Indoor Nanoparticle Emissions and Exposures during Heat-Based Hair Styling Activities

Abstract

Heat-based hair styling activities, such as straightening, curling, and waving, emit volatile and semivolatile chemicals when used with hair care products like creams, lotions, and serums. This study investigates the formation of airborne nanoparticles (6–500 nm) during such activities as a previously unrecognized source of indoor air pollution. Experiments conducted in the Purdue zEDGE Test House revealed that hair styling at temperatures above 300 °F produced indoor nanoparticle concentrations ranging from 10,000 to over 100,000 particles cm^–3, with sub-100 nm particles typically accounting for more than 95% of total number concentrations at temperatures exceeding 360 °F. The primary mechanism for nanoparticle formation was heat-driven volatilization of cyclic siloxanes and various low-volatility constituents in hair care products, followed by nanoparticle nucleation and subsequent growth via condensation and coagulation. Ozonolysis of fragrance additives served as a secondary formation pathway. 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. These findings identify heat-based hair styling as a significant indoor source of airborne nanoparticles and highlight previously underestimated exposure risks. This work advances our understanding of the physical and chemical processes underlying emissions from personal care product use and underscores the need for mitigation strategies to reduce exposure in residential environments.

Introduction

Personal care products (PCPs), widely used for grooming and hygiene and often fragranced, are well-established sources of chemical emissions in residential indoor environments. − These emissions include volatile and semivolatile chemical constituents, some of which can undergo atmospheric oxidation to form secondary products. − Among PCPs, hair care products (HCPs) – such as shampoos, lotions, gels, oils, waxes, and sprays – are especially popular and frequently used for hair cleaning and styling. − A European survey found that 97% of participants use HCPs, with 80% applying them more than once per week and 40% applying them daily. Our previous work using a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) demonstrated that hair styling activities involving HCPs emit large quantities of volatile chemicals, including siloxanes, terpenes, and glycols, into the indoor environment.

While gas-phase emissions from hair care products have been reported, pathways for particle-phase pollutant generation during hair styling remain largely unexplored. Heat-based styling routines, involving high appliance temperatures and the use of HCPs, may create conditions conducive to the formation of particle-phase pollutants. Given the limited research on particle emissions from realistic heat-based hair styling, it remains unclear whether the high surface temperatures of styling appliances, combined with hair oils and HCPs, contribute to indoor particle generation. It has been reported that the brief use of a hair dryer can emit high concentrations of ultrafine particles (UFPs, particle diameter ≤ 100 nm) into the home environment, typically ranging from 10³ to 10⁵ particles cm^–3. − Chamber studies further suggest that 2 min of using a hair dryer and hair straightener can release 10¹⁰–10¹² particles within a size range of 5.6–560 nm. Volatilization of cyclic siloxanes and various low-volatility organics from heated appliance surfaces can drive nanoparticle nucleation and condensational growth. − Additionally, the atmospheric oxidation of these organics represents a secondary pathway for nanoparticle formation. −

Previous research has demonstrated that inhaled airborne nanoparticles can deposit throughout the human respiratory tract. , Long-term exposure to these particles has been associated with adverse health effects, including the accumulation of excessive lung burden. − Notably, sub-100 nm nanoparticles exhibit high deposition fractions (DFs) across the pulmonary, tracheobronchial, and head airways regions of the respiratory system. Pulmonary toxicity studies indicate that nanoparticle exposure can elicit more severe inflammatory and fibrotic responses than larger particles at equivalent doses. Realistic heat-based hair styling routines are typically performed near the nose and mouth, placing them in close proximity to the respiratory system. This positioning can result in elevated short-term exposure to high concentrations of airborne nanoparticles generated during styling. Once inhaled, these particles may deposit on airway surfaces, particularly in the deeper regions of the lungs. Understanding the complex dynamics of nanoparticle deposition in the respiratory tract is, therefore, essential for evaluating the health risks associated with heat-based personal care routines.

Human exposures and potential health risks from nanoparticle emissions can be evaluated using the respiratory tract deposited dose (RTDD) and the respiratory tract deposited dose rate (RTDDR) analysis. , These metrics estimate the deposition pattern of aerosol particles across different regions of the respiratory system, providing valuable insight into inhalation exposure risks. While a few studies have examined RTDDs and RTDDRs in residential indoor environments, they have yet to be applied to nanoparticles emitted during heat-based personal care routines. ,− Modeling RTDDs and RTDDRs requires accurate particle size distribution measurements, which can be obtained using advanced aerosol instruments such as the high-resolution electrical low-pressure impactor (HR-ELPI+). , This instrument provides real-time, high-resolution measurements across a broad particle size range, enhancing data quality and enabling comprehensive analysis of complex aerosol dynamics in residential environments. Its capabilities are especially valuable for investigating particles emitted from industrial processes, combustion sources, and human activities, making the HR-ELPI+ particularly well-suited for studying nanoparticle emissions from heat-based personal care routines. −

While indoor volatile organic compound (VOC) emissions from PCPs and HCPs are well documented, airborne nanoparticle emissions from heat-based hair styling remain largely unreported. These activities, marked by high appliance temperatures, chemically complex styling products, and proximity to the respiratory system, may generate substantial nanoparticle emissions. However, no real-time measurements or exposure assessments have been conducted in full-scale residential settings to date. This study addresses this gap by (a) examining temporal changes in nanoparticle number concentrations and size distributions during realistic heat-based hair styling routines; (b) evaluating the effects of appliance type, surface temperature, HCP type, hair length, and building ventilation on nanoparticle concentrations and size distributions; and (c) quantifying inhalation exposure using RTDD and RTDDR modeling. By providing a detailed characterization of indoor nanoparticle emissions during these routines, this research lays the groundwork for future investigations into their impact on indoor atmospheric chemistry and inhalation toxicity.

