Staying at home stinks. Chemistry can make it smell better, at least

It’s been a long pandemic winter. As warmer weather arrives in the Northern Hemisphere, we can finally crack open some windows. Spring cleaning takes on a special urgency after weeks and months spent hunkered down in our homes with partners, kids, and pets.

All that togetherness. All those smells. It’s the fish cooked two nights ago, the dog’s bed, the litter box, the pile of dirty laundry, and the sofa that has gotten way more use than normal.

At consumer goods giant Procter & Gamble, maker of the Febreze line of air fresheners, an interdisciplinary team of scientists investigates this claustrophobic stew with the aim of making our indoor environment more pleasant and increasing our sense of well-being. The scientists would also like to rebut critics who say spraying chemicals is not the best way to keep a home odor-free and fresh.

In late January, the Febreze team held a media event to showcase how the science that goes into its air-care products makes them both effective and safe. The event was virtual, as we’d all been stuck at home for nearly a year—which only served to highlight the benefits of Febreze, said Morgan Brashear, senior scientist for home care at P&G.

Febreze product developers marry fragrance with functional chemistry that traps or breaks down odor compounds. “We do a lot of consumer research on habits and practices—and what we do that causes odors good and bad—so we can develop technologies to address bad odors,” Brashear said.

The event also gave P&G scientists a chance to describe how they formulate products that are safe for people and pets. That’s important because many of the ingredients in common air fresheners are both volatile and reactive—indeed, that’s how they work. When the products drift outside, they can contribute to the formation of unhealthy smog. And researchers who study indoor air quality say some ingredients react with other chemicals in the home to form substances that could affect human health.

It’s not easy to create a safe product that performs well in the home, explained Steve Horenziak, a P&G research fellow focused on Febreze. “We’re limited in what we can do,” he said, “because we’re not applying a product directly to a source of malodor,” such as a dog or teenager.

Instead, an air freshener uses a small amount of ingredients to do a big job. It has to evaporate at room temperature, deploy reactive chemicals to combat smelly molecules, and leave a scent that can offset different malodors. Some scent chemicals, particularly aldehydes, do double duty by leaving a pleasant scent and reacting with malodorous amine- and sulfur-based molecules.

The workhorse ingredient in Febreze is not a scent but rather the doughnut-shaped starch molecule hydroxypropyl cyclodextrin. It has a cavity the perfect size for capturing small, smelly molecules, Brashear said.

When Febreze launched in 1998 as a line of fabric refreshers, it was embraced by consumers who wanted to rid clothing of the smell of cigarette smoke. Once P&G convinced people to spray Febreze on clothes, Horenziak said, they started spraying it on other hard-to-clean, odor-collecting fabrics like sofas and carpets.

Today, the Febreze line goes beyond fabric refreshers to include air spray, oil diffusers, and wax melt products. And as the list of smells Febreze is called on to vanquish has grown, the chemistry has expanded beyond cyclodextrins.

Many of our least favorite smells are either acidic or basic. Our skin, for example, releases fatty acids, the breakdown products of triglycerides. In contrast, the telltale smell of a cat’s litter box comes from high-pH ammonia. And an ammonia derivative, trimethylamine, is responsible for the stink of raw fish.

Just as a squeeze of lemon can make fish taste less fishy, Febreze uses neutralizing ingredients to eliminate smells, Horenziak told attendees. The products contain a reservoir of buffering chemicals, including sodium citrate and sodium maleate, to counter basic and acidic odors.

In a product demonstration, P&G scientists compared Febreze with a competitor product, SC Johnson’s Glade. A pH indicator showed that Febreze neutralized an ammonia compound, but Glade did not.

C&EN asked SC Johnson to comment on the demo. In an email, the company says Glade has been shown to be effective at reducing malodor perception for ammonia-type odors.

A newer target for P&G’s researchers is cooking smells. One downside of today’s open-floor-plan homes, Horenziak said, is that cooking odors can really travel. For example, aldehydes from fat oxidation are carried away from the stove by convection currents and move around the home. Away from the heat, they fall out of the air and get trapped in soft surfaces, where they slowly release odor over time.

