Seemingly pleasant smells are often associated with good times and a better quality of life. It’s hard to believe that pleasant odors like a backyard barbecue, scented candles, air fresheners, and even Christmas trees can be associated with health risks.

So if pleasant smells can be deceiving, what does fresh, natural, clean air smell like? Surprisingly, that smell isn’t always what the nose and memories associate with freshness.

Thunderstorms and ozone smell

For many people, the smell of clean air is the scent of the air outdoors after a thunderstorm. And unfortunately, that smell is often ozone. While the outdoor scent after thunderstorms may seem “fresh,” ground-level ozone is a pollutant and a health hazard.¹

In a thunderstorm, ozone originates from nitric oxide produced by the electrical charge in the storm. The nitric oxide combines with other airborne chemicals to form ozone, which is pushed close to the ground by the storm.

Additionally, ozone in the stratosphere – which protects the Earth by blocking too much exposure to ultraviolet rays – can be brought to the ground-level by severe thunderstorms. Research conducted in 2014 and published in Geophysical Research Letters found that thunderheads over 50,000 feet in height can tear the boundary between the troposphere and the stratosphere.² When the boundary is torn, ozone in the stratosphere can drop to the lowest level of our atmosphere: the troposphere.

While ozone in the stratosphere is good for life on Earth, it’s terrible on the ground. Ozone is the main ingredient in smog. Inhaling ozone can be dangerous for the respiratory system. Ozone can cause a variety of health problems, including:

• lung damage
• chest pain
• wheezing and coughing
• shortness of breath
• throat irritation
• increased asthma attacks

Household scents

For some people, clean air smells like a natural fragrance that reminds them of the woods, such as pine or cedar. Those who grew up or live in the Hawaiian Islands may associate the scent of fresh flowers with clean air.

In most modern homes, the fresh scent of pine, cedar, lemon, or flowers has been replaced with fragrance designed to replicate those scents. That fresh scent may come from cleaning products that kill germs or from air fresheners that contain a particular scent. Scented candles are popular products for adding a supposedly clean-smelling scent at home.

Air freshener scents, cleaning products, and scented candles can also contain chemicals such as limonene, a volatile organic compound (VOC) that produces the pleasant and familiar smell.³ Limonene gives candles and spray cleaners a lemony smell. A pine scent often comes from pinene, a similar chemical.

Limonene and formaldehyde

Outdoors, airborne limonene and other VOCs react with ozone to form secondary organic aerosols (SOAs) – microscopic particles that become suspended in the air and, like ozone, contribute to the haze commonly known as smog.

Indoors, limonene also becomes aerosolized. The chemical combines with other airborne pollutants to form formaldehyde.

Limonene is not associated with adverse health effects, but formaldehyde is. Formaldehyde exposure can cause:

• watery eyes
• burning sensations in the eyes, nose, and throat
• coughing and wheezing
• nausea
• skin irritation

At sufficient levels, formaldehyde is carcinogenic.

Other “clean” household scents can contain additional chemicals that are equally or even more dangerous than formaldehyde. For example, many detergents and other “fresh scent” detergents contain phthalates. Phthalates are a class of chemicals linked to health problems including:^4,5,6

• asthma
• obesity
• breast cancer
• infertility
• affected thyroid hormone production
• reproductive problems

Clean air has no smell

Despite all of our best intentions to make a home smell clean, the truth is that clean air should smell like nothing at all. The surest sign of clean air in a home is the lack of a masking scent, pleasant or otherwise.

Where odors do exist, remove them rather than trying to mask them with other scents. Instead of trying to remove odors, consider following a few tips for reducing unwanted odors.

• Find unscented options for cleaning products when possible.
• Address accidental stains immediately if you own a pet. Soak up urine with paper, rinse the accident area with clean, cool water, and blot to dry.⁷
• Avoid cleaning with ammonia, a strong scent that may encourage pets to continue to mark an area.
• Ventilate and use a kitchen range hood. Research conducted by the Lawrence Berkeley National Laboratory has demonstrated that well-designed ventilation systems, including high-performance kitchen range hoods that ventilate to the outside, can help reduce cooking odors.⁸
• Use a high-performance air purifier equipped with an optional gas and odor filter will remove additional common airborne pollutants like PM2.5 while removing odors.
• Use a gas and odor purifier. Some gas and odor air purifiers can control a range of chemicals and odors or they can be fitted for removal of specific chemicals.

The takeaway

By addressing odors in the home instead of masking them, the result is a cleaner, healthier living space. Scents that mask odors can only contribute to airborne pollutants while the smell of clean air is the smell of nothing at all.