The electric, plug-in machines pump out plumes of fragrant water vapour. You just have to refill them occasionally with a small amount of water and a few drops of essential oil.
Some essential oils are said to ease insomnia, strengthen the immune system, aid digestion and soothe skin problems. At the very least, they can be rejuvenating or relaxing, depending on the scent.
Some are portable; others make more of a statement in a room.
Diffusers range in price from £30 to £200.
Made by Zen produces a whole range of aromatherapy diffusers. This one has a speaker, a light that dims to nothing and three mist settings – for one hour, two hours and intermittent.
It's both high tech – it's compatible with Bluetooth – and attractive.
For an ultrasonic aromatherapy diffuser, it is calming in the extreme.
This model by Lumie is on the pricier side, but it has a dual purpose. As well as being an aromatherapy diffuser, it is an SAD lamp, claiming to ease symptoms of Seasonal Affective Disorder and improve your sleep.
Two chambers enable you to fill it with essential oils for morning and evening so that you don't need to keep emptying and refilling compartments. You can then set the alarm so that it comes on at particular times.
You can also vary the brightness of the light for different times of day.
This mini diffuser is small but mighty. It comes in four colours and is designed for bedrooms.
It comes with 100ml of piezo oil, from Japan, in a choice of fragrances including rose and natural forest. We found it to be surprisingly powerful – the fragrance escapes through a sliding screen on the top.
You could stay overnight at a spa for this price, but this diffuser will last longer.
Scents include amber, berries, fig tree and wood fire. Just pop the scented capsule into the machine, switch it on and it will diffuse it for 10 minutes, followed by 10 minutes rest, and so on for an hour.
The aroma fills a large living area, making this diffuser particularly good for open-plan rooms.
This simple diffuser is great for a small room. Just add two or three drops of oil into the small ceramic dish, plug it in and let the fragrance diffuse.
Aromatherapy Associates offers a wide range of oils, although they will cost extra.
Our tester found the bowl to be a little wobbly, but thought the diffuser was great value for money.
Ellia makes a host of diffusers and this one stands out for its design: its basket-weave pattern and its mix of glass and wood.
This model also comes with a remote control to change the colour of the light. You can also change the sound it emits, from gentle waves to floating lotus.
It comes with three blended essential oils. Our tester found the fragrance to be noticeable but subtle.
This diffuser is the cheapest in our round-up. It looks neat and minimalist, but it has a range of functions including two speed settings, automatic shut-down, different colour lights and the optional sound of lapping water. The indicator light remains on at all times.
This machine's standout feature is its sleek, vase-like design. It also feels robust and well-made.
You have to buy the oils separately, but they do have a great range.
The mist reaches across large spaces and there are plenty of programming options so you can release the fragrance at different intervals between 30 minutes and three hours. A larger, floor-standing version, in white, is available for £170.
This white dome looks like a very large pebble in a Zen garden and gurgles like a water feature you might find in one.
Our tester found its sounds, aromas and glow to be wonderfully soporific.
It has 30-, 60- and 90-minute settings, so it won't keep running if you fall asleep.
This model comes with a good selection of oils. The scents our tester tried quickly filled the room, but weren't too overpowering. The fragrance lasted a few hours.
Refills are good value, too.
This is a statement piece and is available in different colours.
It’s smaller than the model from Muji and doesn’t produce as much vapour (although it's still very effective).
It's very easy to refill, too.
The oils are quite sweet.
The Alto Diffuser from Made by Zen has it all – subtle scent, plus sounds and light. It’s easy to control, too, with different settings to produce the desired mood.
We also loved the Lumie IRIS 500 Body Clock, with its built-in light-box, while for travel, the Rested Squair Diffuser is a winner.
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