Pine, orange, lavender… what should ‘clean’ really smell like?

On days with air quality warnings, it makes a lot of sense to stay inside. But it’s not necessarily safe to assume the home is always better.

Scientists at Drexel University are finding that the chemicals we use indoors can trigger a surprising amount of pollution.

At the top of the list are two common ingredients in household cleaners: alpha-terpineol, or pine scent, and D-limonene, the smell of oranges. Both compounds are naturally occurring, and individually, pretty harmless.

“The problem happens when they’re reacted with by ozone,” says Drexel engineering professor Michael Waring. “They produce oxygenated compounds that tend to form particulate matter in the indoor air, and they also produce things such as formaldehyde.”

Waring directs the school’s indoor environment research group, and studies the things that sometimes get forgotten when considering the problem of air pollution.

“If you’re interested in things like human health and exposure, you really need to ask yourselves where are people spending the majority of their time,” he says. “And that happens to be inside buildings.”

To learn more about indoor pollutants, the Drexel team uses a large, stainless steel reaction vessel. Precise amounts of the cleaner scents are added, along with ozone, and a variety of instruments measure the number and types of products that result.

It’s not entirely clear how dangerous the newly formed substances might be, but small particles are easily inhaled, and formaldehyde is known to cause cancer.

So far, the group has found that when limonene, the orange scent, is mixed with ozone, it produces three to five times the particulate pollution as the pine scent. It depends on the amount of air exchange and the filtration system in the home, but those pollutants can stick around for up to 10 hours.

Waring and others have found that the oils present on human skin also react with ozone. People, just by sitting and living in a space, it turns out, are constantly altering the indoor air chemistry.

The outside driverThe driving force behind these indoor reactions comes from outside, and is well known to scientists. Ozone is made when nitrogen oxides and volatile organic compounds from car exhaust or power plant emissions, for example, react in sunlight. In addition to reacting with any cleaning products, ozone on its own is harmful to inhale, and is monitored by the EPA.

But homeowners have little control over the amount of ozone in a room. It comes in every time you open the door or window, and seeps through cracks. Better, Waring says, is to just not use the chemicals that ozone loves to react with.

“That smells clean!””I think there’s kind of been a marketing campaign going on in the United States pushing this idea of clean is something you can smell.”

Take, for instance, a Pine-Sol commercial explicitly advertising the strength of its namesake aroma. In the television spot, a janitor using Pine wakes up a sleeping hospital patient.

“Instead, try to think of clean as the absence of smell, and the absence of odor in the air,” advises Waring. “That’s a much healthier environment.”

The fact that the chemicals we use to clean our homes actually pollute the air we breathe is a bit ironic. But it’s not the first counter-intuitive result Waring has found during his career. As a grad student, he investigated how well so-called ion generators worked to clean the air. The machines were popular about 10 years ago and sold at places like the Sharper Image—although they can still be found today. In the process of filtering out particles, the units end up producing ozone.

“The devices could actually operate as net particle generators rather than particle removers,” he says.

Fresh air’s redemptionEven though ozone usually comes from outside, Waring is a big proponent of getting as much outdoor air as possible.

“If you’re in a typical place in Philadelphia [and] you’re not next to a major roadway,” he says, “it’s always pretty good to breathe fresh outdoor air.”

It’s not understood why, but fresh air seems to boost worker productivity. Commercial buildings with higher ventilation rates have fewer employees who call in sick.

And that might even cut down on the need to spray so much cleaner in the first place. For those not ready to give up on the indoor battle with dirt, Waring suggests diluting commercial products or using a mix of water and vinegar.