Materials and Methods

Study Site

Realistic hair styling routines were conducted in a single-zone, mechanically ventilated residential architectural engineering laboratory, the Purdue zero Energy Design Guidance for Engineers (zEDGE) Test House, located on the Purdue University campus in West Lafayette, Indiana, USA. Built on a mobile trailer, the zEDGE Test House is designed according to the Recreational Vehicle Industry Association (RVIA) guidelines, with an interior volume of 60.35 m³, and holds a National Organization of Alternative Housing (NOAH) certification. Exterior and interior photographs of the test house are provided in Figure S1. To maintain a controlled indoor environment, the outdoor air exchange rate (AER) was set at either 3.0 or 6.5 h^–1 using a powered ventilator equipped with two MERV 13 filters for outdoor air supply, a variable-speed bathroom exhaust fan (FV-0511VKS2, Panasonic Eco Solutions of North America, Newark, NJ, USA), and a portable air conditioner with an exhaust duct (QPCA08JAMWG1, Haier, Louisville, KY, USA). With four fans (two personal fans and two box fans) to promote adequate indoor air mixing, the test house can be considered as a completely mixed flow reactor (CMFR). Locations of the four fans inside the test house are illustrated in Figure S2. The nominal indoor air temperature was maintained at 68 °F (20 °C) using a single-zone ductless heating and cooling system (FTX12NMVJU, Daikin North America LLC, Houston, TX, USA) in conjunction with a portable air conditioner. Detailed ventilation conditions for each AER setting, along with their corresponding indoor-outdoor pressure differentials, are provided in Text S1.

Real-Time Measurement of Nanoparticles, VOCs, and O₃ in the Purdue zEDGE Test House

An HR-ELPI+ (Dekati Ltd., Kangasala, Finland) was used in the loft area of the test house to sample emitted particles with aerodynamic diameters (D _a) from 6 to 10,000 nm at 1 Hz. The HR-ELPI+ uses an advanced iterative inversion algorithm to improve the size resolution beyond the standard ELPI+, increasing the number of size bins from 14 to 100. Additional details regarding the operational principle of the ELPI+ can be found in other studies. , The HR-ELPI+ consists of a corona charger, a low-pressure cascade impactor that includes 14 impactor stages, and sensitive electrometers. Oil-soaked sintered collection plates were used to eliminate particle bounce and impactor overloading. During the initial phase of the measurement campaign, the HR-ELPI+ inlet was not connected to a sampling line to minimize particle deposition losses. For subsequent experiments, a 1.2 m conductive silicone tube was connected to the inlet. The open end of the sampling line was hung in the lower level of the test house (kitchen area) in close proximity to the locations of the participants. For experiments conducted with a sampling line, particle deposition losses were calculated and corrected in the data analysis.

A PTR-TOF-MS (PTR-TOF 4000, Ionicon Analytik Ges.m.b.H., Innsbruck, Austria) was used to measure VOC mixing ratios at a frequency of 1 Hz, with an inlet sampling rate of 100 sccm, utilizing hydronium (H₃O⁺) as the reagent ion. Major detected compounds included siloxanes (C₆H₁₈O₃Si₃, C₈H₂₄O₄Si₄, C₁₀H₃₀O₅Si₅, C₁₂H₃₆O₆Si₆, C₈H₂₄O₂Si₃, C₁₀H₃₀O₃Si₄, and C₁₂H₃₆O₄Si₅), terpenes (C₁₀H₁₆, C₁₀H₁₄O, C₁₀H₁₆O, C₁₀H₁₈O, C₁₀H₂₀O, and C₁₀H₂₂O), and glycols (C₃H₈O₂, C₃H₈O₃, and C₆H₁₄O₂). Due to the absence of a fast gas chromatograph at the PTR-TOF-MS inlet, it was not possible to distinguish isomers with identical chemical formulas. Consequently, the mixing ratios reported in this study represent the total contribution of all of the isomers with the same formula. Table S1 summarizes the detected VOCs, including their m/z values, proton transfer reaction rate constants, and tentative isomer identifications. A perfluoroalkoxy (PFA) sampling line (3/8 in. OD) equipped with a polytetrafluoroethylene (PTFE) membrane filter (1 μm pore size) was positioned at the center of the test house for air sampling. Details on daily calibration of the PTR-TOF-MS, conversion of raw ion signals to mixing ratios, and data analysis procedures are provided in our previous studies. , Indoor atmospheric ozone (O₃) mixing ratios were measured using a photometric nondispersive ultraviolet (UV at 254 nm) absorption-based O₃ analyzer (Serinus 10, ACOEM Ecotech, Melbourne, Australia). The experimental layout for all instruments in the zEDGE Test House is shown in Figure S2.