P&G calls this migration the cycle of malodor. It upgraded the Febreze formula to add the functional polymer polyethylenimine to soak up aldehydes from cooking fats.

As the company adds new ingredients, it avoids others. One category on the “no” list is phthalates, a family of fragrance carriers that are considered to be endocrine-disrupting compounds associated with harm to reproductive health, according to health scientists and regulators. P&G instead uses dipropylene glycol, a low-toxicity alcohol, to dissolve solid fragrance ingredients like vanillin.

And while competing air freshener sprays also do not contain phthalates, Brashear said Febreze stands out for not using volatile organic compounds (VOCs), such as isobutane or propane, as propellants. Instead, nitrogen keeps the water-based contents under pressure.

With its 7,500 R&D employees, P&G has the scientific heft to do its own safety testing. For example, it works to ensure that substances in air fresheners cannot enter the lungs. Febreze is formulated to deploy particles of 85–120 µm, small enough to smell but too large to enter the respiratory tract.

Human safety isn’t the only issue: 88% of surveyed P&G customers live in households with dogs, and 11% share space with cats. Because dogs chew on everything, they have high exposure to home care products, P&G veterinary scientist Daniel S. Marsman said at the media event. Cats have a different risk profile because of how their livers metabolize chemicals and the way their kidneys concentrate urine, Marsman explained.

Despite P&G’s efforts to ensure safety, some academic researchers who study urban indoor air quality say scented air fresheners are not the best way for people to neutralize bad odors.

“I don’t use air fresheners or anything that contains fragrances,” says Shelly L. Miller, a professor of mechanical engineering at the University of Colorado Boulder. She says it’s too challenging to evaluate the long and constantly changing lists of ingredients in scented products. In addition, scientists are finding that ingredients react with chemicals in the air and in other products to form new substances, some of which may be hazardous.

For example, fragrances contain terpenes like linalool and limonene that react with indoor ozone and hydroxyl radicals to produce formaldehyde and ultrafine particles (Int. J. Hyg. Environ. Health 2020, DOI: 10.1016/j.ijheh.2019.113439). Terpenes also react with chlorine from common cleaning products to produce volatile reaction products and chlorine-containing particles (Environ. Sci. Technol. 2019, DOI: 10.1021/acs.est.9b04261).

Miller says the best way to eliminate odor is to remove it by using an air cleaner with an activated carbon or potassium permanganate filter. As part of the Healthy Homes program for Fort Collins, Colorado, Miller works to educate residents on nontoxic freshening and cleaning methods to reduce exposure to chemicals that can cause health problems.

Air fresheners and other consumer products also put VOCs into the outdoor air. In California, emissions from consumer products are the largest single human-caused source of VOCs contributing to smog formation, says Melanie J. Turner, information officer for the California Air Resources Board (CARB).

CARB has restricted VOCs in consumer products for 30 years. As California’s population grows, total consumer product use has also increased. So the state is now proposing rules to reduce the allowed VOC content in air fresheners from 20% to 10% by weight in 2023 and to 5% in 2027. Other limits target solid products and systems that automatically disperse fragrance.

The P&G scientists said that not using a volatile propellant helps Febreze stay under the VOC limits. The company’s R&D team is also researching whether terpenes in its products contribute to the formation of smog outdoors or formaldehyde or other pollutants indoors.

Meanwhile, Horenziak found, in a review of research studies, that malodor itself has negative effects on human behavior and well-being (Atmosphere 2020, DOI: 10.3390/atmos11020126). Living with foul odors can contribute to depression, anger, and stress and even cause physical symptoms like headache and nausea. In one study, a bad smell made people perform worse on cognitive tasks like proofreading.

So it’s no surprise that the year of working from home has meant strong sales for Febreze. “People are getting a sense of control over their environment,” Brashear said. “We anticipate that feeling will be sticking around, even when things open up and we have more freedom.”