PAHs, dominated by naphthalene, made up roughly 87-92% of total PAH mass from scented candle combustion across all tested ventilation and humidity conditions, and were mostly in the gas phase rather than bound to particles.
Moderate evidence.
Caveats
Single scented-candle formulation, small chamber study; benzo[a]pyrene itself was non-detect in nearly every condition, so toxicity-weighted risk is driven by other high-potency congeners, not BaP.
Indoor air / Measured in people or real products
Burning four unscented paraffin candles at once drove living-room NOx and HONO to 46.86 ppb and 1.63 ppb, about ten times higher than a single lavender-scented candle (0.02 ppb NOx, 0.23 ppb HONO) burned in the same room.
Moderate evidence.
Caveats
Single home, one scented and one unscented candle product; the unscented scenario burned roughly 22x more total wax mass (4 candles vs. 1), so the room-concentration gap partly reflects source size, not just chemistry.
Source: Deng 2026, Environment International Link to this fact
Indoor air / Measured in people or real products
Once normalized per gram of wax burned, the scented candle's HONO emission rate (170.77 mg per kg of wax) was an order of magnitude higher than the unscented candles' (24.14 mg/kg) or incense sticks' (52.50 mg/kg), and higher than HONO from vehicle exhaust in a traffic tunnel (88-115 mg/kg fuel).
Moderate evidence: One scented candle product tested; emission index relies on a mass-balance model of room volume and air exchange, not direct stack sampling.
Source: Deng 2026, Environment International Link to this fact
Indoor air / Measured in people or real products
Incense smoke was far more chemically complex than candle smoke: over 100 distinct VOCs, including classic wood-smoke markers (acetonitrile, methoxyphenols) and a set of nitrogen-ring compounds (pyrroles) that were completely absent from either candle's emissions.
Moderate evidence: One incense brand vs. one scented and one unscented candle product.
Source: Deng 2026, Environment International Link to this fact
Indoor air / Measured in people or real products
Ammonia, a byproduct rarely discussed for indoor combustion, rose 3-9 ppb above background during candle and incense burning (to 30-38 ppb total) and 4-10 ppb above background during cooking (to 30-44 ppb total), in both cases a statistically significant increase over pre-combustion levels.
Moderate evidence: Single-home measurements; cooking-related ammonia rise could not be cleanly separated from concurrent human-occupancy ammonia emissions.
Source: Deng 2026, Environment International Link to this fact
Indoor air / Other evidence
Burning scented candles releases VOCs, SVOCs, and particulate matter, including polycyclic aromatic hydrocarbons (benzo[a]pyrene, naphthalene, anthracene, pyrene) classified as carcinogenic.
Moderate evidence.
Caveats
This is a secondhand restatement in a two-page opinion piece, not a new measurement; our own chamber primaries (Salthammer 2021, Andersen 2021) independently confirm PAH/VOC output but find it strongly formulation-dependent rather than universal to all scented candles.
Source: Singh 2023, Environmental Science & Technology Link to this fact
Indoor air / Other evidence
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.
Caveats
Small, uncontrolled, non-blinded case series with multiple co-occurring intervention elements per home (product substitution, education, counseling, and in one case new chemical storage); cannot isolate the VOC-reduction effect from other changes.
Source: Rincón et al. 2024, Primary Health Care Research & Development Link to this fact
The industry / Industry source
The fragrance industry's aggregate consumer exposure model did not include any estimate of inhaled fragrance exposure until 2017, when it added an inhalation route for air fresheners, scented candles, and personal-care sprays only — 23 years after the industry's 1994 safety package dismissed inhalation testing.
Moderate evidence.
Caveats
This is a modeled inhalation exposure fraction from a compartmental model run at single default parameter values, not measured breathing-zone data; the model's own validation against real chemical concentrations is reported in a companion paper (Safford et al. 2017) not yet in this evidence base.
Source: Comiskey 2017 (RIFM/Creme Global), Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
The industry / Industry source
Some of the amount-per-use values feeding this industry exposure model come from very small studies — 10 Dutch adults for bar-soap-in-the-shower amounts, and a single non-distributional value for scented-candle amount per day — rather than large, representative samples.
Moderate evidence.
Caveats
Small external studies feeding a large aggregate model is a data-provenance observation, not itself evidence the resulting exposure estimates are biased in a particular direction; the paper does not report sensitivity analysis on these specific inputs.