Protocol for Realistic Heat-Based Hair Styling Experiments Using HCPs and Heating Appliances

Participants (n = 7) were invited to conduct realistic hair styling routines (n = 21) during the measurement campaign, following a Purdue University Institutional Review Board (IRB)-approved experimental protocol (IRB-2022-1573). These experiments involved various HCPs (n = 5) and heating appliances (n = 3). The HCPs represented different product types, including hair creams, serums, lotions, and sprays. Detailed information about the HCPs, including their ingredients, density, and amount dispensed per pump, is provided in Table S2.

Participants used three types of heating appliances: hair straighteners, curlers, and wavers. Both short-haired (above the shoulder) and long-haired (below the shoulder) participants were invited to replicate their typical hair styling routines. The experiments were performed by using various combinations of HCPs, heating appliances, surface temperatures, and outdoor AERs to simulate realistic indoor scenarios. According to the approved IRB protocol, the HCPs and heating appliances were chosen by the participants for each individual experiment. A comprehensive summary of the experimental IDs, experimental conditions, product use, and associated participants is presented in Table S3.

Participants were instructed to perform realistic hair styling in five steps: (1) divide hair into four sections (left rear, left front, right rear, right front) before the experiment starts; (2) enter the test house and stay for a period of time to establish background emissions; (3) preheat the heating appliance surface to desired temperature; (4) apply 2 pumps of a HCP to one hair section and heat this hair section with the chosen heating appliance, then repeat this step for the remaining three hair sections; (5) after finishing step (4), wrap up, and exit the test house.

For participants with long hair, experiments were designed with a 10 min background period with one or two participants in the test house to account for human-related particle and VOC emissions, followed by a 25 min source period and a 60 min decay period. AERs were controlled at either 3.0 or 6.5 h^–1 throughout the experiment to assess the impact of building ventilation conditions on nanoparticle concentrations. During the source period, participants preheated the heating appliance for 3 min to the desired temperature. Hair styling was performed for 20 min by applying the selected HCP to each hair section and styling for 5 min per section, following the order: left rear, left front, right rear, right front. After completing the styling routine, participants had 2 min to wrap up the tools and vacate the test house, which remained unoccupied for the 60 min decay period. The sequence for long hair experiments is illustrated in Figure S3a.

For participants with short hair, the protocol was similar, with the following adjustments: two participants were present in the test house, and the source period was reduced to 15 min. The AER was fixed at 6.5 h^–1. During the source period, participants preheated the appliance and conducted 10 min of styling, applying the chosen HCP to each hair section, and heating it for 2.5 min per section in the same order as the long hair protocol. The sequence for short hair experiments is illustrated in Figure S3b.

Calculation of Respiratory Tract Deposited Dose Rates and Doses

The human respiratory tract is typically divided into three regions: the head airways (HA) region, also referred to as the extra-thoracic region (extending from the nose and mouth to the larynx); the tracheobronchial (TB) region (spanning from the trachea to the terminal bronchioles); and the pulmonary (P) region, also known as the alveolar interstitial region (comprising the respiratory bronchioles, alveolar ducts, and alveoli). − In this study, the Multiple-Path Particle Dosimetry (MPPD) model (version 3.04, Applied Research Associates Inc., Albuquerque, NM, USA) was adopted to obtain the DFs for D _a ranging from 6 to 500 nm in each respiratory tract region (Figure S4). DFs represent the ratio of the number of particles depositing in each region of the respiratory tract to the number of particles of the same size entering the tract, and are dependent on particle size. − The MPPD model simulates the deposition and clearance of both monodisperse and polydisperse aerosols in the respiratory tracts of humans and laboratory animals for particles ranging from ultrafine (1 nm) to coarse (100 μm) in size. −

During each hair care routine, the size-resolved, number-based respiratory tract deposited dose rate (dRTDDR _N /dlogD _a; min^–1) for each region of the respiratory tract can be calculated as follows:

$$\frac{d{\mathrm{RTDDR}}_N}{d\log D_{\mathrm{a}}}=Q\times \mathrm{DF}\times \frac{dN}{d\log D_{\mathrm{a}}}$$ 1

where Q is the human inhalation rate for an adult engaged in light activity (1.25 m³ h^–1), DF is the size-resolved deposition fraction, and dN/dlogD _a is the measured particle number size distribution function. Both DF and dN/dlogD _a are functions of the particle aerodynamic diameter, D _a. The size- and time-integrated, number-based respiratory tract deposited dose (RTDD _N ) is calculated by integrating the size-resolved dose rate, dRTDDR _N /dlogD _a, over the particle diameter range of D _a = 6–500 nm and then further integrating the result over time.

Results and Discussion

Impact of Heating Temperature on Indoor Nanoparticle Emissions during Hair Straightening

Previous studies have shown that nanoparticle emissions from heated metal surfaces are strongly correlated with temperature. To assess the influence of heating temperature on nanoparticle emissions during realistic hair care routines, hair straightening experiments were conducted using the same hair care product (HCP, product C) under a nominal AER of 6.5 h^–1. Experiments were performed at three surface temperatures: 410 °F (210 °C), 300 °F (148.9 °C), and 70 °F (21.1 °C) (Figure a–c). The results include temporal variations in nanoparticle number size distributions (first row), size-integrated number concentrations (second row), VOC mixing ratios (third row), indoor O₃ mixing ratios (fourth row), and cumulative RTDD _N (fifth row).

