Fragrance Guide

Part 4 of Fragrance: Dispersal Method

What is evaporation?

Evaporation is when a liquid scent drifts into the air on its own over time, similar to how a puddle evaporates on a sunny day. This is how reed diffusers, scented gels and beads, sachets, and potpourri work. It’s also what happens after you spritz a scent from a regular pump or trigger bottle: the water evaporates into the air.

Fan diffusers speed up the process. They blow air across a pad soaked in scent. Think of using a small fan to help the scent evaporate faster.

Evaporation adds the fewest extra things to your air. There’s no flame or propellant-forced mist, so what you breathe is mostly the scent molecules themselves. That doesn’t make it risk-free: scent molecules still react with ozone indoors no matter how they get into the air (more on that in the scent section above).

Plug ins and wax warmers

Both of these use heat without a flame. Wax warmers melt scented wax cubes. Plug-in air fresheners actually use heat and a wick that draws up scented liquid.

Wax melts are often marketed as the cleaner alternative to candles, which makes some sense because you’re not burning something inside your house, however, they may actually release more of a specific type of air pollutant, terpenes. Purdue researchers measured more monoterpenes released from scented wax melts than from scented candles. The probably is that those terpenes reacted with indoor ozone to form nanoparticles, and the researchers estimated a person could inhale a similar amount as from candles or gas stoves. The authors also say more toxicology research is needed on what breathing these particles actually does.

Plug-ins use heat, too, but unlike wax warmers are designed for constant output, even when you’re not home. They deliver a steady stream of scent compounds into the room.

What is nebulized or ultrasonic diffusion?

Ultrasonic and nebulizing diffusers work differently from evaporation, including evaporative fan-based diffusers. They push tiny droplets into the air.

Ultrasonic diffusers hold water with a few drops of oil. A plate vibrates at a high frequency and breaks the water into a fine mist that carries the oil with it. Anything else in the water goes airborne too. The EPA notes these devices are very efficient at releasing the minerals in tap water into the air, and a Virginia Tech study found tap water in an ultrasonic humidifier raised room particle counts nearly 13 times, while distilled water raised them only slightly. That research was on humidifiers, which use the same technology. If you use an ultrasonic diffuser, use distilled water.

Nebulizers use no water at all. They turn undiluted fragrance oil into an aerosol, so you get a more concentrated output.

Both produce small airborne particles. Chamber studies have measured particles in the 10–100 nanometer range from ultrasonic diffusers, which is small enough to reach deep into the lungs. The research here is early, and the health effects of routinely breathing nanoparticles from diffusers aren’t well characterized. I think caution is reasonable.

What are aerosols?

Technically, an aerosol is any suspension of fine liquid or solid particles in air, so every spray makes one. But in normal everyday use, “aerosol” means a pressurized can that uses gas to propel the contents out as a very fine mist. Think spray paint, or canned room sprays like Febreze Air.

The difference from pressurized bottles and regular bottles is how long the particles stay in the air. In a chamber study of cleaning sprays, particles from pressurized cans took about twice as long to settle as particles from regular spray bottles. More time in the air means more time to breathe it in, so it’s more important that the scent is safe.

What are propellants?

Propellants are the compressed gases in aerosol cans that force the scent out as a powerful mist. In the “olden days,” this gas was CFCs, or chlorofluorocarbons. The US banned CFCs as aerosol propellants in 1978 because they damage the ozone layer. Brands mostly switched to hydrocarbon propellants so today that usually means propane, butane, and isobutane, or compressed gases like nitrogen or CO2. Since propellants are released into the air along with the product, what they are can impact your indoor air quality. Even in they come out in very small quantities with each spray, if you use a spray very often, it can potentially add up.

What is the difference between nitrogen and other gas propellants?

Hydrocarbon propellants like propane, butane, and isobutane are the most common in consumer aerosols. They’re flammable and add to your indoor VOC load, though a single spray delivers a small amount.

The bigger concern is contamination. Starting in 2021, an independent lab found benzene, a known human carcinogen, in many personal care aerosols. The FDA pointed to the propellant isobutane as one possible source. Those were deodorants, dry shampoos, and sunscreens, not air fresheners. But they show the broader problem: aerosols contain several ingredients besides the scent, and full disclosure is rare.

Nitrogen is the most inert option because it doesn’t react with much. It’s less common because it doesn’t keep pressure as steady as the can empties. If a safety data sheet lists nitrogen as the propellant, that’s the better choice among aerosols. Skipping aerosols for home fragrance altogether is the simpler move when alternatives exist.

And yes, it’s true that one spray won’t hurt you on its own. But, we’re constantly exposed to poor air quality, so if you can make this change without added stress, I think it’s a good idea.

Why is burning a candle bad for you?

Burning anything indoors (candles, incense, palo santo, sage, even food on the stove) creates combustion byproducts. For candles, that includes carbon monoxide, carbon dioxide, nitrogen oxides, formaldehyde, benzene, PAHs, PM2.5, and ultrafine particles. Burning also releases the fragrance compounds themselves, whether they’re natural or synthetic.

PM2.5 is the same pollutant behind wildfire smoke advisories and outdoor air quality warnings. Chronic exposure at high levels is associated with cardiovascular disease, respiratory disease, and emerging research links it to cognitive decline. That said, occasional candle use in a ventilated room is a very different exposure from the outdoor and occupational studies behind those associations. One large chamber study found most calculated indoor levels from candles were well below guidance values. Still, the direction of the evidence is a reasonable basis for keeping indoor combustion to a minimum, especially since you likely have other daily combustion sources like cooking or gas heating.

If you do burn candles, ventilation is helpful.

Is incense worse than candles?

Yes. Burning incense is smoldering combustion, and produces more than 45 mg of particles per gram burned, compared with about 10 mg for cigarettes. Incense smoke also contains carbon monoxide, nitrogen dioxide, sulfur dioxide, benzene, toluene, xylenes, aldehydes, and PAHs. A French study found benzene and formaldehyde emissions from scented candles were far lower than from incense, and incense cones put out more formaldehyde than sticks.