The standard routine of protecting hair from heat damage actually turns your bathroom into a dangerous emissions zone, as scientists have found that just 10-20 minutes of styling using common products can result in tens of billions of ultra-fine particles being inhaled directly into the lungs - similar to standing next to a busy road during rush hour or smoking a few cigarettes.
Purdue University researchers have discovered what happens to hair products when exposed to heat, providing the first quantitative evidence that daily hair care routines may be a significant source of indoor nanoparticle (or, more specifically, ultrafine particle) emissions. In a controlled experiment in a purpose-built microhouse—called the Zero Energy Design Guide for Purdue Engineers (zEDGE), which was designed to replicate real-world contaminant testing conditions—the researchers recruited three subjects and performed their normal hair care routines seven times, using their usual products and tools.
Participants applied their own styling products—sprays, serums, creams and protectants, typically applied before or during heat treatments—and used tools including straighteners, curling irons and blowdryers, with the straightener taking up most of the time. Tool temperatures are set to commonly used levels ranging from 150 °C (302 °F) to 230 °C (446 °F). While the names of the products were not revealed, the main takeaway is their ingredients; you can read the details in the study's supplementary material.
Although the study was small, the repeated experiments were still tightly controlled and the testing was advanced enough to clearly determine the emission patterns and chemical fingerprints of these common real-life modeling processes. During these 10-20 minute hairstyling sessions, the researchers continuously measured air quality using scanning mobility particle size spectrometers (SMPS) and condensed matter particle counters (CPC), which can detect and count particles as small as a few nanometers. The researchers then used gas chromatography and mass spectrometry to identify the chemical composition of these emissions.
They found that the nanoparticles released upon heating were primarily composed of condensed volatile organic compounds (cVOCs). A significant number of these are cyclic volatile methylsiloxanes (cVMS), especially decamethylcyclopentasiloxane (D5), a silicone commonly used in hair care formulations to promote smooth and shiny hair. When heated, these siloxanes evaporate and condense into ultra-fine particles that float in the air and are easily inhaled. The amount of D5 inhaled during each styling session is estimated to be 1-17 milligrams. While this may not sound like much, the key is that it is granular and the contact frequency is high.
Heating hair products breaks down silicon compounds such as D5 into ultra-fine particles smaller than 100 nanometers, which are perfect for reaching the deepest parts of the lungs and even entering the bloodstream. They can accumulate in the body (bioaccumulate). In terms of what ends up in the lungs, a 20-minute styling session releases as many particles as smoking several cigarettes.
"It's really concerning," said lead researcher Nusrat Jung, an assistant professor in Purdue's Lyles School of Civil and Architectural Engineering. "The amount of nanoparticles inhaled using regular store-bought hair care products was far higher than we expected."
Over the course of 21 treatments, each 10- to 20-minute treatment releases tens of millions of ultrafine particles per cubic centimeter of air, and the stylist inhales approximately 10 billion ultrafine particles.
Tiny house used by scientists to conduct real-world hairstyling experiments Purdue University
The role of heat in this hazard also deserves attention. The amount of ultrafine particles released increases dramatically once tool temperatures rise above around 150°C (302°F) to around 230°C (446°F) – so flat clips (or hair straighteners) are the most polluted, while hair dryers are the least polluted (although their concentrations are still much higher than normal indoor concentrations).
"The formation of nanoparticles in the atmosphere is particularly sensitive to these applications of heat," said PhD researcher Jianghui Liu. "Heat is the primary driver—cyclic siloxanes and other low-volatility components volatilize, nucleate, and grow into new nanoparticles, most of which are smaller than 100 nanometers."
We already know that ultrafine particles are a huge health (and environmental) risk because of their ability to penetrate deep into the alveoli and enter the bloodstream. Early research has linked exposure on this scale to oxidative stress, respiratory inflammation, and neurological and neurodevelopmental problems.
This latest study builds on researchers' 2023 study that examined harmful chemicals in hair care products that are released into the air during the styling process.
"When we first looked at emissions from hair care products in hot weather, we focused on the volatile chemicals released from them, and what we found was already quite concerning," Jung said. "But when we looked more closely using aerosol instruments typically used to measure exhaust fumes, we found that these chemicals can produce bursts of 10,000 to 100,000 nanoparticles per cubic centimeter."
The good news is, you don't have to throw away your hair straighteners right away. Some practical measures can significantly reduce exposure to this invisible hazard – including using a bathroom exhaust fan when styling (experiments have shown that this can reduce nanoparticle levels by more than 90%), and using hair tools at temperatures below 150°C (302°F). The researchers also noted that silicone-free products would help limit the number of nanoparticles produced through high temperatures.
"By detailing the emission characteristics of these indoor nanoparticles during personal care, our study lays the foundation for future studies of their effects on indoor atmospheric chemistry and inhalation toxicity," said Jung. “This type of research has never been done before, so until now, the public knows very little about the potential health risks of daily hair care habits.”
She added: "By filling these research gaps, future studies can provide a more complete understanding of emissions and exposures associated with thermal hairstyling, thereby helping to improve indoor air pollution assessments and mitigation strategies."
The research was published in the journal Environmental Science and Technology.