1.

Time-resolved evaluation of hair straightening activities with different heating temperatures under an AER ∼ 6.5 h^–1. First row: nanoparticle number size distributions from 6 to 500 nm; second row: size-integrated nanoparticle number concentrations (N) from 6 to 500 nm; third row: mixing ratios of siloxanes (sum of C₆H₁₈O₃Si₃, C₈H₂₄O₄Si₄, C₁₀H₃₀O₅Si₅, C₁₂H₃₆O₆Si₆, C₈H₂₄O₂Si₃, C₁₀H₃₀O₃Si₄, and C₁₂H₃₆O₄Si₅), terpenes (sum of C₁₀H₁₆, C₁₀H₁₄O, C₁₀H₁₆O, C₁₀H₁₈O, C₁₀H₂₀O, and C₁₀H₂₂O) and glycols (sum of C₃H₈O₂, C₃H₈O₃, and C₆H₁₄O₂); fourth row: mixing ratios of indoor atmospheric O₃; fifth row: cumulative, number-based adult respiratory tract deposited doses (RTDD _N ). (a) (A, B, C, D, E): 410 °F (Exp. 18A); (b) (F, G, H, I, J): 300 °F (Exp. 18B); (c) (K, L, M, N, O): 70 °F (Exp. 18C).

The first row shows a sudden increase in nanoparticle number size distributions after one min of preheating the hair straightener at 410 and 300 °F, with number size distributions (dN/dlogD _a) reaching peaks of ∼6 × 10⁵ and ∼8 × 10³ particles cm^–3, respectively. The second row reveals multiple spikes in size-integrated number concentrations (N) for both temperatures, likely corresponding to the repeated application of heat to individual hair sections. The highest spikes in N reached ∼5 × 10⁵ and ∼2 × 10⁴ particles cm^–3 at 410 and 300 °F, respectively. In contrast, at 70 °F, where the hair straightener was not heated, indoor nanoparticle emissions were minimal during both the preheating and styling periods, with a peak N of only ∼1 × 10³ particles cm^–3.

Nanoparticles emitted at all temperatures primarily consisted of sub-100 nm UFPs. While larger particles (D _a = 100–500 nm) were observed during the 20 min hair styling periods, UFPs typically accounted for more than 95% of total number concentrations. A detailed summary of mean sub-100 nm and sub-500 nm size-integrated nanoparticle number concentrations for each experiment is provided in Table S4.

In addition to differences in nanoparticle emissions, VOC emission profiles varied across hair care routines, reflecting both temperature effects and product use. As shown in the third row, four distinct peaks were observed at each temperature, corresponding to the heating of the four hair sections. Among the VOCs, siloxanes were the dominant emitted species across all temperatures, with peak mixing ratios of ∼350, 250, and 200 ppb for 410, 300, and 70 °F, respectively. Compared with the high siloxane concentrations, the contributions of terpenes and glycols were relatively minor, with peak mixing ratios for both species generally below 20 ppb across all three temperatures. The higher siloxane concentration at 410 °F indicates that the increase in heating temperature may result in an increase in siloxane emissions. Further analysis about the relationship between siloxane emissions and HCP use during heat-based hair styling routines at different temperatures can be found in our previous study.

Based on the experimental results, it is expected that nanoparticle emissions during heat-based hair styling activities are mainly dependent on the surface temperature of the appliance being used. At 410 and 300 °F, nanoparticle emissions during the styling period are primarily attributed to the volatilization of chemicals present on the hair under high heating temperatures. Previous studies have indicated that cyclic siloxanes can vaporize at elevated temperatures and subsequently nucleate into nanoparticles as they cool away from the heating source. Nanoparticle emissions are likely attributed to the desorption and subsequent nucleation of low-volatility chemicals present in HCP formulations, which desorb from both the heating appliance surfaces and hair strands during styling routines. ,

Since the hair straightener used in the experiments offers only three temperature settings, additional hot plate experiments were conducted in the test house using the same HCP (product C) at temperatures of 200 °F (93.3 °C), 250 °F (121.1 °C), 370 °F (187.8 °C), 450 °F (232.2 °C), and 500 °F (260 °C) to further explore the correlation between heating temperature and nanoparticle emissions (Figure S5). Nanoparticles reached high concentrations (dN/dlogD _a > 10⁵ particles cm^–3) only when the hot plate temperature was at or above 370 °F. Significantly lower nanoparticle emissions were observed at temperatures at or below 250 °F, with dN/dlogD _a values less than 10³ particles cm^–3. The increase in emissions at temperatures above 300 °F likely corresponds to the boiling points of specific ingredients in the HCPs. Table S5 summarizes the boiling points of selected compounds in each HCP. For Product C, the five selected ingredients, decamethylcyclopentasiloxane (D5), menthol, propylene glycol, dimethiconol, and phenyl trimethicone, have boiling points ranging from 333 to 509 °F. These values suggest that while some volatilization can occur at lower temperatures, substantial nanoparticle formation is expected only when heating exceeds 300 °F.

