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Learn · Part Three — Learning origins · Lesson 15 of 16

Diffusing essential oil in a room: what a diffuser releases

Ultrasonic diffusers also release fine particles, about five times more with tap water. Essential oil meeting ozone forms formaldehyde. What to watch in air-conditioned rooms, with babies, cats and birds.

Diffusing essential oil in a room is the most common way essential oils are used in Vietnam. It is also the least explained: the diffuser comes with a slip of paper that says "add a few drops", and that is all. This article covers what the slip leaves out. What a diffuser actually releases into the air, why a closed, air-conditioned room is the place to pay attention, and when to switch the machine off.

Diffusing does not clean the air

The first thing to know: a diffuser adds substances to the air in the room. It does not filter, absorb or remove anything already there. The room smells nicer because another scent sits on top, and because the nose turns its attention to the new one.

That does not make diffusing a bad thing. It only means the right question is not "how much do I diffuse to make the air clean", but "how much should I add".

Four kinds of device, four ways of putting oil in the air

  • Ultrasonic diffusers (the commonest, with a water tank). A plate vibrating at high frequency breaks water and oil into a fine mist. What comes out is not just oil vapour but tiny water droplets carrying oil.
  • Waterless nebulisers. They draw neat oil through a narrow nozzle and break it into particles. With no water to dilute it, far more oil goes into the air, and far more is used.
  • Oil burners (candle or electric). They warm a dish of water with oil on it. Heat drives off the lightest compounds first, so the scent shifts hour by hour. A candle burner also adds the candle's smoke and soot.
  • Reeds, stones, wood. Oil soaks into a surface and evaporates on its own. No particles, slow and even, but it cannot be switched off: it is always releasing.

What an ultrasonic diffuser puts into a room

In 2021 a group at the University of Toronto measured the emissions of a household ultrasonic diffuser in a test chamber. Three findings are worth knowing:

  1. The diffuser produces fine particles. Besides oil vapour, it releases very small particles, mostly under 1 micrometre (PM1). The amount varied widely with the oil: with lemon oil the emission rate was about twenty times that with lavender or eucalyptus.
  2. Tap water raised particle emissions about fivefold compared with deionised water. The cause is the minerals in tap water. As each droplet dries, the minerals are left behind as airborne particles. It is the same "white dust" familiar from ultrasonic humidifiers.
  3. Modelling showed that ordinary use can substantially raise fine particles indoors, especially when a room exchanges little air with the outside.

The study measured one device under laboratory conditions. The numbers in your room will differ. But the direction of the result is clear.

When essential oil meets ozone

Many essential oils, citrus and conifer oils above all, are rich in limonene and α-pinene. Both react quickly with ozone in the air. The products include formaldehyde and ultrafine particles, things people want in a room even less than the oil itself.

Indoor ozone comes from outdoor air, from some printers, and especially from ozone deodorisers and ionising air purifiers, which many households use. Do not run an ozone-generating device at the same time as, or in the same room as, an essential oil diffuser.

Closed, air-conditioned rooms

Most wall-mounted air conditioners in Vietnam only cool and recirculate the air in the room. They bring in almost no fresh air from outside. A bedroom with the door shut and the air conditioner on all night has a very low air exchange rate. Whatever the diffuser releases stays there a long time.

This is also why your nose stops reporting the scent after a few minutes, while the machine keeps releasing at a steady rate. The feeling that "the smell has gone" does not mean the air is free of it.

A more sensible way to diffuse

  1. Use distilled or bottled purified water, not tap water, in an ultrasonic diffuser.
  2. Diffuse intermittently. Run it for about thirty minutes to an hour, then rest it, rather than leaving it on all night. Many machines have a timer; use it.
  3. Less than you think. Start at the lowest amount in the machine's instructions. If you can still clearly smell it when you walk into the room, that is enough.
  4. Open a door slightly or run an extractor fan in a small room, especially an air-conditioned one.
  5. Choose an oil whose composition you know. A cheap fragrance blend sold as "essential oil" can contain anything. What is an essential oil helps you tell them apart.
  6. Do not use old, oxidised oil, citrus oils opened long ago above all. The storage page explains why.
  7. Clean the tank often. Water left standing for days in a machine is where microbes grow, and an ultrasonic diffuser will spray them out with the mist.

Young children, people with respiratory conditions, and pets

  • Babies under three months: do not diffuse essential oil in the baby's room. For older children, avoid oils rich in menthol and camphor near them. Details on the children and pregnancy page.
  • People with asthma or a respiratory condition: strong scent and fine particles can both be irritants. Ask your doctor first.
  • Cats: a cat's liver lacks a group of enzymes that people use to break down many essential oil components. If you diffuse, leave the door open so the cat can leave, and keep it short and light.
  • Birds: do not diffuse in a home with birds. A bird's respiratory system is far more sensitive than ours.
  • Dogs: a dog's nose is far more sensitive than ours. A level pleasant to you may be overpowering to a dog.

The pets page has more.

Sources

  • Schwartz-Narbonne, Du and Siegel (2021). Volatile organic compound and particulate matter emissions from an ultrasonic essential oil diffuser. Indoor Air 31(6), 1982–1992.
  • Nazaroff and Weschler (2004). Cleaning products and air fresheners: exposure to primary and secondary air pollutants. Atmospheric Environment 38(18), 2841–2865.
  • Destaillats et al. (2006). Indoor secondary pollutants from household product emissions in the presence of ozone: a bench-scale chamber study. Environmental Science & Technology 40(14), 4421–4428.
  • Tisserand and Young (2014). Essential Oil Safety, 2nd edition. Churchill Livingstone.

Further reading for this part

A shared list for the whole of Part Three — Learning origins, not a note for every sentence. We state what each source was used for.

  1. Young Soo Joung and Cullen R. Buie, 2015. Aerosol generation by raindrop impact on soil The MIT study that filmed how raindrops fling up bubbles carrying material from the soil into the air. It underpins the explanation of the smell of first rain: rain does not create the smell, it spreads it.
  2. Cây cỏ Việt Nam The botanical identification source for the species in the dictionary. Used together with Những cây thuốc và vị thuốc Việt Nam, and we say so when the two record things differently.

One thing must be said plainly. We have not read every work above in the original; most of the knowledge comes through secondary literature and reviews. We say so rather than let the list suggest a depth of research we have not reached.

Last updated: October 2026