You might want to think twice about setting the mood with a scented wax melt (or at least open a window at the same time): a new study finds that these flame-free candles can create levels of indoor air pollution on par with diesel engines and gas stoves.
The team behind the study, from the US and Germany, discovered that volatile organic compounds called terpenes, given off by wax melts and other fragrance products, interact with ozone to form a concerning number of nanoscale particles.
While the impact of these kinds of particles on health isn't yet fully clear, they're small enough to get inside the lungs. Previous studies have linked these fine particles, also known as nanocluster aerosol, with issues like cardiorespiratory disease.
"To understand how airborne particles form indoors, you need to measure the smallest nanoparticles – down to a single nanometer," says civil engineer Brandon Boor, from Purdue University.
"At this scale, we can observe the earliest stages of new particle formation, where fragrances react with ozone to form tiny molecular clusters. These clusters then rapidly evolve, growing and transforming in the air around us."
To detect these tiny particles, the researchers bought out the heavy duty equipment: a high-resolution particle size magnifier-scanning mobility particle sizer (PSMPS) and a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) were used to check indoor nanoparticle size in a 'house lab' environment.
Scented wax melts are often marketed as less polluting than candles, because there's no flame and no combustion – but these findings show they can be just as bad as candles when it comes to clogging up the air (and potentially our airways).
Even with no flames or smoke, scented wax melts can release more volatile organic compounds than candles, because they have more fragrance and melt over a larger surface area when heated.
There could be billions of these particles in your respiratory system after 20 minutes of exposure to scented products, according to the experiments run for this study – and we don't yet know what that might be doing to our lungs.
"Fragranced products are not just passive sources of pleasant scents – they actively alter indoor air chemistry, leading to the formation of nanoparticles at concentrations that could have significant health implications," says civil engineer Nusrat Jung, from Purdue University in the US.
The study team hopes their work will push forward research into indoor air pollution – something that isn't typically as extensively studied as pollution outdoors, despite potentially being just as hazardous to our health.
Earlier work from the same team suggests oil diffusers, air fresheners, disinfectants, and other scented products give off plenty of nanoparticles too. Buildings, heating systems, and air conditioners should be designed with this indoor nanoparticle pollution in mind, the researchers suggest.
"A forest is a pristine environment, but if you're using cleaning and aromatherapy products full of chemically manufactured scents to recreate a forest in your home, you're actually creating a tremendous amount of indoor air pollution that you shouldn't be breathing in," says Jung.
The research has been published in Environmental Science & Technology Letters.