Source: Comiskey 2017 (RIFM/Creme Global), Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
The industry / Other evidence
In this industry model, the highest simulated exposure to vanillin came from body lotion (prestige range, P95 8.79 mg/kg bw/day), eau de parfum (8.34) and eau de toilette (6.34), while plug-in air freshener and scented candles — despite having the highest simulated in-product concentrations of vanillin — produced comparatively low exposure (P95 1.81 and 0.81 mg/kg bw/day) because there is no direct skin contact and the modeled inhalation fraction is low.
Moderate evidence: Modeled, not measured; industry-funded (RIFM); pattern shown for vanillin, generalized by the authors to the other 7 ingredients tested.
Source: Safford 2017, Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
The industry / Measured in people or real products
The fragrance industry's own study found that grasping a scented candle for a total of 100 seconds transfers only about a quarter of a microgram per square centimeter of cinnamic aldehyde and eugenol to skin, which the authors say leaves a 1,000-fold or larger safety margin below the industry's own dermal sensitization threshold.
Moderate evidence.
Caveats
Industry-funded (RIFM and fragrance-industry co-authors); the comparison threshold ('NOEL-HRIPT') itself comes from an unpublished 2006 RIFM/IFRA technical dossier, not independently reviewed data. The paper itself states two different safety-factor numbers for cinnamic aldehyde in the same paper (~1,000 in its own table vs ~2,000 in its discussion text), an unresolved internal inconsistency.
Source: Api 2007 (RIFM), Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
The industry / Measured in people or real products
In the same candle, d-limonene transferred to hands in undetectable amounts in every subject and every round tested, while cinnamic aldehyde and eugenol transferred consistently — the study authors could not explain the difference and did not test whether the missing d-limonene had oxidized inside the candle into breakdown products that are known allergens.
Moderate evidence.
Caveats
Industry-authored (RIFM); the authors themselves raise, but did not test, in-candle auto-oxidation of d-limonene as one possible explanation, citing separate evidence that limonene's oxidation products are allergenic.
Source: Api 2007 (RIFM), Regulatory Toxicology and Pharmacology Industry-funded Link to this fact
The industry / Review of other studies
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.
Caveats
The critique also identified a real methodological point (chamber concentrations do not equal room concentrations), which the responding authors accepted; industry authorship does not by itself make that specific point wrong.
Source: Kim et al. 2016, Journal of Hazardous Materials Link to this fact
The industry / Review of other studies
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.
Caveats
This is a documented instance of a trade association prompting language softening and deletion of a specific hazard finding in an academic paper's corrigendum; it does not mean the underlying revised emission-factor numbers (which went up, not down) are wrong.
Source: Kim et al. 2016, Journal of Hazardous Materials Link to this fact
The industry / Animal study
Scented candles are marketed and used in hotels, restaurants, and spas for a calming, stress-relieving aromatherapy effect, but in a mouse model, scented candle fume exposure did not significantly reduce the stress hormone corticosterone that chronic social-disruption stress had raised.
Moderate evidence.
Caveats
The study exposed mice to candle combustion fumes, not a controlled scent-only stimulus, so it cannot separate a candle's psychological/olfactory calming effect (untested here) from its combustion-product toxicity; a human user's subjective calming experience is not ruled out by this animal biomarker result.
Source: Chandrasekaran 2021, Current Research in Toxicology Link to this fact
What you can do / Review of other studies
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: Expert advice; the salon point rests on a single exposure keeping skin flared up to ~4 weeks (clinical observation).
What you can do / Measured in people or real products
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: Demonstrated for purpose-built research candles across 4 wax types and 5 fragrance families, not verified for any specific retail candle brand.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
What you can do / Measured in people or real products
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.
Caveats
Comparison is described qualitatively in the text ("on average a factor of 3-5 higher"), not given as a precise table value; based on the ignition-phase behavior of the chamber's continuous particle monitor, not a home-scale test.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
What you can do / Animal study
If burning scented candles indoors, ventilate the room (open window or extraction) rather than burning in a sealed, air-conditioned space, and prefer shorter burn times: in this study the worst inflammatory and histopathological outcomes came from the longest daily exposure (6 h/day) in a fully sealed room.
Moderate evidence: Inferred from a rat model with an unmeasured indoor dose; the specific ventilation rate needed to meaningfully reduce exposure was not tested.
Source: Mohamed et al. 2025, Frontiers in Public Health Link to this fact
Information, not medical advice. See also: myths we won’t tell you.