Air fresheners and wax melts can make your indoor air more dirty than outside

When you walk outside, you might be concerned about how a nearby idling car or a faraway factory are polluting the air you breathe. But when you’re inside, the products you use to make your home smell good—like wax melts, air fresheners, or diffusers—warrant the same worries. These products create nanoparticles that pollute your indoor air, at times even making the air inside your home more polluted than the urban outdoors.

Researchers at Purdue University have been studying how everyday products create air pollution inside our homes. In a lab that resembles a tiny house—called the Purdue zero Energy Design Guidance for Engineers (zEDGE) lab—they study the emissions that come from cleaning products, essential oil diffusers, air fresheners, scented wax melts, disinfectants, and even deodorant and hair care products. 

Though you might burn some wax melts or use an essential oil diffuser as a way to mimic the fresh, clean air of a forest or a field of flowers, it’s actually those fragrances that cause pollution. These scented household products emit volatile organic compounds that can then react with the indoor air, including with oxidants like ozone.

The result are nanoparticles that, though just a few nanometers in size—or even as small as 1 nanometer—can impact your health. (A human hair, for contrast, is about 80,000 nanometers). These tiny particles can penetrate deep into our lungs and move to other organs; breathing nanoparticles has been associated with health impacts on respiratory, cardiovascular, and neurological systems. 

This “tiny house lab,” which sits outside of Purdue’s Delon and Elizabeth Hampton Hall of Civil Engineering, allows researchers to comprehensively study indoor air quality. [Photo: Kelsey Lefever/Purdue University]

“People spend 90% of their time indoors, yet indoor air pollution remains largely unregulated and understudied,” Nusrat Jung, an assistant professor in Purdue’s Lyles School of Civil and Construction Engineering, said via email. Jung and Brandon Boor, also a civil engineering professor, have been working in the zEDGE lab together to study the formation of these tiny particles indoors and compare them to outdoor air. 

Scientists have been studying indoor air more frequently, especially during the pandemic; gas stoves have become the most prominent culprits of indoor air pollution, along with other types of combustion. But Jung and Boor’s work shows that we need to pay attention to other sources of indoor particulate matter, too. “Our research shows that indoor nanoparticles can form and grow at rates much faster than those observed outdoors, largely due to the significant release of reactive fragrances from scented household products,” Boor says. 

Purdue University engineers Nusrat Jung, left, and Brandon Boor study the impact of everyday products and activities on a home’s air quality. [Photo: Kelsey Lefever/Purdue University]

In their latest study, published earlier this month, they looked specifically at wax melts—which are often touted as a safer, cleaner option than candles and incense, the burning of which produces pollutants. But even though wax melts are flame-free, they still release nanoparticles in concentrations comparable to candles, gas stoves, and diesel engines. 

This emphasizes Jung and Boor’s general findings that scented household products release pollution, “often exceeding levels found in heavily polluted outdoor environments,” Jung says. Those pollution levels even occur with ostensibly “natural” products like plant-based cleaners or essential oils. They’ve found that using “citrus-scented air fresheners, essential oil diffusers, wax warmers, and terpene-based cleaners can elevate indoor nanoparticle concentrations by 10 to 1,000 times above typical urban outdoor levels,” she adds.

The inside of Purdue’s tiny house lab has various sensors and equipment to accurately and precisely measure pollutant emissions from common household activities in real time. [Photo: Kelsey Lefever/Purdue University]

Proper ventilation is key. But if you’re opening a window to dilute your indoor air pollution, you could also be adding in more outdoor ozone, which then reacts with those fragrances to generate even more nanoparticles. Instead, air purifiers could help reduce pollution levels. Choosing fragrance-free products could also reduce indoor air pollution. 

Jung and Boor say they will continue to research how everyday scented products impact our indoor air quality. They also hope to better understand what specific ingredients are causing those nanoparticles to form, as well as what role is played by ventilation, air circulation, and other chemicals already in our air. 

“The findings from our Purdue zEDGE test house are not just academic; they have real implications for building design, ventilation strategies, consumer product formulations, and public health guidelines,” Jung says. “As we continue to push the boundaries of indoor air quality research, our goal is to empower people with the knowledge and tools to create healthier indoor environments. Because the air we breathe inside should be just as much of a priority as the air outside.”

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