Why you should rethink your scented candles

What’s released into indoor air when we light scented candles?

The past two years turned us into mini experts on aerosols and indoor air quality as we educated ourselves about viral transmission and Covid spread. Which makes this a perfect time to consider other particles that we breathe indoors, especially the ones we have more control of. 

I'll open this discussion with an admission: Scented candles are a pet peeve of mine. Most people love them, I know. They're a favorite gift, they’re often beautiful, and it’s a personal annoyance that I know isn’t shared by many. The scented candle industry is big and growing and worth billions. Scented candles are marketed as mood enhancers, destressors and also as (aroma)therapeutic. 

But in my personal opinion scent preference is highly personal – simply a matter of taste. A scent that’s pleasing to you may be nauseating to someone else. If you’re baking cookies or sautéing garlic your home will naturally carry these ingredients’ scent for a while as a side effect. But lighting a chocolate cinnamon candle is a deliberate scent-adding decision, and while you may like that scent others may not.

And here’s the thing with smelling stuff: Unlike our vision and hearing, in which only waves stimulate our receptors and nothing material enters our body, with smell you’re actually internalizing molecules. If you’re smelling something, these are actual particles that enter your nose, and it’s not just aroma that’s delivered. Burning stuff around the house produces combustion gasses that may affect the quality of the air – and modern homes are better insulated so what’s indoors lingers.

So let’s examine what’s carried in the air with these scents.

What’s burning?

Candles are made of fuels for burning, such as wax, a wick, and to that are added fragrances and sometimes colors.

The fragrance is what we seek, but a lit candle releases not just fragrance but also solvents (the oils in which the aroma is dissolved) and these solvents’ emissions – which are many times odorless – may be air pollutants. Even a completely natural and safe-to-eat material changes when it’s burned, it transforms into something else.

A recent study in Environment International examines 24 types of candles with different fuels and fragrances that represent the candles currently in the market. The candles were tested in a ventilated and well mixed 8 meter (26 feet) square emissions test chamber. The 4 types of candle fuels tested were palm, paraffin wax, soy and stearin, each of these with five different scents, including floral, fresh, fruit, oriental and spice, and edibles, or in an unscented version. Candles with scent had a 5 percent fragrance load, which is the industry average.

And the results: There were multiple combustion products emitted, and while most of these molecules’ concentrations were below the levels considered concerning, several scented candles produced nitrogen dioxide (NO2), acrolein and benzopyrenes above the levels considered safe by groups such as the World Health Organization and the US Environmental Protection Agency. Unscented candles emitted lower emissions of combustion byproducts than candles with fragrance. As to the fuels of which the candle burns: There was no fuel that showed a better emissions profile.

A study in the Journal of Hazardous Materials testing 12 samples from 6 candle types (5 scented, 1 unscented as control) in the unlit and lit condition found that toxic compounds like formaldehyde were emitted in large quantities from some scented candles, especially when lit. The researchers measured a number if emitted pollutants known to have deleterious effects on human health, and compared these to the recommended upper limit set by OSHA (Occupational Safety and Health Administration) and ACGIH (American Conference of Governmental Industrial Hygienists), and for most compounds the level was below that upper limit, but for formaldehyde the level in three of the samples exceeded it considerably. The authors conclude: “The use of scented candles was found to have serious consequences as a significant source of certain pollutants, especially some light hydrocarbon species. The 76 of the most commonly released compounds from scented candles appear to vary greatly in their emission profiles by a simple criterion – whether the candle is lit or not.”

Another study of unlit vs, lit candles finds that an unlit candle emits weak odor, which increases as much as ten fold when lit, and the lit candles also release volatile organic compounds, especially benzene, benzaldehyde, and benzophenone. The authors note that measuring the exact quantity of the compounds is fraught with artifacts, and that the benzene emissions were low.

A study published in 2014, which was financed by the Research Institute for Fragrance Materials Inc. tested 9 scented candles in small and large chambers, to resemble a bathroom or a living room, respectively. There were measurable non-fragrance combustion chemicals such as benzene and formaldehyde in the chambers after burning the scented candles, but those were below the concentration limits set by the World Health Organization. The formaldehyde and benzene formed as a result of combustion are considered a carcinogen – so it’s perhaps small comfort that the levels measured in these experiments were low and considered safe by research financed by an affiliated industry. 

Does the benefit outweigh the risk?

The issue of diffused scent is not limited to candles. Scents and air fresheners are all around us, and are part of the identity of many retail environments. That lovely aroma that you associate with a hotel chain, a fashion brand and an ice cream parlor are actively diffused into the air, to make your stay pleasant and to make you stay longer. To compensate for our Covid masks blocking the aroma some retailers nowadays pump more scent into the air. Scent, its designers argue, plays a significant role in marketing just about anything.

We can’t really control the scents or air quality outside our home. At home, however, products designed to fragrance the air have no essential role. We’re intentionally releasing into our home a mostly unknown chemical mix, especially when we burn these aroma sources.

There’s risk in that, and it’s real, even if it’s small. 

The benefits of a burning scented candled are highly subjective. Do they outweigh that risk for you and others sharing your space? That would really depend on how much you enjoy scented candles. When it comes to inhaling toxic chemicals, less, I think, is always better, even if the amounts are rather small.

Dr. Ayala