At 70 °F, where no heat was applied, indoor nanoparticle emissions are likely driven by the atmospheric oxidation of selected organic compounds in the HCPs. Previous studies have shown that the ozonolysis of terpenes can lead to the formation of secondary organic aerosol (SOA). − Certain terpene species are reported to have high reaction rate constants with indoor O₃. − As shown in the fourth row, indoor O₃ levels ranged from 20 to 40 ppb before the experiments, and a moderate decrease in O₃ mixing ratios (∼15 ppb) was typically observed during the hair straightening period. While measured terpene mixing ratios were relatively low, the observed O₃ depletion suggests that some degree of terpene ozonolysis may have occurred. Therefore, SOA formation via this pathway may have contributed to the low-level nanoparticle emissions observed at 70 °F, although the extent of this contribution remains uncertain.

It has been reported that SOA can also form from the oxidation of VOCs by other atmospheric oxidants such as nitrate (NO₃), chlorine (Cl), and hydroxyl radicals (OH). ,− However, O₃ was established as the primary oxidant in this study. A previous investigation indicated that NO₃ and Cl are typically not dominant oxidants in indoor environments due to their relatively low concentrations. Furthermore, measurements using laser-induced fluorescence–fluorescence assay by gas expansion (LIF–FAGE) showed limited detection of OH radicals in the test house, suggesting that SOA formation via OH-initiated VOC oxidation was likely minimal under the conditions tested. In addition, no hydroperoxyl radicals (HO₂) were detected above the instrument’s detection limit in this study. Because HO₂ is a key intermediate in terpene ozonolysis, its absence indicates a limited level of SOA formation. While terpene ozonolysis may have contributed to some SOA formation at lower temperatures (e.g., 70 °F), its overall contribution was likely minor compared to the substantial nanoparticle production observed at higher temperatures via heat-induced volatilization of HCP ingredients.

Since hair styling activities typically occur near the occupant’s breathing zone, it is critical to evaluate potential nanoparticle deposited doses in the respiratory tract during these events. The fifth row presents the cumulative RTDD _N for the experiments, showing that higher nanoparticle emissions result in greater deposited doses across all respiratory tract regions. At 410 °F, an estimated 6.49 × 10¹⁰ particles were deposited in the respiratory tract per occupant. This number decreased to 8.81 × 10⁸ at 300 °F and 2.27 × 10⁸ at 70 °F.

Impact of HCP Type on Indoor Nanoparticle Emissions during Hair Straightening

As previously discussed, the heating temperature plays a key role in influencing nanoparticle emissions, even more so than VOC emissions, when using the same HCP during styling. However, the chemical composition of HCPs varies considerably, and VOC emissions are highly dependent on the specific product used. To examine the relationship between VOC and nanoparticle emissions across different HCPs, four products (D–G) were used to simulate hair straightening routines at 370 °F (187.8 °C) under a nominal AER of 6.5 h^–1. Figure a–d presents the temporal variations in indoor nanoparticle number size distributions (first row), size-integrated nanoparticle number concentrations (second row), VOC mixing ratios (third row), indoor O₃ mixing ratios (fourth row), and the associated cumulative RTDD _N (fifth row) for products (D–G). Additional temporal data for experiments not included in Figures and are provided in Figures S6 and S7.

2.

Time-resolved evaluation of hair straightening activities at 370 °F with different HCPs under an AER ∼ 6.5 h^–1. First row: nanoparticle number size distributions from 6 to 500 nm; second row: size-integrated nanoparticle number concentrations (N) from 6 to 500 nm; third row: mixing ratios of siloxanes (sum of C₆H₁₈O₃Si₃, C₈H₂₄O₄Si₄, C₁₀H₃₀O₅Si₅, C₁₂H₃₆O₆Si₆, C₈H₂₄O₂Si₃, C₁₀H₃₀O₃Si₄, and C₁₂H₃₆O₄Si₅), terpenes (sum of C₁₀H₁₆, C₁₀H₁₄O, C₁₀H₁₆O, C₁₀H₁₈O, C₁₀H₂₀O, and C₁₀H₂₂O) and glycols (sum of C₃H₈O₂, C₃H₈O₃, and C₆H₁₄O₂); fourth row: mixing ratios of indoor atmospheric O₃; fifth row: cumulative, number-based adult respiratory tract deposited doses (RTDD _N ). (a) (A, B, C, D, E): Product D (Exp. 19A); (b) (F, G, H, I, J): Product E (Exp. 19B); (c) (K, L, M, N, O): Product F (Exp. 19C); (d) (P, Q, R, S, T): Product G (Exp. 19D).

There was no noticeable change in nanoparticle emissions across different HCPs when the samples were tested under the same heating temperature. Nanoparticle profiles (first and second row) for products D–G exhibited similar emission magnitudes as observed in Figure a, with peak dN/dlogD _a values around 3 to 5 × 10⁵ particles cm^–3 and peak N values ranging from 2 to 3 × 10⁵ particles cm^–3. In contrast to the consistent nanoparticle trends, VOC emissions varied significantly across the HCPs (third row). For products D, F, and G, siloxanes were the dominant emitted VOCs, contributing to over 80% of total emissions with peak mixing ratios of 570, 82, and 133 ppb, respectively. For product E, glycols were the dominant species, accounting for 59% of the total emissions, while siloxanes contributed 40%. The peak siloxane and glycol mixing ratios for product E were 402 and 549 ppb, respectively. Terpene emissions were minor for products D–F, contributing less than 1.2% of the total VOCs. Product G showed the highest terpene emissions, accounting for 19% of the total VOC profile. Despite the presence of terpenes, their relatively low concentrations and the moderate O₃ depletion observed across all styling events (fourth row) suggest that nanoparticle formation via ozonolysis was limited.

