Cleanliness may be next to godliness, but a new study suggests it could have an unexpected downside: A few minutes of mopping indoors with a fresh-scented cleaning product can generate as many airborne particles as vehicles on a busy city street. The finding suggests custodians and professional cleaners may be at risk of health effects from frequent exposure to these suspended tiny particles, known as aerosols.
“I was absolutely amazed that mopping produced potentially harmful aerosols at similar rates to those generated by traffic on a busy street,” says Nicola Carslaw of the University of York who investigates indoor air pollution but was not involved with the study. “The people who should be paying particular attention to this paper are NIOSH, the National Institute for Occupational Safety and Health,” adds Glenn Morrison, an environmental scientist at the University of North Carolina, Chapel Hill, who was also not involved. “There is a lot of particle formation during these cleaning events, even under conditions that we would consider very normal.”
The air in homes, schools, and offices can sometimes be dirtier than the air outdoors, even in cities with pollution problems. Any kind of burning—candles, incense, cigarettes—is bad. Gas stoves and cooking food also spew unhealthy particles into the air, which can cause asthma and other health problems when breathed in. Other significant sources of indoor air pollution include personal care and cleaning products, whose fragrances contain volatile organic chemicals that react with ozone in the air to form small particles known as aerosols.
One molecule of concern is limonene, a commonly added to cleaners and furniture polish to help remove oil and grease. The lemon-scented molecule reacts readily with ozone, an outdoor pollutant that is the main ingredient in smog. When ozone wafts into buildings, it reacts with limonene and similar molecules called monoterpenes, turning them into peroxides, alcohols, and other molecules that grow into airborne particles. Small particles can lodge deep in the lungs, irritating cells and—at high enough exposure—leading to health problems, such as asthma. In vulnerable people, particulate air pollution can cause heart attacks and strokes.
Previous studies found cleaning products can generate such pollution. But not all of these experiments were realistic or detailed. The new study was designed to reveal, minute by minute, what happens with airborne reactions during a typical floor cleaning. Researchers brought their lab instruments into a room with an air volume of 50 cubic meters, about half the size of a semitrailer container. In the morning, they mopped the floor for 12 to 14 minutes with a terpene-based cleaner. Then, they used state-of-the-art instruments to monitor the molecules and particles as they reacted over the next 90 minutes. “This is the first study that really looked at the entire chemical process under realistic indoor conditions,” says co-author Philip Stevens, an atmospheric chemist at Indiana University, Bloomington.
After the researchers collected data from the room, they calculated how many particles smaller than half a micron a person there would inhale during mopping. Using a standard computer model, the team reports today in Science Advances that an average person would breathe in about 1 billion to 10 billion nanoparticles each minute. That’s equivalent to vehicle traffic on a busy street in a typical U.S. or European city. It’s also about the same as cooking with a gas stove or lighting a candle.
The researchers also detected short-lived molecules called radicals, like hydroxyl and hydroperoxyl, that are known to drive reactions that create particles outdoors. But the new research shows they can also form indoors, from reactions between monoterpenes and ozone. “The rate at which it occurs indoors is surprising,” says co-author Colleen Rosales, a postdoc at the University of California, Davis. Carslaw adds that this “really important finding” should raise concerns about indoor air quality.
What about opening windows? The ventilation in the laboratory, similar to a typical office building, was not powerful enough to remove the particles. Ventilation can also be a double-edged sword, researchers say. It removes particles, but it can also bring in more dangerous ozone from outdoors.
Keeping ozone levels below one part per billion—either by reducing ventilation or using activated carbon air filters—would help reduce particle formation, the researchers say. So would cleaning in the morning or evening, when ozone levels tend to be lower, and avoiding products with limonene or other kinds of terpenes. Portable air filters can also reduce the concentration of particles inside rooms, says co-author Brandon Boor, a civil engineer at Purdue University.
Fortunately, time also helps: In the hours after cleaning, the newly formed particles grow in size, after which they settle out of the air. While resting on surfaces, the particles are harmless.
The larger problem, Boor says, is the scarcity of regulations for the design and operation of buildings—and the use of various common chemicals inside them—with respect to air quality. “We need to pay closer attention to what’s going on in indoor environments.”