A room that smells like citrus, pine, or flowers is often associated with cleanliness. However, those familiar smells may indicate chemical reactions taking place in the air.
Research led by Brandon Boor at Purdue University found that fragrance compounds released by both conventional cleaners and essential oil-based botanicals can react quickly indoors and generate nanoparticles. If inhaled, some of these extremely small particles can penetrate deep into the lungs.
Researchers say exposure can be reduced by choosing unscented products, improving ventilation with exhaust fans or open windows, and avoiding ozone-generating devices while using scented cleaning products.
The researchers presented their results at the American Chemical Society (ACS) fall meeting during the “Healthy Indoor Spaces: Bridging the Microbiome and Chemistry” symposium at McCormick Place.
“Importantly, cleaning removes viruses and bacteria from surfaces, but it can also create invisible air pollution. There is no visible dust or smoke in the air, but these particles are forming.” -Brandon Boor
Cleaning can create invisible nanoparticles
“We show that indoor ozone reactions with cleaning fragrances produce nanoparticles that carry a respiratory dose comparable to or greater than what would be experienced when being outside on a busy highway,” says Boor, an assistant professor of Civil and Construction Engineering at Purdue University who studies indoor air quality. “The particles are different in terms of their composition, but the total dose may be higher. You don’t see smoke, dust or haze in the air. Instead, you think the air smells great, so it must be clean.”
Boor began researching how cleaning agents and chemical disinfectants affect indoor environments during the COVID-19 pandemic with colleague Nusrat Jung, an assistant professor of civil and construction engineering at Purdue. One feature quickly stood out: many of the products people use to clean and disinfect indoor spaces contain strong fragrances.
“This is usually to create a pleasant olfactory landscape in the interior space,” says Boor. “But clean air shouldn’t smell like highly concentrated citrus. In fact, it shouldn’t smell like anything.”
Scientists who study the atmosphere have long known that compounds released by plants can react with ozone. Terpenes such as pinene from pine trees, for example, can participate in reactions that generate small particles in the air. Over time, those particles can combine and grow until they become large enough to help seed clouds.
This chemistry progresses relatively slowly in forests because the amount of terpenes in the outside air is generally low.
Cleaning products release high levels of terpenes
Inside, the situation can be very different. Scented cleaners release terpenes when aromatic compounds evaporate from sprayed droplets or clean surfaces.
Cleaning liquids typically contain compounds including pinene, limonene (lemon), thymol (thyme), and linalool (lavender). Their concentrations during cleaning can be much higher than levels normally measured outdoors. Boor says airborne terpene concentrations inside a room during cleaning can increase to tens or even hundreds of times those found in a forest.
To examine what happens under realistic conditions, researchers tested conventional scented liquid products as well as disinfectant wipes and sprays containing botanicals inside a model home on Purdue’s campus.
The tiny house includes a functional kitchen, hardwood floors and a bathroom, allowing researchers to recreate everyday household cleaning activities. Their experiments demonstrated that the same basic chemistry responsible for nanoparticle formation outdoors can occur indoors much more rapidly and at much higher concentrations, with potentially important consequences for human exposure.
Billions or trillions of particles can form
Routine tasks such as mopping floors, spraying countertops, and wiping surfaces with scented products generate billions or trillions of particles, and the total depends on the product.
Most were nanoparticles or ultrafine particles measuring only 1 to 30 nanometers in diameter. Because such small particles often fall outside the detection range of home air quality monitors, people may have no indication that particle concentrations have increased.
The researchers found that ordinary cleaning can temporarily raise levels of ultrafine particles above those measured outdoors.
Its small size is important from a health point of view. Ultrafine particles can deposit throughout the respiratory tract and reach deep regions of the lungs. Once there, they can contribute to irritation and inflammation of the respiratory system. Some may also have the potential to enter the bloodstream.
One of the biggest surprises was the speed of the process. Particle formation and growth occurred within minutes.
“By the time you finish cleaning an indoor space, you’ve already formed a lot of nanoparticles and inhaled them,” Boor says.
Ozone can intensify the formation of particles indoors
More recently, Boor and Purdue professor Ernest Blatchley studied what happens when scented surface cleaners are used at the same time as far-ultraviolet (UV-C) germicidal lamps designed to disinfect indoor air.
The combination created particularly favorable conditions for the formation of nanoparticles.
The lamps interact with oxygen in the air and produce ozone. During the experiments in the tiny house, ozone concentrations increased to approximately 20 to 40 parts per billion. These levels were comparable to, but somewhat lower than, the outdoor ozone concentrations measured when the experiments were conducted.
With elevated levels of ozone and high concentrations of terpenes present, nanoparticle production became even more intense. That combination raised additional concerns about the amount of particles occupants could inhale.
How to reduce exposure during cleaning
Boor emphasizes that the goal is to help consumers make informed decisions rather than discourage cleaning. Cleaning is still important to remove viruses and bacteria from surfaces, but several simple measures can reduce exposure to secondary contamination produced during the process.
The researchers recommend:
- Choose unscented or low-scented products.
- Avoid applying several scented products in the same cleaning session.
- Turn on exhaust fans or open windows to ventilate the space.
- Do not clean surfaces with scented products simultaneously while using ozone-generating devices, such as far-UV-C lamps.
“Importantly, cleaning removes viruses and bacteria from surfaces, but it can also create invisible air pollution,” says Boor. “There is no visible dust or smoke in the air, but these particles are forming.”
Boor thanks all the graduate students who worked with him on the tiny house experiments along with the support of the undergraduate students.
The research was funded by a grant from the Faculty Early Career Development Program (CAREER) of the National Science Foundation and the Alfred P. Sloan Foundation.
Qualification Formation of indoor atmospheric nanoparticles from scented cleaning products.
Abstract Volatile scented chemicals, including surface cleaning agents and botanical disinfectants, are widely used indoors and represent a major source of reactive organic emissions. These products are commonly applied in homes and workplaces to clean and disinfect surfaces to reduce the presence of viruses and bacteria. However, its role in driving indoor atmospheric chemistry and nanoparticle formation remains poorly constrained. This presentation investigates the impact of the use of scented cleaning products on the nucleation, growth, and human exposure of airborne nanoparticles to secondary pollutants in indoor environments. Field and laboratory experiments were carried out in controlled residential and office environments using real-time, high-resolution measurements of volatile organic compounds and nanoparticle size distributions extending to the nanocluster aerosol regime (1-3 nm). Surface cleaning and disinfection activities produced rapid increases in terpene and terpenoid mixing ratios (10-1000 ppb), often exceeding levels observed in outdoor forested environments. These compounds reacted with interior oxidants, particularly ozone, to initiate intense nanoparticle nucleation and growth events. Observed nucleation rates (~105 cm-3 yes-1) and condensational growth rates (up to 300 nm h-1) exceeded typical outdoor values by orders of magnitude, resulting in transient indoor nanoparticle number concentrations of 105-108 centimeter-3. The rapid growth of nanoparticles enabled survival to sizes that deposit efficiently throughout the human respiratory system, producing inhalation dose rates comparable or higher than those from primary combustion sources such as traffic emissions. Both conventional and botanical cleaning products generated complex multiphase exposure scenarios involving reactive gases and secondary organic aerosols. These findings identify cleaning and disinfection of indoor surfaces as key drivers of indoor atmospheric nanoparticle formation and highlight the need to improve building ventilation, air cleanliness, and product formulation to mitigate exposure to secondary pollutants.