A standard routine to protect hair from heat damage is actually turning your bathroom into a dangerous emissions zone, as scientists find that just 10-20 minutes of styling with common products results in some 10 billion ultrafine particles being inhaled straight to the lungs – akin to standing next to a busy road in peak hour or smoking several cigarettes.
Purdue University researchers have uncovered just what happens when hair-styling products meet heat, providing the first quantitative evidence that everyday haircare routines can be a significant source of indoor nanoparticle (or, more specifically, ultrafine particle) emissions. Conducting controlled experiments in a purpose-built tiny house – known as the Purdue zero-Energy Design Guidance for Engineers (zEDGE), aimed to recreate real-world conditions for testing such pollutants – the scientists recruited three people to undertake seven normal hairstyling routines with products and tools they'd use.
Participants used their own styling products – sprays, serums creams and protectants typically applied before or during heat treatment – and used tools including flat irons, curling irons and blow dryers, with flat irons making up the bulk of the sessions. Tool temperatures were set to commonly used levels, ranging from 150 °C (302 °F) to 230 °C (446 °F). While the products weren't named, it's more what's in them; you can read the breakdown in the study's supplementary material.
While it's a small study size, repeated experiments were nonetheless carefully controlled and the advanced testing was more than enough to establish clear emission patterns and chemical fingerprints resulting from these common real-world styling sessions. During these 10-20-minute hairstyling routines, researchers continuously measured air quality with scanning mobility particle sizers (SMPS) and condensation particle counters (CPC), which can detect and count particles as tiny as just a few nanometers in size. Gas chromatography and mass spectrometry were then used to identify the chemical composition of these emissions.
What they found was that, through heating, the nanoparticles released were largely composed of condensed volatile organic compounds (cVOCs). A significant amount were cyclic volatile methyl siloxanes (cVMSs), especially decamethylcyclopentasiloxane (D5) – a silicone commonly used in hair-care formulations that promote smoothness and shine. When heated, these siloxanes vaporize and condense into ultrafine particles, which are then floating around in the immediate air and are readily inhaled. Estimated inhalation exposure to D5 ranged from 1-17 mg per styling session. While this might not sound like a lot, the key is the fact it's in particle form and frequency of exposure.
Heating hair products breaks down silicone compounds like D5 into ultrafine particles smaller than 100 nm in size – perfect for reaching the deepest parts of the lungs and even entering the bloodstream. And they have the ability to build up (bioaccumulate) in our body. And in terms of what ends up in our lungs, a 20-minute styling session can release particle numbers akin to smoking several cigarettes.
"This is really quite concerning," said lead researcher Nusrat Jung, an assistant professor in the Lyles School of Civil and Construction Engineering at Purdue University. "The number of nanoparticles inhaled from using typical, store-bought hair-care products was far greater than we ever anticipated."
Across the 21 sessions, a 10–20 minute routine released tens of millions of ultrafine particles per cubic centimeter of air, with the stylist inhaling around 10 billion.
It's also worth noting the role heat plays in this hazard. The amount of ultrafine particles released increased sharply once tool temperatures exceeded around 150 °C (302 °F) to 230 °C (446 °F) – so flat irons (or hair straighteners) were the worst offenders, and blow-dryers were the least polluting (but still had elevated levels well above normal indoor concentrations).
"Atmospheric nanoparticle formation was especially responsive to these heat applications," said PhD researcher Jianghui Liu. "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."
By now we know that ultrafine particles are a huge health (and environmental) concern, because of their ability to penetrate deep into the lungs' alveoli and make their way into the bloodstream. Earlier studies have linked exposures at this scale to oxidative stress, respiratory inflammation, and neurological and neurodevelopmental problems.
This latest study builds on the researchers 2023 findings that looked into the harmful chemicals of hair-care products released into the air during styling sessions.
"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."
The good news is that you don't have to throw out the flat iron just yet. Practical steps can dramatically cut exposure to this invisible hazard – including using the bathroom's exhaust fan during styling (which, in experiments, cut the nanoparticle content by more than 90%) and operating hair tools at temperatures below 150 °C (302 °F). The researchers also noted that silicone-free products will help limit how many nanoparticles are generated through heat exposure.
"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.
"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," she added.
The research was published in the journal Environmental Science & Technology.
Source: Purdue University via Phys.org
