The 4 Best Essential Oil Diffusers of 2025

During testing, we looked at how our essential oil diffuser picks affected indoor air quality. We tested multiple essential oils and blends, using four ultrasonic diffusers—the 300 mL Urpower diffuser, the 500 mL Asakuki model, the Vitruvi Stone Diffuser, and the 100 mL Urpower 2nd Generation model—and the Organic Aromas Raindrop 2.0 nebulizer. In a roughly 200-square-foot bedroom, senior staff writer Tim Heffernan measured how the machines affected the number of airborne particles with the same TSI AeroTrak 9306 particle counter that he uses in testing for our guide to air purifiers. He closed the windows and shut off his apartment’s ventilation system during the tests, and he filled the diffusers with distilled water to ensure that no dissolved minerals made it into the air and affected the results. He added the oils in the manufacturer-recommended quantities.

As we expected, all the machines raised the number of fine particles in the air—after all, they work by doing exactly that. The numbers in the chart below show the increase in particles of a 0.3-micron diameter (0.3 micron being the HEPA test standard):

Airborne particulates: Ultrasonic diffusers vs. nebulizers

For each test, Tim ran the particle counter for five minutes with the diffuser or nebulizer off to get a baseline reading of the air quality in the room, after which he turned the diffuser or nebulizer on for 30 minutes at its highest continuous setting. He placed the diffuser or nebulizer on his bedside table, with the particle counter a few feet away on the headboard. After 15 minutes, he moved the particle counter to a second table on the far side of the bed, about 8 feet away, to see if distance affected the airborne-particle count.

We found two main takeaways from our findings. First, the nebulizer showed a much greater increase in particulate levels in the air than the diffusers. That isn’t surprising: Nebulizers create a fine mist of pure essential oil droplets, while diffusers create a mist composed mostly of water with a small amount of essential oil mixed in. (Most diffuser makers recommend about four or five drops of essential oil per 100 mL of water.) The water rapidly evaporates and, as a result, does not show up in the particle counts; what’s left over are the relatively sparse oil droplets.

Second, the diffusers created a fairly uniform concentration of airborne particles throughout the room, while the nebulizer concentrated them in its immediate vicinity—note the dropoff for the nebulizer at minute 20, after Tim moved the particle counter away from it.

So, does any of this matter? It’s well established that fine particles of soot, smoke, and dust have negative health impacts when inhaled, but what about fine droplets of essential oils?

Tim spoke with Sachiko Koyama, an assistant research professor at the Indiana University School of Medicine who co-authored a comprehensive review of the ways in which essential oils’ impacts on the body are affected by the mode of delivery (by inhalation, topically, or by ingestion). In general, she told us, aromatherapeutic uses of essential oils can be considered safe, because the oils themselves are not inherently dangerous when inhaled and because their concentration in the air is low. Koyama noted that much higher concentrations are used in the established practice of smell training, which may help to restore a sense of smell to people who have lost it (including some who lost their sense of smell post–COVID-19). Koyama did note that some people are allergic to components found in essential oils, and that overexposure to essential oils may create sensitivity to them in people who previously were not negatively affected. In addition, as mentioned above in “Who this is for,” there are also anecdotal reports of essential oils negatively affecting the breathing of asthmatics and non-asthmatics alike. And as Wirecutter senior staff writer Kaitlyn Wells reports, some airborne essential oils can be dangerous to pets. But “if you don’t have a specific allergy,” Koyama said, “for most people I don’t think it will cause a problem.”