Scented cleaning products may be creating billions or even trillions of tiny, invisible nanoparticles indoors, which could travel deep into the lungs, according to a study led by Brandon Boor at Purdue University. The research found that fragrance compounds in both traditional cleaning products and those based on botanical essential oils can react quickly with indoor ozone to form these nanoparticles. When inhaled, these particles may deliver a respiratory dose similar to or even greater than that from standing near heavy traffic. The findings were shared at the fall meeting of the American Chemical Society during a symposium titled Healthy Indoor Spaces: Bridging the Microbiome and Chemistry at McCormick Place. The study focused on terpenes, a group of organic compounds commonly found in cleaning products. Examples include pinene, limonene, thymol, and linalool, which are often used for their pleasant scents and antimicrobial properties. These terpenes can react with ozone, a common indoor pollutant, to form nanoparticles. These reactions happen quickly, with the formation and growth of particles occurring within minutes. The researchers tested both scented conventional liquid cleaners and disinfectant sprays and wipes containing botanical ingredients in a model home on Purdue's campus. They found that the same chemical processes that create nanoparticles outdoors can occur indoors at a much faster rate and with higher concentrations. The research also showed that using scented surface cleaners in combination with germicidal far-UV (UV-C) lamps, which are designed to disinfect indoor air, can create conditions that promote nanoparticle formation. These lamps interact with oxygen in the air to produce ozone, which can increase nanoparticle production. The study's authors recommend using low-fragrance or fragrance-free cleaning products, avoiding the use of multiple scented products during the same cleaning session, and ensuring proper ventilation by using exhaust fans or opening windows. They also advise against using ozone-generating devices while cleaning with scented products. The findings highlight the potential health risks associated with indoor air quality and the chemical interactions that can occur in everyday environments. While the study does not directly link these nanoparticles to specific health effects, it underscores the importance of being aware of the invisible pollutants that may be generated by common household products. The research adds to the growing body of knowledge about how indoor environments can impact human health in ways that are not always immediately obvious.