The air inside your home may be more polluted than outside

Scented wax melts have gained popularity as a stylish and aromatic alternative to traditional candles and incense. These small, fragrant wax pieces release pleasant aromas when heated without an open flame.

Marketed as noncombustion, smoke-free, and nontoxic, they are often perceived as safer options for indoor aromatherapy. However, recent research challenges this assumption, revealing that scented wax melts may contribute to indoor air pollution by emitting volatile organic compounds (VOCs) and generating nanoparticles.

Despite their flame-free nature, scented wax melts can release higher quantities of VOCs than scented candles. This is due to their high fragrance concentration and the increased surface area of the melted wax. Manufacturers promote this feature, emphasizing how effectively wax melts distribute scents indoors. However, these VOCs, primarily monoterpenes and monoterpenoids, react with atmospheric ozone (O3), even at low concentrations.

When these compounds interact with ozone indoors, they trigger secondary chemical reactions that lead to the formation of new airborne particles. In environments with low nitrogen monoxide (NO) levels, which is typical of indoor spaces, this reaction promotes new particle formation (NPF). While wax melts do not produce visible smoke, they facilitate the creation of nanoscale pollutants, potentially affecting indoor air quality.

Researchers at Purdue University have conducted comprehensive evaluations of this process in a controlled residential setting.

Using advanced instruments like a high-resolution particle size magnifier-scanning mobility particle sizer (PSMPS) and a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS), scientists have demonstrated that scented wax melts significantly increase indoor nanoparticle concentrations. The levels of these nanoparticles are comparable to those produced by scented candles, gas stoves, diesel engines, and natural gas engines.

The fresh scent of pine trees in a forest is invigorating, but recreating that aroma indoors with chemical products introduces unintended pollutants. Air fresheners, wax melts, floor cleaners, and deodorants release volatile chemicals that react with ozone, forming nanoscale particles capable of penetrating deep into the lungs. These newly formed nanoparticles can spread to other organs, raising concerns about potential health risks.

"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," said Nusrat Jung, an assistant professor at Purdue’s Lyles School of Civil and Construction Engineering.

Nanoparticles just a few nanometers in size can enter the respiratory system, reaching the bloodstream and internal organs. Civil engineering professors Jung and Brandon Boor are among the first researchers to study how these airborne nanoparticles form indoors. By analyzing the earliest stages of new particle formation, they have observed how fragrance compounds react with ozone to form molecular clusters that evolve and grow rapidly.

In their research conducted at the Purdue Zero Energy Design Guidance for Engineers (zEDGE) lab, a controlled residential environment, Jung and Boor use state-of-the-art air quality instruments to monitor emissions from household products.

The lab, built in 2020, replicates a typical home while providing real-time data on air quality. Their studies indicate that many everyday scented products release volatile compounds that alter indoor air chemistry, resulting in nanoparticle formation at potentially harmful levels.

Scented wax melts, often advertised as nontoxic, contribute to indoor air pollution in ways similar to candles. Wax melts release terpenes—the compounds responsible for their scents—in higher concentrations than many candles, increasing terpene levels in the air.

When these terpenes interact with ozone, nanoparticle formation is triggered. Surprisingly, the nanoparticles from wax melts match or exceed those from combustion sources, highlighting the need to examine noncombustion products as sources of nanoscale pollutants.

Jung and Boor’s findings also show that other common household products, including essential oil diffusers, disinfectants, air fresheners, and scented sprays, produce significant nanoparticle levels. Their research extends to cooking emissions, revealing that gas stoves generate nanoparticles in large quantities.

A single kilogram of cooking fuel releases 10 quadrillion nanoparticles smaller than 3 nanometers—matching or exceeding emissions from internal combustion engines. Exposure to these particles indoors may be 10 to 100 times greater than exposure to vehicle exhaust on a busy street.

Scented chemical products, gas stoves, and combustion engines all generate high concentrations of nanoparticles smaller than 3 nanometers, known as nanocluster aerosol. Within 20 minutes of exposure to scented products, between 100 billion and 10 trillion of these particles can deposit in the respiratory system.

These findings emphasize the importance of further research into nanoparticle formation and exposure risks associated with heavily scented indoor products.

Jung and Boor’s work is shaping the future of indoor air quality research. Collaborating with industry partners, they are testing new air quality measurement instruments in Purdue’s tiny house lab before they become commercially available. The controlled residential setting provides more accurate data than traditional chamber-based research methods.

"When companies see top-tier research coming out of Purdue, they want to be part of it," Jung said. "And if they have an innovative product, they want experts to push it to its limits."

One of the advanced instruments used in their studies is the particle size magnifier-scanning mobility particle sizer (PSMPS), developed by GRIMM AEROSOL TECHNIK, a DURAG GROUP company. This device allows researchers to detect nanoparticles as small as a single nanometer at the moment of formation.

By collecting high-resolution data on new particle growth, Jung and Boor have published groundbreaking studies comparing indoor and outdoor air pollution levels. Their findings contribute to a growing body of evidence that emphasizes the need for improved indoor air quality monitoring and regulation.

Beyond scented products, Jung and her team have investigated how common household activities, such as hair care routines, impact indoor air quality. They found that certain chemicals, especially cyclic volatile methyl siloxanes—prevalent in hair products—linger in the air in significant amounts. A single hair care session can lead to inhalation of 1-17 milligrams of these chemicals.

While toxicologists will need to further assess the long-term health effects of exposure to these chemicals and nanoparticles, Jung and Boor’s research underscores the importance of reevaluating indoor air quality standards. Their work aims to bridge the gap between research and real-world applications, ensuring that building designs and HVAC systems prioritize healthier indoor environments.

"Indoor air quality is often overlooked in the design and management of the buildings we live and work in, yet it has a direct impact on our health every day," Boor said. "With data from the tiny house lab, we aim to bridge that gap—transforming fundamental research into real-world solutions for healthier indoor environments for everyone."

Note: Materials provided above by The Brighter Side of News. Content may be edited for style and length.

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