A new study shows that using scented products indoors changes the chemistry of the air, creating as much air pollution as car exhaust outdoors. Researchers say inhaling these nanoscale particles could have serious health effects.
When you hear or see the word "air pollution," you most likely think of factory and car exhaust. These are outside pollution - pollution outside your home. But have you ever thought that using seemingly harmless products like scented non-flammable candles in the rooms where you live can also contribute to air pollution?
The new Purdue University study is the latest in a series of Purdue-led studies that examine how scented products, in this case non-burning candles, can be a significant source of nanoparticles that are small enough to penetrate deep into the lungs, posing potential risks to respiratory health.
"The forest is a pristine environment, but if you use cleaning and aromatherapy products filled with chemically manufactured scents to recreate the look of a forest in your home, you're actually creating a lot 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 Architectural Engineering and co-corresponding author of the study.
Scented wax melts are marketed as a flameless, smokeless, non-toxic alternative to traditional candles and are a safer way to scent your home or office. To assess the veracity of these claims, the researchers comprehensively measured the nanoparticles formed when melted wax was heated in a mechanically ventilated test house. The tiny house is actually a construction engineering laboratory called the Zero Energy Design Guide for Engineers (zEDGE) Laboratory at Purdue University. Designed and engineered to test the energy efficiency of large buildings, the lab is filled with sensors that monitor the impact of daily activities on indoor air quality.
"To understand how airborne particles are formed indoors, you need to measure the smallest nanoparticles -- as small as a nanometer," said Brandon Boor, an associate professor of civil engineering at Purdue University and another corresponding author of the study. "At this scale, we can observe the earliest stages of new particle formation, when fragrances react with ozone to form tiny clusters of molecules."
The researchers knew from previous studies that the formation of the new nanoparticles begins when terpenes - the aromatic compounds responsible for the smell of plants and herbs alike - are released from the melt and react with ozone (O3) in the indoor atmosphere. They found that activities such as mopping the floors with terpene-rich cleaners, using citrus-scented air fresheners, or applying scented personal care products like deodorant inside a zEDGE room emitted pulses of terpenes into the indoor air within five minutes. Conversely, using an essential oil diffuser or peeling a citrus fruit results in a gradual increase in terpenes.
In this study, heating scented wax greatly increased the number of new particles formed in indoor air, especially those smaller than 100 nanometers. The resulting atmospheric concentration is more than 1 million nanoparticles per cubic centimeter (106cm-3), which is comparable to concentrations emitted by conventionally lit candles (106cm-3), gas stoves (105-107cm-3), diesel engines (103-106cm-3), and natural gas engines (106-107cm-3). In contrast, there are no significant terpene emissions when heating unscented wax melts.
The researchers also studied respiratory deposition dose rate (RD), an effective method for studying air pollution that measures the number of particles deposited in the respiratory tract over time. They found that new particle formation triggered by indoor use of scented wax melts produced a median RD1.18 to 100nm particles of 29 billion particles per minute (2.9x1010min-1). In other words, there are approximately 483 million particles per second. Most of the particles formed by melted scented wax are deposited in the upper respiratory tract. Their small size means they can migrate between cells and enter the bloodstream, potentially reaching organs such as the brain.
While scientists have begun to explore the effects of inhaled terpenes on human health, there is still much more to discover that requires further research. For now, though, the findings can inform building design and ventilation.
"Our research shows that fragrance products are not just passive sources of pleasant scents, they actively alter the chemical composition of indoor air, leading to the formation of nanoparticles at concentrations that may have significant health consequences," Jung said. "These processes should be considered in the design and operation of buildings and their HVAC (heating, ventilation and air conditioning) systems to reduce our exposure."
The research was published in the journal Environmental Science & Technology Letters.