Scented wax melts emit harmful nanoparticles

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Feb 26 2025

Despite being marketed as a cleaner alternative, scented wax melts emit high levels of volatile organic compounds that react with indoor ozone, producing pollutant nanoparticles that may pose health risks.

Study: Flame-Free Candles Are Not Pollution-Free: Scented Wax Melts as a Significant Source of Atmospheric Nanoparticles. Image Credit: Tanya Greene / Shutterstock.com

Scented wax melts (SWMs) are promoted as a safer alternative to burning aromatherapy candles and incense. However, a recent study published in Environmental Science & Technology Letters indicates that SWMs release a similar level of polluting nanoparticles (NP) to burning candles and even diesel engines.

What are SWMs?

SWMs are small pieces of highly fragranced wax that are heated to release the odor indoors. As compared to traditional wax candles, SWMs do not burn and, as a result, are considered cleaner, safer, smoke-free, and nontoxic options.

Despite these potential benefits, SWMs emit volatile organic compounds (VOC) in amounts that exceed emissions from traditional scented candles. The increased release of VOCs is due to the concentrated fragrance in SWMs and direct wax heating, as compared to evaporation from the melted wax pool.

VOCs released from SWMs can include monoterpenes and monoterpenoids, which are the oxidized form of monoterpenes. Monoterpenes react very rapidly with ozone, even when ozone is at very low concentrations, which can lead to new particle formation (NPF) in the air. During NPF, particle nuclei are formed, which grow rapidly into NPs in the absence of high nitrogen monoxide (NO) levels.

About the study

To date, few studies have investigated NPF with indoor SWM use. In the current study, SWMs were placed in a test house with mechanical ventilation. The background proportion of ozone in indoor air was 9.7-29.9 parts per billion (ppb), whereas NO levels ranged from 0.6 to 0.8 ppb.

The distribution of NPs within the house air was measured using a high-resolution particle size magnifier-scanning mobility particle sizer (PSMPS). The researchers also assessed terpenes as a proportion of air using a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS).

Study findings

SWMs release high amounts of terpenes that react with indoor ozone. This leads to the formation of peroxy and hydroperoxy radicals that react with each other and NO. Thus, even as ozone is consumed, NO concentrations remain stably low.

At higher NO concentrations, other peroxy radical reactions predominated. With low NO levels, ozone-terpene reactions continued, causing them to auto-oxidize into highly oxygenated molecules.

Furthermore, low NO levels led to intense particle nucleation of less than three nanometers (nm) in size. Nucleation rates exceeded typical rates for outdoor atmospheric particles by orders of magnitude, which was likely due to ozone-driven terpene oxidation.

SWMs that emitted higher terpene levels were associated with rapid growth of nucleated particles. The maximum growth in size coincided with peak terpene emissions and mirrored the ozone-mediated terpene reaction graph.

Thus, particle growth is likely due to terpene release by SWMs and their reaction with ozone. This hypothesis was supported by the lack of terpene release and particle formation with an unscented wax melt.

Ventilation also contributes to the rate of particle growth. For example, lower ventilation with outdoor air promotes higher terpene concentrations indoors, thereby resulting in greater particle growth.

All terpenes are not equally reactive with ozone. Linalool and d-limonene, for example, are more reactive as compared to menthol and menthone. In the absence of volatile condensable vapors, such small particle nuclei rapidly decrease in their concentration.

SWMs that release lower terpene concentrations contributed to nucleation of particles.

High particle production

Although no combustion was occurring, the NP spectrum and rate of production resembles the production of particles from sources of actual combustion. These include burning scented candles, as well as gas stoves, diesel engines, and natural gas engines.

Expressed as particles per minute, the mean net particle production rate at 1.75 nm during these NPF events was 6.4 × 10¹² min–1, about three times higher than the reported particle emission rate at 2.3 nm during indoor candle combustion.”

SWM induces NPF, which subsequently exposes the respiratory system to significant amounts of NP pollutants that can directly deposit into the upper respiratory tract at a median rate of 2.9 × 10¹⁰/min. These particles can travel in the blood to deposit in and potentially harm organs like the brain and liver.

Alarmingly high deposition rates have also been recorded in the lungs. The dose rate of deposited NPs in the respiratory tract is similar to reported deposited doses from combustion sources.

Conclusion

Our results challenge the perception of scented wax melts as a safer alternative to combustion-based aromatherapy.”

The study findings emphasize the importance of investigating particle formation from SWMs  and assessing their potential adverse effects on the human body and environment.