Wax melts are products that scent a room by electrically warming and melting fragranced wax without a flame. Even when there is no visible smoke, can evaporated fragrance compounds react in the air to create new particles?
In 2025, Satya S. Patra and colleagues closely measured the air during scented-wax use in a test house at Purdue University in the United States. Their aim was to investigate whether a flameless product could generate extremely small particles, how those particles grew and the extent of potential respiratory exposure.
- Measuring fragrance vapors and particles together inside a house
- Fragrance compounds react in air to form particles
- Increasing ventilation had different effects for different particle sizes
- The amount entering the respiratory tract was estimated from measurements
- “Flameless” alone does not establish the impact on air
- Summary
- References
Measuring fragrance vapors and particles together inside a house
The team used a wax warmer in a full-scale test house equipped with mechanical ventilation. Alongside scented products, they also tested wax without added fragrance. Multiple instruments measured airborne particles over a diameter range of approximately 1.18–572.5 nanometers. One nanometer is one millionth of a millimeter.
They simultaneously tracked volatile compounds released as fragrance, including terpenes, as well as ozone, nitrogen oxides and other substances. They also compared two ventilation conditions introducing outdoor air at 3.0 and 6.5 house volumes per hour. This was an experiment with controlled ventilation settings, not an assumption that every room in an ordinary home would reach the same concentrations.
Fragrance compounds react in air to form particles
New formation of extremely small particles was observed during every measurement activity using scented wax. Within minutes of switching on the warmer, particles smaller than three nanometers began to increase. Elevated particle levels persisted after heating stopped, which the researchers interpreted as continuing reactions involving terpenes remaining in the air.
The proposed central mechanism is the reaction between terpenes released from the wax and indoor ozone. If these reactions produce low-volatility substances, molecules can cluster to form particle nuclei, which may grow as more material accumulates. The observed particles were not smoke from burning wax but secondary particles formed by reactions of emitted compounds in the air.
The unscented-wax comparison supports this explanation. The unscented wax tested showed neither substantial terpene emissions nor new particle formation and growth, and the particle distribution did not change markedly from the air before use. With scented wax, estimated rates of terpene–ozone reactions also corresponded with particle formation and growth rates.
Not all newly formed particles grew larger, however. Growth beyond three nanometers occurred only under some conditions with high terpene emissions. The authors considered differences in compound reactivity relevant, but did not measure the low-volatility substances involved in growth themselves, leaving details of the reaction products unresolved.
Increasing ventilation had different effects for different particle sizes
In comparisons during particle formation using three scented waxes, particles of 3–100 nanometers were more abundant under lower ventilation. Median concentrations were approximately 49,000 particles per cubic centimeter of air at 3.0 air changes per hour and 2,700 at 6.5 air changes per hour.
In contrast, median concentrations of the still smaller 1.18–3-nanometer particles were approximately 1.4 million and 2.3 million, respectively. Increasing ventilation did not reduce particle counts uniformly. The authors discussed the combined roles of fragrance concentration, particle growth and the reduction in number caused by particles sticking together.
Ventilation does more than remove existing particles: it also affects indoor reaction conditions. This experiment cannot establish that increasing ventilation reduces every particle size by the same proportion or that a particular ventilation rate ensures safety.
The amount entering the respiratory tract was estimated from measurements
Using the measured particle sizes and numbers, the team also modeled how many particles would deposit in the nose, airways, lungs and other respiratory regions when an adult breathed. During particle formation, the median whole-respiratory-tract deposition rate for 1.18–100-nanometer particles was approximately 29 billion particles per minute. This was about three orders of magnitude above the estimate for background air before the experiment.
These were not particle counts obtained by examining human lungs. They were exposure estimates generated by feeding indoor-air measurements into a model, with most particles estimated to deposit in the upper airways. The paper raises concern about respiratory burden because such high particle-number-based exposures may occur.
The authors also compared their findings with particles reported in previous studies of candles, gas stoves and engines, noting that particle number concentrations and estimated deposition from scented wax could be comparable to combustion-related particles. However, those studies differed in measurement locations and particle-size ranges. Similar particle counts do not mean identical chemical composition, toxicity or disease risk.
“Flameless” alone does not establish the impact on air
This was not a clinical trial investigating symptoms or disease incidence, and the toxicity of the particles produced was not directly tested. Reactions vary with conditions such as ozone in the test house and the wax formulation, so health risks cannot be generalized into one estimate for every product and setting.
Nevertheless, the authors argued that the absence of combustion alone is insufficient reason to regard these products as a safer way to fragrance indoor air. In light of the measured particle numbers and estimated respiratory deposition, they concluded that detailed identification of emitted compounds and toxicity testing of the resulting particles are needed next. Uncertainty about health effects must be distinguished from the measurable changes that occurred in the air.
Summary
In this test house, warming scented wax generated nanometer-sized particles through reactions between emitted terpenes and indoor ozone. Even without combustion, reactions in the air after a product releases its fragrance can lead to particle exposure.
The study highlights the need to evaluate not only what fragrance products emit but also what those constituents become indoors and how much people may inhale. The high particle numbers provide a reason for closer assessment, not an answer quantifying the risk of a particular disease or ranking safety against combustion products.
References
Patra, S. S., Jiang, J., Liu, J., Steiner, G., Jung, N., & Boor, B. E. (2025). Flame-Free Candles Are Not Pollution-Free: Scented Wax Melts as a Significant Source of Atmospheric Nanoparticles. Environmental Science & Technology Letters, 12(2), 175–182. DOI: 10.1021/acs.estlett.4c00986 · Full original article · Publisher PDF shared by the authors.
Original article: © 2025 The Authors. Published by American Chemical Society. CC BY-NC-ND 4.0. The research was supported by the US NSF, Purdue University and ASHRAE. Author Gerhard Steiner is an employee of GRIMM Aerosol Technik, a manufacturer of measurement instruments, and disclosed a related financial conflict of interest.
