When you clean with a scented product, the smell isn’t the end of it. When ozone is present, some scent chemicals keep reacting in the air and make new compounds that were never in the bottle. Below is what researchers have measured so far, and where the evidence stops.
Evidence key Strong Moderate Weak How we grade
For many chemicals, exposure happens indoors
Across decades of US monitoring where people carried personal air samplers, the air they breathed held more of almost every hazardous chemical than outdoor air did: exposure happens indoors, not at the outdoor monitoring station. Strong evidence. Weisel 2002
A chemical released indoors is roughly 1,000 times more likely to be breathed in than the same amount released outdoors (exact when comparing the low end of each range; only ~100x at the high end, since the ranges overlap at 10⁻³), so cleaning products and air fresheners (about 1% of California's smog-forming emissions) may deliver an inhaled dose on the same scale as all outdoor sources combined. Strong evidence. Nazaroff & Weschler 2004
Scent chemicals are a signature of indoor air
Limonene was 13x higher indoors than outdoors in European offices and classrooms, one of the two highest indoor/outdoor ratios of 23 chemicals measured. Strong evidence. Geiss 2011
Limonene was detected in every childcare room studied and was more than 1,000 times higher indoors than outdoors, so it came from products used inside. Strong evidence. Hoang 2016
The chemistry after you clean
Researchers keep finding the same reaction. Scent chemicals such as limonene and pine oil react with ozone indoors to make new pollutants, including formaldehyde and fine particles, that keep forming for hours after cleaning ends. Strong evidence. Nazaroff & Weschler 2004
A test in a real house found it too. In a real test house, using a limonene-scented foam cleaner exactly as the label directed produced a burst of 45,000-150,000 tiny new particles per cubic centimeter within minutes, but only when ozone was in the air. Strong evidence. Rossignol 2013
Scented cleaners react with ozone indoors and release bursts of tiny new particles; every study able to separate them found this. Strong evidence. Salonen 2024
In one test room, the change lasted long after the cleaning stopped. After 2-7 minutes of cleaning with a terpene cleaner, indoor air chemistry stayed altered for 10-12 hours because the scent chemicals soaked into walls and floors and came back out. Moderate evidence. Singer 2006
What this does and doesn’t mean
What you breathe after using a scented cleaner is not only what was in the bottle: the scent chemicals keep reacting in the air to make new compounds. Strong evidence. Chen & Hopke 2010
Researchers haven’t yet shown how much of this harms people at everyday home levels. Many of these measurements come from lab chambers and test rooms. We list the overstatements we won’t make about indoor air on our myths page.
Air fresheners
About 1 in 6 adults (17%) in four countries said air fresheners or deodorizers make them unwell; of those, half reported breathing problems. Moderate evidence. Steinemann 2019
41% of US adults with asthma reported health problems from air fresheners, and 36.7% said they can't use a public restroom that has one. Moderate evidence. Steinemann 2018
What you can do
- Choose fragrance-free cleaners. Look them up in EWG’s Guide to Healthy Cleaning, and use our label decoder to check the label.
- Fix odors at the source instead of covering them up. The California Department of Public Health recommends enforced fragrance-free workplace policies, no air fresheners or fragranced cleaners, and better ventilation instead of odor masking. Moderate evidence. Weinberg 2017
- Take used wipes and paper towels out of the room when you’re done. Leaving used paper towels in the room after cleaning increased the solvent and scent chemicals released by 35-100%. Moderate evidence. Singer 2006, Indoor Air
Every claim above shows its evidence grade and names its source, with a link wherever one exists. See all the indoor air facts. Information, not medical advice.