Scented products cause indoor air pollution on par with car exhaust

Using scented products indoors changes the chemistry of the air, producing as much air pollution as car exhaust does outside, according to a new study. Researchers say that breathing in these nanosized particles could have serious health implications.

When you hear or see the words ‘air pollution,’ you most likely think of things like factories and car exhaust. That’s pollution that is out there – outside your house. But have you thought about how you’re contributing to air pollution inside of where you live by using seemingly innocuous products like scented, non-combustible candles?

New research by Purdue University, the latest in a series of Purdue-led studies, examined how scented products – in this case, flame-free candles – are a significant source of nanosized particles small enough to get deep into your lungs, posing a potential risk to respiratory health

“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 in Purdue’s Lyles School of Civil and Construction Engineering and co-corresponding author of the study’s.

Scented wax melts are marketed as a flameless, smoke-free, non-toxic alternative to traditional candles, a safer way of making your home or office smell nice. To assess the truth of these claims, the researchers comprehensively measured the nanoparticles formed when they warmed wax melts in their mechanically ventilated test house. The tiny house is actually an architectural engineering laboratory called the Purdue Zero Energy Design Guidance for Engineers (zEDGE) lab. Designed and engineered to test the energy efficiency of a larger building, it’s full of sensors that monitor the impact of everyday activities on indoor air quality.

“To understand how airborne particles form indoors, you need to measure the smallest nanoparticles – down to a single nanometer,” said Brandon Boor, associate professor in civil engineering at Purdue and the study’s other corresponding author. “At this scale, we can observe the earliest stages of new particle formation, where fragrances react with ozone to form tiny molecular clusters.”

The researchers knew from their previous research that new nanoparticle formation was initiated by terpenes – aromatic compounds that determine the smell of things like plants and herbs – released from the melts and reacting with indoor atmospheric ozone (O₃). They’d found that activities such as mopping the floor with a terpene-rich cleaning agent, using a citrus-scented air freshener, or applying scented personal care products like deodorant inside the zEDGE house resulted in pulsed terpene emissions to the indoor air within five minutes. Conversely, using essential oil diffusers or peeling citrus fruits caused a more gradual increase in terpenes.

In the present study, heating the scented wax contributed significantly to the number of new particles formed in the indoor air, particularly those smaller than 100 nanometers (nm). The resulting atmospheric concentrations were over one million nanoparticles per cubic centimeter (10⁶ cm^-3), which is comparable to concentrations emitted by traditional lighted candles (10⁶ cm^-3), gas stoves (10⁵ – 10⁷ cm^-3), diesel engines (10³ – 10⁶ cm^-3), and natural gas engines (10⁶ – 10⁷ cm^-3). By comparison, there were no significant terpene emissions when unscented wax melts were heated.

The researchers also examined respiratory tract deposited dose rates (R_D), a useful way of studying air pollution that measures the number of particles deposited in the respiratory tract over time. They found that the new particle formation triggered by using scented wax melts indoors produced a median R_D for 1.18 to 100 nm particles of 29 billion per minute (2.9 x 10¹⁰ min^-1). That’s about 483 million particles per second. The majority of scented-wax-melt-formed particles were deposited in the upper airways. Their small size means they can migrate between cells and enter the bloodstream, potentially reaching organs such as the brain.

While scientists have begun exploring the impact of inhaled terpenes on human health, there is still much more to learn that can only be uncovered through further research. For now, though, the study’s findings can inform building design and ventilation.

“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,” said Jung. “These processes should be considered in the design and operation of buildings and their HVAC [heating, ventilation, and air-conditioning] systems to reduce our exposures.”

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

Source: Purdue University

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