Scented products release indoor air pollutants

Many find comfort in the familiar scents of pine, lavender, or citrus. However, introducing these fragrances indoors through scented candles, wax melts, air fresheners, or floor cleaners can have unexpected effects.

Scientists at Purdue University have discovered that these everyday products release nanoscale particles into the air. These particles are so small that they can reach deep into the lungs and potentially spread to other organs.

Unlike the natural scents of a forest, these artificial fragrances interact with ozone inside buildings, creating invisible pollutants.

For years, indoor air pollution has received little attention. While people focus on smog, vehicle emissions, and industrial pollutants, they often assume their homes provide a safe escape.

Yet, researchers now suggest that indoor environments may contain harmful airborne particles at levels comparable to outdoor pollution.

Scented products transform indoor air

Many people assume that air fresheners and scented cleaning products improve indoor air quality.

These products give the illusion of freshness, masking odors and filling the home with pleasant smells. However, their chemical makeup often leads to unexpected reactions once they enter the air.

Nanoparticles form when fragrance compounds interact with ozone. Ozone naturally enters buildings through ventilation systems, windows, and doors. Once inside, it reacts with volatile organic compounds (VOCs) from scented products, triggering chemical changes that result in new pollutants.

Nusrat Jung, an assistant professor in Purdue’s Lyles School of Civil and Construction Engineering, researches indoor air quality and the impact of household products.

“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 Jung.

Health risks of nanoparticles

Unlike visible dust or smoke, nanoparticles are far too small to be seen with the naked eye. These tiny particles, measuring just a few nanometers in size, can easily bypass the body’s natural defenses. They penetrate deep into the respiratory system and may even spread to other organs.

Jung and fellow civil engineering professor Brandon Boor have conducted meticulous research on indoor air pollution. Their studies compare the formation of nanoparticles inside homes to atmospheric processes that occur outdoors.

The team found that household products release nanoscale particles that are just as concerning as some outdoor pollutants.

Indoor air pollution

“To understand how airborne particles form indoors, you need to measure the smallest nanoparticles – down to a single nanometer,“ said Professor Boor.

“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 track how these particles develop, Jung and Boor use a specialized research facility called the Purdue Zero Energy Design Guidance for Engineers (zEDGE) lab. This “tiny house lab” is a fully functioning residential space designed to monitor indoor air quality.

Built in 2020, the lab is equipped with advanced sensors to measure how common household activities affect air composition.

Indoor scents and air pollution

Jung and Boor’s research suggests that many everyday household products are not as safe as previously assumed.

While their potential health effects require further study, the scientists have repeatedly found that fragranced products react with indoor air to form nanoparticles at high concentrations.

“Our research shows that 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,” Jung said.

These findings indicate that household air quality should be an essential consideration in building design and HVAC system operation. Many current ventilation systems do not adequately account for the nanoparticle pollution created by scented chemical products.

Surprising impact of wax melts

In one of their recent studies, Jung and Boor examined the pollution caused by scented wax melts. These products, often advertised as nontoxic because they do not burn like candles, were found to pollute indoor air just as much as traditional candles.

Wax melts contain high concentrations of fragrance oils, which release terpenes when warmed. Terpenes are the chemical compounds responsible for many natural scents, such as those found in citrus fruits and pine trees. However, when terpenes react with ozone indoors, they trigger the rapid formation of nanoparticles.

Despite lacking combustion, wax melts produce nanoparticle pollution comparable to burning candles. This discovery suggests that non-combustion sources of indoor pollution deserve more attention.

Other fragranced chemical products, such as essential oil diffusers, disinfectants, and air fresheners, were also found to generate significant amounts of nanoscale particles.

Cooking and indoor pollution

While scented products are a major source of nanoparticle pollution, they are not the only contributors. Boor’s research also found that cooking, especially on gas stoves, releases large quantities of nanoparticles into the air.

Just one kilogram of cooking fuel releases 10 quadrillion particles smaller than three nanometers. This emission rate matches or even exceeds the pollution from internal combustion engine vehicles.

A person cooking on a gas stove indoors could inhale 10 to 100 times more of these sub-3 nanometer particles than they would from standing on a busy city street.

Still, the levels of nanoparticles produced by scented chemical products rival or surpass those emitted from gas stoves and cars. Within just 20 minutes of exposure to scented products, between 100 billion and 10 trillion nanoparticles could be deposited in the respiratory system.

How indoor air gets polluted

To deepen their understanding of indoor air pollution, Jung and Boor are collaborating with industry partners.

Their tiny house lab provides a more realistic testing environment than traditional chamber-based air quality studies. As a result, companies use the lab to test new air quality measurement instruments before bringing them to market.

“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 latest instruments tested in the lab is a particle size magnifier-scanning mobility particle sizer (PSMPS) developed by GRIMM AEROSOL TECHNIK, a DURAG GROUP company. This cutting-edge tool allows researchers to detect nanoparticles as small as a single nanometer as soon as they form.

Comparing indoor and outdoor air

Thanks to high-resolution data from the tiny house lab, Jung and Boor have been able to compare indoor and outdoor air pollution with unprecedented accuracy.

Since indoor air pollution remains largely unregulated and understudied, these findings are critical for understanding how indoor environments impact health.

The researchers have also expanded their studies to include the effects of personal care routines on indoor air quality. Jung and her students discovered that common hair care products release significant amounts of cyclic volatile methyl siloxanes, a type of chemical that lingers in the air long after use.

In a single hair care session at home, a person can inhale between 1 and 17 milligrams of these chemicals.

Future of indoor air quality research

Toxicologists will need to investigate how inhaling complex mixtures of volatile chemicals and nanoparticles affects human health. The researchers hope their findings will lead to improved monitoring, regulation, and control of scent-related indoor air pollution.

“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.”

The study is published in the journal Environmental Science & Technology Letters.

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