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A typical morning hair-care routine can expose you to as much immediate nanoparticle pollution as standing in dense highway traffic, report Purdue University engineers.

A Purdue research team led by Nusrat Jung, an assistant professor in the Lyles School of Civil and Construction Engineering, and her Ph.D. student Jianghui Liu, found that a 10–20-minute heat-based hair care routine exposes a person to upward of 10 billion nanoparticles that are directly deposited into their lungs. These particles can lead to serious health risks such as respiratory stress, lung inflammation and cognitive decline.

The team's findings are in Environmental Science & Technology.

"This is really quite concerning," Jung said. "The number of nanoparticles inhaled from using typical, store-bought hair-care products was far greater than we ever anticipated."

Until this study, Jung said, no real-time measurements on nanoparticle formation during heat-based hair styling had been conducted in full-scale residential settings. Their research addresses this gap by examining temporal changes in indoor nanoparticle number concentrations and size distributions during realistic heat-based hair styling routines.

"By providing a detailed characterization of indoor nanoparticle emissions during these personal care routines, our research lays the groundwork for future investigations into their impact on indoor atmospheric chemistry and inhalation toxicity," Jung said. "Studies of this kind have not been done before, so until now, the public has had little understanding of the potential health risks posed by their everyday hair care routines."

What makes these hair care products so harmful, Liu said, is when they are combined with large amounts of heat from styling appliances such as curling irons and straighteners. When combined with heat exceeding 300 degrees Fahrenheit, the chemicals not only rapidly release into the air but also lead to the formation of substantial numbers of new airborne nanoparticles.

"Atmospheric nanoparticle formation was especially responsive to these heat applications," Liu said. "Heat is the main driver—cyclic siloxanes and other low-volatility ingredients volatilize, nucleate and grow into new nanoparticles, most of them smaller than 100 nanometers."

In a study Jung in 2023, her team found that heat significantly increased emissions of volatile chemicals such as decamethylcyclopentasiloxane (aka D5 siloxane) from hair care routines. D5 siloxane in particular was identified as a compound of concern when inhaled.

"When we first studied the emissions from hair care products during heat surges, we focused on the volatile chemicals that were released, and what we found was already quite concerning," Jung said. "But when we took an even closer look with aerosol instrumentation typically used to measure tailpipe exhaust, we discovered that these chemicals were generating bursts of anywhere from 10,000 to 100,000 nanoparticles per cubic centimeter."

Jung said that D5 siloxane is an organosilicon compound and is often listed first or second in the ingredient lists of many hair care products, indicating it can be among the most abundant ingredients. It has become a common ingredient over the past few decades in many personal care products due to its low surface tension, inertness, high thermal stability and smooth texture.

According to the European Chemicals Agency, D5 siloxane is classified as "very persistent, very bioaccumulative." And while the test results on laboratory animals are already concerning, Jung said, there is little information on its human impact. The chemical in wash-off cosmetic products has already been restricted in the European Union because of this.

"D5 siloxane has been found to lead to adverse effects on the respiratory tract, liver and nervous system of laboratory animals," Jung said previously. However, under high heat, cyclic siloxanes and other hair care product ingredients can volatilize and contribute to the formation of large numbers of airborne nanoparticles that deposit efficiently throughout the respiratory system. These secondary emissions and exposures remain far less characterized than the primary chemical emissions.

"And now it appears that the airborne hazards of these products—particularly 'leave-on' formulations designed to be heat-resistant, such as hair sprays, creams and gels—are even greater than we expected," Liu said.

According to the report, respiratory tract deposition modeling indicated that more than 10 billion nanoparticles could deposit in the respiratory system during a single hair styling session, with the highest dose occurring in the pulmonary region—the deepest part of the lungs. Their findings identified heat-based hair styling as a significant indoor source of airborne nanoparticles and highlight previously underestimated inhalation exposure risks.

As for how to avoid putting oneself at risk of inhaling mixtures of airborne nanoparticles and volatile chemicals, Jung and Liu said the best course of action is simply to avoid using such products—particularly in combination with heating devices. If that is not possible, Jung recommends reducing exposure by using bathroom exhaust fans for better room ventilation.

"If you must use hair care products, limit their use and ensure the space is well ventilated," Liu said. "Even without heating appliances, better ventilation can reduce exposure to volatile chemicals, such as D5 siloxane, in these products."

To more fully capture the complete nanoparticle formation and growth process, Jung said future studies should integrate nano-mobility particle sizing instruments capable of detecting particles down to a single nanometer. The chemical composition of these particles should also be evaluated.

"By addressing these research gaps, future studies can provide a more holistic understanding of the emissions and exposures associated with heat-based hair styling, contributing to improved indoor air pollution assessments and mitigation strategies," Jung said.

Gathering the data

Jung and Liu's experimental research was conducted in a residential architectural engineering laboratory that Jung designed: the Purdue zero Energy Design Guidance for Engineers (zEDGE) tiny house.

The zEDGE lab is a mechanically ventilated, single-zone residential building with a conditioned interior. A state-of-the-art high-resolution electrical low-pressure impactor (HR-ELPI+) from Jung's laboratory was used to measure airborne nanoparticles in indoor air in real time, second by second. In parallel, a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) was used to monitor volatile chemicals in real time.

The hair care routine emission experiments were conducted during a measurement campaign in zEDGE over a period of several months, including three experiment types: realistic hair care experiments that replicate actual hair care routines in the home environment, hot plate emission experiments that explore the relationship between the temperature of the hair care tools and nanoparticle formation, and surface area emission experiments that investigate how hair surface area impacts nanoparticle emissions during hair care events.

For the realistic hair care routine emission experiments, participants were asked to bring their own hair care products and hair styling tools to replicate their routines in zEDGE. Prior to each experiment, the participants were instructed to separate their hair into four sections. The hair length of each participant was categorized as long hair (below the shoulder) or short hair (above the shoulder). The sequence of each experiment consisted of four periods, to replicate a real-life routine.

After hair styling, the participants had two minutes to collect the tools and leave zEDGE; this was followed by a 60-minute concentration decay period in which zEDGE was unoccupied, and the HR-ELPI+ monitored the decay in indoor nanoparticle concentrations. The experiments and subsequent analysis focused on the formation of nanoparticles and resulting exposure during and after active hair care routine periods.