In addition to this realistic hair styling study conducted in a residential setting, a chamber study by Schripp et al. also reported nanoparticle emissions from heating a hair straightener under comparable experimental conditions. In that study, approximately 10¹⁰ particles, primarily in the sub-100 nm size range (measurement range: 5.6–560 nm), were emitted within 2 min of heating a hair straightener to 200 °C (392 °F) under an AER of about 3.0 h^–1. The highest emission rate occurred in the nucleation mode at approximately 10 nm, with nanoparticles growing to about 400 nm when significant levels of VOCs and semi-VOCs (SVOCs) were present in the air. Similarly, in this study, nanoparticle growth was observed, likely driven by the condensation of low-volatility constituents in the HCPs and by nanoparticle coagulation.

It was also observed that 6–10 nm nanoparticle number concentrations were relatively low compared to those in the 10–100 nm range. This may be attributed, in part, to instrumental limitations of the HR-ELPI+. Although the HR-ELPI+ includes a filter stage designed to target particles as small as 6 nm, its lowest impactor cut-point is 16 nm, limiting its ability to accurately resolve number size distributions for particles smaller than 10 nm. In addition, the chemical characteristics of the emitted volatile and low-volatility compounds from the HCPs likely influenced the observed nanoparticle nucleation and condensational growth process. In this study, it is possible that emitted cyclic siloxanes and other low-volatility organic compounds first underwent nucleation and then condensed onto newly formed nanoparticles, promoting their growth to larger sizes. To better understand the mechanisms driving nanoparticle generation during heat-based hair styling, future studies should employ aerosol mass spectrometry to characterize the real-time molecular composition of the emitted particles.

These findings confirm that indoor nanoparticle emissions during hair styling are primarily driven by heat, reinforcing the conclusion that volatilization followed by nucleation and condensation of chemical constituents is the dominant mechanism of particle formation. Furthermore, a potential correlation between the presence of low-volatility compounds, particularly from siloxane-based HCPs, and nanoparticle formation is suggested, warranting further investigation in future studies.

In the case of alcohol-based HCPs, a previous study on benzyl alcohol suggests that its contribution to SOA formation may be significant, potentially occurring through adsorption or absorption of oxidation products. As shown in Table S5, the alcohol ingredients present in HCPs can have high boiling points exceeding 400 °F (204 °C), requiring elevated heating temperatures for significant volatilization. The primary and secondary mechanisms driving emissions from alcohol-based HCPs are likely influenced by the physical and chemical properties of individual ingredients and their subsequent atmospheric reactions, which could be further explored by using the experimental framework presented in this study.

Since nanoparticle emissions showed little variation across products at similar temperatures, the cumulative RTDD _N values were also comparable, with the majority of the dose occurring during the 20 min hair styling period (fifth row). Under well-ventilated conditions (AER ∼ 6.5 h^–1), it took at least 35 min for nanoparticle concentrations to decay to background levels. An occupant remaining in the space for 60 min after hair styling could receive a total deposited dose of approximately 3.66–5.05 × 10¹⁰ particles. Previous studies have reported geometric mean AERs in American residential homes to be below 1.5 h^–1, − suggesting that occupants in typical residential settings are likely to receive even higher nanoparticle doses than those estimated under the experimental conditions of this study.

Impact of Hair Styling Appliances on Nanoparticle Size Distributions and Respiratory Tract Deposited Dose Rates

Using deposition fractions (DFs) and measured nanoparticle size distribution functions (dN/dlogD _a), number-based respiratory tract deposited dose rates can be calculated as shown in eq . Figure a–c presents a size-resolved analysis of 20 min hair styling periods using a hair straightener, curler, and waver, operated at comparable heating temperatures (370 °F (187.8 °C), 360 °F (182.2 °C), and 380 °F (193.3 °C), respectively) for nanoparticles ranging from 6 to 500 nm. The analysis includes measured nanoparticle number size distributions (first row), log-normal fitted nanoparticle number size distributions (second row), and size-resolved respiratory tract deposited dose rates (dRTDDR _N /dlogD _a) (third row) for each hair styling appliance. Additional experimental results not shown in Figure are provided in Figures S8–S14. Mode peaks, standard deviations, and number concentrations for all log-normal fitted nanoparticle size distributions are summarized in Table S6. Peak dRTDDR _N /dlogD _a values for each respiratory tract region are detailed in Table S7.

3.

Size-resolved analysis of nanoparticle emissions and exposures during 20 min hair styling routines using different heating appliances operating at similar surface temperatures, for nanoparticles ranging from 6 to 500 nm. First row: nanoparticle number size distributions; second row: log-normal fitted nanoparticle number size distributions; third row: median respiratory tract deposited dose rates (number-based) (dRTDDR _N /dlogD _a). (a) (A, B, C): experiment using a hair straightener at 370 °F (Exp. 19A); (b) (D, E, F): experiment using a hair curler at 360 °F (Exp. 24A); (c) (G, H, I): experiment using a hair waver at 380 °F (Exp. 27B).

