Scenting your home: what candles, incense and diffusers do to the air

No home fragrance cleans the air. A candle, an incense stick, an air freshener spray or a diffuser adds compounds to a room; none of them takes anything away. Public health guidance in the UK and the US follows the same order: cut back on the source first, then ventilate.

That does not mean giving up the smell of home. The US Environmental Protection Agency's page on improving indoor air quality says the most effective approach is usually to eliminate individual sources of pollution or reduce their emissions, with better ventilation and air cleaning as the other two strategies. An occasional candle followed by an open window is a very different exposure from incense every evening in a closed bedroom.

Why doesn't home fragrance clean the air?

Home fragrance works by releasing odour molecules into the room: it covers a smell, it does not remove it. The EPA's page on volatile organic compounds and their impact on indoor air quality lists aerosol sprays and air fresheners among household sources of VOCs, and notes that concentrations of many VOCs are consistently higher indoors, up to ten times higher, than outdoors.

The Chief Medical Officer for England reached the same point in his 2022 annual report on air pollution, in a list of suggestions for reducing exposure indoors:

The same report says that products such as air fresheners, candles, cleaning products and personal care products can often be removed, swapped for lower-emission options or used less, and that where they are needed, good ventilation while using them reduces exposure.

Candles, incense, sprays, diffusers: what does each one release?

Home fragrances do not all behave the same way, and burning changes a lot. The table sums up what the official sources in this class report.

What did a French study measure from candles and incense?

In 2017, a French study of pollutants from candles and incense by CSTB, LCE and INERIS, co-funded by the environment agency ADEME, burned nine incense sticks, nine scented candles and one catalytic lamp sold in France, under realistic conditions in an experimental house. Its English abstract reports that levels during and just after burning incense were much higher than with scented candles: benzene, toluene, formaldehyde, acetaldehyde, acrolein, polycyclic aromatic hydrocarbons and particles.

Candles released far fewer volatile compounds but smaller particles, and their formaldehyde faded more slowly after the flame went out, suggesting the release carries on after extinction. Candles also gave off more nitrogen oxides than incense.

The conclusion is measured, which is what makes it useful. No situation of concern is expected from the most common uses, but frequent or long use, several products at once, a small room or poor ventilation can push exposure above the usual reference values, mainly with incense. The priority substances it names are benzene, fine particles, acetaldehyde, formaldehyde and acrolein.

How should you ventilate when you burn a candle or incense?

Ventilate while it burns and afterwards. In its 2020 guideline on indoor air quality at home, the National Institute for Health and Care Excellence (NICE) asks professionals to tell people that using candles, household sprays or aerosols may lead to poor indoor air quality, and that they should think about increasing ventilation with extractor fans or by opening windows where possible and safe.

For people at higher risk, NICE goes further: pregnant women and people caring for babies under 12 months should reduce their use of household sprays and air fresheners, avoid or reduce candles where possible, and keep the room well ventilated. The Chief Medical Officer's report adds a practical figure for spray cleaners, paints and similar products: increase ventilation during use and for at least 30 minutes afterwards. And whatever the air quality, a lit candle should never be left alone in a room.

Are essential oil diffusers safer because they are natural?

No. In its 2020 statement on essential-oil based sprays and diffusers, France's health safety agency ANSES notes that these products, often sold as able to "purify" the air, emit volatile organic compounds, some of which may be irritant or sensitising even when they are of natural origin, and some of which react with the ozone naturally present in the air. French poison control centres recorded eye, nose and throat irritation and respiratory effects under normal use, mostly minor.

ANSES recommends keeping sprays, diffusers and bottles out of the reach of young children, as you would detergents or medicines. With the poison control centres, it advises against essential oils for people with respiratory conditions, particularly asthma, and for women who are pregnant or breastfeeding, and sends any question to a pharmacist. How fragrance allergens are labelled is a separate subject, covered in the class on allergens and IFRA standards.

How can you scent a room without burning or spraying?

A raw material set down in a room has a scent of its own: strips of sweet orange peel drying on a saucer, a piece of cedarwood, a tear of frankincense left unburnt. They release odour molecules too, but without combustion, so without smoke. This is not purification, just a quieter scent that is still a source, so the ventilation habit still applies.

Cedar has long been a household smell. In 1995, the FAO's book on flavours and fragrances of plant origin noted that cedarwood oils went into soap perfumes, household sprays, floor polishes and insecticides. Much of that "cedar" is juniper, as the class on precious woods in perfumery explains, and the cedarwood note describes the material itself.

Frankincense, the resin burned as incense, can be smelled cold as well, as a tear or a chip in a small dish. Smelling it cold and then recalling the smoke of a stick is a direct way to notice what the flame adds.

Exercise: smell your home when you come back in

Do this on an ordinary day, without lighting, spraying or putting anything on your skin.

1. Before you go out, close the door of your main room and write one sentence about what you smell there. You may smell almost nothing: that is olfactory fatigue, explained in the class on training your nose.
2. Stay out for at least half an hour. When you come back, smell the room from the doorway and note what stands out: cooking, laundry, wood, dust, yesterday's candle.
3. Open the windows wide for ten minutes, close them, wait five minutes and smell again from the doorway.
4. Now put a single material in the room: fresh orange peel on a saucer, a piece of cedarwood if you have one, or a tear of frankincense, unlit. Step out for a while, come back, smell.
5. Compare it with what you remember of a candle or incense stick burning in the same room, then share your notes in the Home Fragrance & Candles circle.

The exercise shows two things: your home's smell reads better after airing, and a single material can be enough to give a room its signature.