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More households are using various air-freshening products, including incense sticks, scented candles and diffusers, to get rid of unpleasant odors at home or change the mood. However, a growing number of studies have found that these products, used to create pleasant scents, can instead pollute indoor air to levels comparable to cigarette smoke and release various carcinogens. Harmful substances can linger for long periods, particularly in enclosed spaces with poor ventilation or small homes, posing a serious threat to respiratory health. Extra caution is therefore needed.

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Researchers at Imperial College London recently examined indoor air quality in 112 homes in London and looked at the relationship between the use of scented products and levels of fine particulate matter (PM2.5).

They found that when incense sticks were used, hourly PM2.5 concentrations reached as high as 1,293 ug/m3. This is about 17 times the level classified as "very poor" in South Korea, which applies when the daily average concentration of fine particulate matter exceeds 76 ug/m3.

Diffusers were also associated with indoor particulate matter concentrations. The highest hourly PM2.5 concentration observed in connection with diffuser use in the study was 858 ug/m3, higher than the peak recorded for scented candles.

A South Korean study also found that incense sticks emitted more particulate matter than scented candles. Researchers from Eulji University and Seoul National University tested five types of scented candles and five types of incense sticks purchased online in 2024 in an enclosed space. The average PM2.5 concentration measured near the products was 17.4 ug/m3 when scented candles were burned and 1,901.3 ug/m3 when incense sticks were burned. The concentration measured for incense sticks was about 109 times higher than for scented candles.

Harmful chemicals produced during combustion

Incense sticks are made by mixing wood powder with fragrance and adhesives. When lit, they produce smoke and fine particles through incomplete combustion. The combustion process can release various chemicals, including benzene, formaldehyde, toluene and carbon monoxide. Benzene and formaldehyde are classified as Group 1 carcinogens by the International Agency for Research on Cancer (IARC).

Scented candles can also release pollutants when burned. In an experiment by South Korean researchers, benzene and toluene concentrations were also found to rise when scented candles were burned. A 2003 study in Taiwan found that burning one incense stick emitted 32.6 to 52.7 mg of PM2.5. That amount is equivalent to the emissions from about two to four cigarettes.

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Professor Kwon Hyuksoo of the Allergy Department at Asan Medical Center also appeared recently on the YouTube channel Jisik Hansang and said that he does not keep air fresheners, scented candles, incense or other scented products at home.

"Air fresheners, scented candles, incense - things that are burned. In particular, lighting something to produce smoke is toxic to our bodies," Professor Kwon said. "Burning anything releases unusual chemicals. But when we set it up because we like the scent, huge amounts of fine dust spread through the air."

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He also identified volatile organic compounds (VOCs) in diffusers and air fresheners as ingredients that warrant caution. "Some VOCs are not dangerous, but certain substances can irritate the nose and bronchial tubes and cause inflammation," Professor Kwon said. "Some air fresheners contain phthalates. Some studies have found that these substances can cause asthma and bronchitis, and prolonged inhalation can disrupt hormonal and immune balance."

Ventilation Is More Important in Enclosed Spaces

Pollutants can build up indoors because they cannot escape outside. The Imperial College London study also found greater exposure associated with scented products in smaller homes or homes without adequate ventilation. Extra caution is needed when using these products in environments with limited space and windows that are difficult to open frequently, such as studio apartments.

Pollutants do not disappear immediately after scented products are used. In a 2021 study, researchers from Sungkyunkwan University examined indoor air quality during everyday activities in actual homes, including cooking, cleaning and using scented candles. In many cases, particulate matter concentrations remained above the study's benchmark even 30 minutes after the activities had ended.

Scented products may also generate other indoor pollutants. The U.S. Environmental Protection Agency (EPA) says that fragrance compounds released by some scented products can react with other substances in indoor air and produce pollutants such as particles and formaldehyde. This is why the EPA identifies removing or reducing pollution sources as the most effective way to improve indoor air quality.

Professor Kwon also stressed the importance of managing indoor spaces to prevent odors rather than masking them with other scents. "Natural is best. There is no need to add a scent deliberately," he said. "Adding a scent means covering one scent with another, doesn't it? It's better to manage things so that there is no odor in the first place. Something like charcoal may also absorb odors."

Ventilation is also essential. Professor Kwon said indoor air pollution can be worse than outdoor pollution and advised people to ventilate even on days when fine dust levels are high. "Numerous studies have shown that indoor air pollution is much more serious than outdoor air pollution," he said. "If you don't ventilate indoors for a few hours, the air can be worse than outside." He recommended opening all the windows in the home at once for brief periods of no more than five minutes, three to four times a day.

This content was produced with the assistance of AI translation services.