Aroma Candles - Are Wax Melt (non-burning) Better than Regular Candles? No

I know that there are some big aroma candle users out there, so this is for you.  A recently released study showed that aroma wax melts (perfumed wax heated to the point of melting to release the 'aromas') produce more health impacting chemicals similar to those produce by a diesel engine...not that many are have a diesel engine to improve relaxation while sitting in a soothing bath with rose pedals. (This is too close to my ongoing fear of getting picked off by a speeding forklift in some overly dark warehouse).

"Our findings reveal that terpenes released from scented wax melts react with indoor atmospheric ozone (O3) to initiate new particle formation (NPF) events, resulting in significant indoor atmospheric nanoparticle concentrations (>106 cm–3) comparable to those emitted by combustion-based scented candles, gas stoves, diesel engines, and natural gas engines. We show that scented wax melt-initiated NPF events can result in significant respiratory exposures, with nanoparticle respiratory tract deposited dose rates similar to those determined for combustion-based sources."

So there you go, don't be buying one of those fancy plug-in wax melts. You can get the same terpene high with those cheap burning aroma candles and at least get the flickering flame.

While this work focused on wax melts, I would guess the same would apply for those plug-in scented oil units that have become popular.

As for me, I have gone unscented.  I think about all those chemicals added to provide some artificial smell....like laundry detergent, or worse yet, drier sheets.  Clean is the absence of smell.  As a former boss used to say, don't use agents that mask the smell, because if not properly clean, we want to know.

Environmental Science & Technology LettersVol 12/Issue 2
https://pubs.acs.org/doi/full/10.1021/acs.estlett.4c00986
Flame-Free Candles Are Not Pollution-Free: Scented Wax Melts as a Significant Source of Atmospheric Nanoparticles
Satya S. PatraJinglin JiangJianghui LiuGerhard SteinerNusrat Jung*Brandon E. Boor*

Abstract

Scented wax melts are being popularized as a safer, nontoxic alternative to traditional candles and incense for indoor aromatherapy. We performed field measurements in a residential test house to investigate atmospheric nanoparticle formation from scented wax melt use. We employed a high-resolution particle size magnifier-scanning mobility particle sizer (PSMPS) and a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) for real-time monitoring of indoor atmospheric nanoparticle size distributions and terpene mixing ratios, respectively. Our findings reveal that terpenes released from scented wax melts react with indoor atmospheric ozone (O3) to initiate new particle formation (NPF) events, resulting in significant indoor atmospheric nanoparticle concentrations (>106 cm–3) comparable to those emitted by combustion-based scented candles, gas stoves, diesel engines, and natural gas engines. We show that scented wax melt-initiated NPF events can result in significant respiratory exposures, with nanoparticle respiratory tract deposited dose rates similar to those determined for combustion-based sources. Our results challenge the perception of scented wax melts as a safer alternative to combustion-based aromatherapy, highlighting the need for further research on the toxicological properties of the newly formed nanoparticles to better understand their environmental health implications.

Introduction

Scented wax melts have become a popular, stylish, and aromatic alternative to traditional candles and incense. These small pieces of scented wax release pleasant fragrances into homes, offices, and other indoor environments when heated without an open flame. Marketed as noncombustion, smoke-free, and nontoxic, scented wax melts are perceived as safer and cleaner options for indoor aromatherapy. (1) The scented wax melt market is rapidly expanding, as evidenced by its prominent presence in the product lines of numerous leading home fragrance companies.

Despite being flame-free and smoke-free, scented wax melts can emit quantities of volatile organic compounds (VOCs) that are larger than those of traditional scented candles (2) due to their higher fragrance concentration and the direct heating of wax, maximizing the melted wax surface area. (1) Manufacturers often highlight this advantage, claiming that scented wax melts quickly and efficiently fill indoor spaces with aroma. However, these VOCs, primarily composed of monoterpenes and their oxygenated derivatives, monoterpenoids, are highly reactive with atmospheric ozone (O3), even at low O3 concentrations. (3,4) Studies report that indoor sources emitting monoterpenes and monoterpenoids can react with indoor atmospheric O3 to induce new particle formation (NPF; nucleation and subsequent growth of nanoparticles) in environments with low nitrogen monoxide (NO) concentrations. (3−11) Because scented wax melts do not involve combustion, they likely do not emit NO, suggesting possibilities for secondary reactions that could initiate NPF, despite being smoke-free. However, this phenomenon in indoor environments remains unexplored.

Here, we present the first comprehensive evaluation of nanoparticle formation using scented wax melts in a full-scale, mechanically ventilated residential test house. Utilizing advanced measurement techniques, including a high-resolution particle size magnifier-scanning mobility particle sizer (PSMPS) (12) and a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS), (13) we quantitatively demonstrate that secondary chemistry during scented wax melt use can form and grow indoor atmospheric nanoparticles, significantly enriching the sub-100 nm particle concentration in the indoor environment. Surprisingly, the indoor atmospheric nanoparticle concentrations from noncombustion-based scented wax melts were similar to those emitted by combustion-based scented candles, gas stoves, diesel engines, and natural gas engines. Furthermore, the adult nanoparticle respiratory tract deposited dose rates while using scented wax melts rivaled those determined for combustion-based sources. Therefore, contrary to the perception of being a safer alternative, NPF events from noncombustion scented wax melts result in nanoparticle formation comparable to primary nanoparticle emissions from combustion-based sources.