According to the first row, the majority of emitted particles during hair straightening and curling fall within the UFP size range, with prominent peaks between 20–30 and 25–35 nm, respectively. In contrast, hair waving predominantly emits larger nanoparticles, with the size distribution peaking at around 100 nm. The peak values of the median dN/dlogD _a for hair straightening, curling, and waving were 1.72 × 10⁵, 8.11 × 10⁴, and 6.20 × 10³ particles cm^–3, respectively. Interestingly, hair waving generates significantly fewer UFPs than the other two appliances, with sub-100 nm particles accounting for only 70% of total particle emissions versus 95% and 99% for hair straightening and curling, respectively. While hair straightening and curling produce a substantial number of sub-30 nm UFPs, these particles are nearly absent during hair waving. As shown in the second row, the dN/dlogD _a curves reveal bimodal and trimodal characteristics for hair straightening and waving, respectively, whereas hair curling exhibits a single prominent mode. The shift in particle number size distributions toward larger diameters during hair waving suggests significant nanoparticle growth, likely driven by enhanced condensation of low-volatility vapors onto the newly formed nanoparticles. −

The particle number size distributions are strongly correlated with the respiratory tract deposited dose rates (dRTDDR _N /dlogD _a); however, high particle number concentrations at specific sizes do not necessarily result in high dose rate values (third row). Regardless of the styling appliance used, the pulmonary (P) region exhibited the highest dRTDDR _N /dlogD _a values among the three respiratory tract regions for nanoparticles from 6 to 500 nm. For hair straightening and curling, the dRTDDR _N /dlogD _a peaks aligned closely with the peaks of the measured particle number size distributions. However, hair waving deviated from this trend, with its dRTDDR _N /dlogD _a peak occurring at around 60 nm. This discrepancy arises because dose rates are influenced not only by dN/dlogD _a but also by the deposition fractions (DFs). As shown in Figure S4, between 20 and 100 nm, DFs for the pulmonary region are highest among the three respiratory tract regions, with a peak near 30 nm. In contrast, DFs for 100–500 nm particles are generally lower across all regions. Although hair waving emitted a greater proportion of particles larger than 100 nm compared to straightening and curling, the peak dose rates remained concentrated in the sub-100 nm range, highlighting the greater deposition efficiency of UFPs in the human respiratory tract.

To contextualize the nanoparticle emissions from heat-based hair styling activities, Table presents a comparison of reported particle number concentrations from various indoor activities, both heat-based and nonheat-based, measured using real-time aerosol instrumentation. Nonheat-based activities, such as mopping and disinfecting surfaces with fragranced consumer products, predominantly generate UFPs below 50 nm, with number concentrations in the range of 10⁴–10⁵ particles cm^–3, comparable to those from heat-based hair styling routines. ,, It is important to recognize that different aerosol measurement techniques characterize nanoparticles using different particle diameter definitions. The HR-ELPI+ measures particles based on their aerodynamic diameter (D _a), whereas the scanning mobility particle sizer (SMPS) and the particle size magnifier–scanning mobility particle sizer (PSMPS) report particle sizes according to their electrical mobility diameter (D _em). Discrepancies between these sizing methods arise due to size-dependent variations in the particle effective density (ρ_eff), which is influenced by the particle composition and morphology. Despite these differences, prior studies have shown reasonable agreement between the two measurement techniques. ,

1. Comparison of Measured Nanoparticle Number Concentrations Across Heat-based and Nonheat-based Indoor Activities.

indoor activity	aerosol instrument	measured particle size range (nm)	N (cm–3)
heat-based hair styling	HR-ELPI+	6–500	103–105
electric stove cooking	SMPS	15–685	105
gas stove cooking	PSMPS	1.18–3	105–106
terpene mopping	SMPS, PSM	1.2–100	105
thymol-based disinfecting	HR-ELPI+	10–100	104
ethanol-based disinfecting	HR-ELPI+	6–300	104
laser printing	SMPS	200–500	104–105

In addition to the real-life indoor activities summarized in Table , a screening study of PCPs reported that the use of fragranced body lotion and hair treatment can emit approximately 10⁴ particles cm^–3 with diameters larger than 8 nm. Another chamber study found that terpene-based perfumes can release between 10³ and 10⁵ particles cm^–3 within the 20–1000 nm size range. These findings demonstrate that the use of PCPs has the potential to generate nanoparticles, with number concentrations on the order of 10³–10⁵ particles cm^–3.

Respiratory Tract Deposited Doses during Heat-Based Hair Styling Activities

Deposited nanoparticle doses are critical for assessing human exposure and potential inhalation toxicity. To illustrate how different factors influence these doses, Figure a–d presents the total number-based respiratory tract deposited doses (RTDD _N s) during the hair styling period, comparing variations by heating temperature, hair length, and the use of two different hair care products (Products D and E). RTDD _N values represent the size- and time-integrated deposited dose, calculated from dRTDDR _N /dlogD _a over the styling period. A summary of RTDD _N values for all experiments, limited to the active hair styling (source) period and excluding the subsequent decay period, is provided in Table S8.

4.

Total, number-based adult respiratory tract deposited doses (RTDD _N ) throughout the hair styling (source) periods. (a) Experiments using product C and a hair straightener at different surface temperatures: 410 °F (Exp. 18A), 300 °F (Exp. 18B), and 70° (Exp. 18C); (b) experiments using product C and a hair straightener at 370 °F for different hair lengths under different AERs (values presented for short hair at AER ∼ 6.5 h^–1 are the values of Exp. 25C; values presented for long hair at AER ∼ 6.5 h^–1 are the mean values of Exp. 31A – 31C; values presented for long hair at AER ∼ 3.0 h^–1 are the mean values of Exp. 32A – 32C); (c) experiments using product D and different heating appliances at similar temperatures: hair straightener (370 °F, Exp. 19A), hair curler (360 °F, Exp. 24A), and hair waver (380 °F, Exp. 27B); (d) experiments using product E and different heating appliances at similar surface temperature: hair straightener (370 °F, Exp. 19B), hair curler (360 °F, Exp. 24C), and hair waver (380 °F, Exp. 27C).

Across all scenarios, the pulmonary (P) region consistently received the largest fraction of total deposited doses (48–54%), followed by the tracheobronchial (TB) and head airways (HA) regions. The total number of nanoparticles deposited during hair straightening at 410 °F (4.65 × 10¹⁰ particles) was 2 orders of magnitude higher than at 300 °F (4.61 × 10⁸ particles) and over 600 times higher than at 70 °F (7.08 × 10⁷ particles) (Figure a). For context, the number of nanoparticles deposited during 20 min of hair straightening at 410 °F is equivalent to 20–200 min of exposure to traffic-related nanoparticles inside a vehicle cabin or 30–100 min of exposure to ambient urban aerosol in a typical North American city.

Assuming that the heating time required for short hair styling is half that of long hair, the total nanoparticle doses for short and long hair straightening were estimated to be 4.57 × 10⁹ and 1.61 × 10¹⁰ particles under an AER ∼ 6.5 h^–1, respectively. This indicates that long hair straightening results in a 70% higher dose compared to that of short hair under identical ventilation conditions. Furthermore, a lower AER can extend the residence time of emitted cyclic siloxanes and various low-volatility organic compounds from the HCPs, influencing subsequent nucleation and condensation dynamics. A previous study reported that longer residence times promote nanoparticle nucleation, leading to elevated nanoparticle concentrations. Consistent with this finding, we observed that reducing the AER to ∼3.0 h^–1 increased the total dose to 2.39 × 10¹⁰ particles, which is 33% more than the total dose at an AER ∼ 6.5 h^–1 (Figure b). In addition to increasing the nanoparticle dose during hair styling, a lower AER also significantly slowed the nanoparticle concentration decay, prolonging the time required for nanoparticles to return to background levels (Figure S7). This suggests that poorly ventilated residential environments can contribute to additional exposure during the decay period.

Figure c,d shows that total RTDD _N s were comparable across different HCPs. However, nanoparticle doses were consistently higher when using a hair straightener (2.61 × 10¹⁰ to 3.02 × 10¹⁰ particles) compared to a curler (8.24 × 10⁹ to 1.21 × 10¹⁰ particles) or a waver (3.08 × 10⁸ to 6.87 × 10⁸ particles) at similar heating temperatures, regardless of HCP type. While hair waving emitted significantly fewer nanoparticles than straightening and curling, this does not necessarily imply lower exposure risk. A small number of large particles may carry substantially greater mass than numerous small particles, underscoring the importance of size-resolved evaluations to fully assess the human health implications of different hair styling techniques.

Future Directions

This study marks a significant advancement in understanding indoor nanoparticle emissions and associated human exposures resulting from the routine use of HCPs and heat-based styling tools. Our findings reveal consistently high nanoparticle concentrations (dN/dlogD _a ∼ 10⁵ particles cm^–3) at heating temperatures above 300 °F, with individual occupants potentially inhaling and depositing ∼10¹⁰ nanoparticles across their respiratory tract, even under well-ventilated conditions. Nanoparticles generated during heat-based styling likely consist of a complex mixture of cyclic siloxanes and various low-volatility organic compounds originating from HCP formulations, as well as their potential atmospheric oxidation products. Future research should employ online aerosol mass spectrometry to enable real-time molecular characterization of the emitted particles, providing deeper insight into the chemical complexity of emissions and their implications for indoor atmospheric chemistry. Such measurements would enable the identification of key molecular constituents involved in nanoparticle nucleation and condensational growth processes. Furthermore, the lower detection limit of the HR-ELPI+ (6 nm) excludes nanocluster aerosol with diameters between 1 and 3 nm, which are important in the initial stages of nanoparticle formation. To more comprehensively capture the complete nanoparticle number size distribution, future studies should integrate nano mobility particle sizing instruments capable of detecting particles below 6 nm. 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.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c14384.

• Photos and layouts of the Purdue zEDGE Test House, illustration of experimental sequences, m/z values for tentatively identified VOCs, nanoparticle deposition fractions, detailed descriptions of all emission experiments, summary of cumulative deposited dose values for each experiment, and additional figures not included in the main manuscript, such as temporal variations in nanoparticle concentrations, size distributions, and size-resolved respiratory tract deposited dose rates (PDF)

The authors declare no competing financial interest.