Here's the answer before we get into the detail: nothing. Clean indoor air is largely odourless. The scents we associate with pleasant homes — the candle, the diffuser, the spray — are almost entirely the smell of chemicals we've added to the air ourselves.
It's worth understanding what those products are actually doing, because the research on home fragrance is more substantial than most people realise.
Scented Candles
Most commercial candles are made from paraffin wax, a petroleum byproduct. When burned, paraffin releases volatile organic compounds including benzene — a known human carcinogen with no established safe exposure level — and toluene, a neurotoxin with regulatory exposure limits set by the US EPA and OSHA.
The fragrance compounds add a separate layer of concern. Synthetic fragrance oils can contain hundreds of undisclosed chemicals. A single "fragrance" ingredient on a label may represent a complex mixture that includes phthalates — endocrine-disrupting chemicals linked to hormone disruption and reproductive harm.
Fine particulate matter (PM2.5) is also generated during burning. These particles are small enough to bypass the upper respiratory tract and reach deep lung tissue, where they can cross into the bloodstream. Research has documented polycyclic aromatic hydrocarbons in candle emissions, several of which are classified as probable or known carcinogens.
The main variables are wax type, fragrance type, and ventilation:
- Paraffin wax produces more VOCs than soy or beeswax
- Synthetic fragrance oils carry more chemical risk than pure essential oils
- A well-ventilated room reduces but does not eliminate exposure
Better choice: beeswax or soy with pure essential oil fragrance and a cotton wick, in a ventilated room.
Reed Diffusers
Reed diffusers are commonly seen as a safer, flame-free option. They are lower risk than candles — but they are not neutral.
The liquid in a typical reed diffuser is 70–90% carrier solvent — usually dipropylene glycol or ethanol — with fragrance compounds making up the remainder. Both off-gas continuously at room temperature. A 2024 study measuring emissions in a full-scale chamber found continuous VOC release throughout the day.
The more significant concern is secondary chemistry. Terpene compounds in fragrance oils react with ozone in indoor air to form secondary pollutants including formaldehyde. This reaction continues after the diffuser is in place.
Because a reed diffuser operates 24 hours a day, VOCs can accumulate in poorly ventilated modern homes over time — particularly in bedrooms where people spend seven to nine hours overnight.
Better choice: phthalate-free formulations, used in a ventilated room. Position matters — diffuser placement affects VOC distribution throughout a room.
Incense Sticks and Cones
Of the six products covered here, incense has the strongest evidence base for harm. The findings on particulate matter alone are striking.
Published research documents that incense burning produces particulate matter at a rate greater than 45mg per gram burned. For cigarettes, the equivalent figure is around 10mg per gram. These particles are predominantly PM2.5 — fine enough to bypass the upper respiratory tract, reach deep lung tissue, and cross into the bloodstream.
Real-world measurements confirm the scale of exposure. One study recorded PM2.5 concentrations of 1,850 micrograms per cubic metre in residential apartments during incense burning under poor ventilation. The WHO indoor air quality guideline for PM2.5 is 15 micrograms per cubic metre as an annual mean.
VOC emissions are also significant. Research measuring benzene found that burning incense raised indoor levels by a factor of 73. Products with a wooden core were the primary driver.
The clinical literature links regular incense exposure to:
- Increased asthma risk and respiratory tract inflammation
- Upper respiratory tract and lung cancer with long-term exposure
- Bladder cancer risk, as benzene is excreted via the kidneys
- Elevated cord blood IgE in newborns of mothers with high incense exposure
Better choice: natural plant resins such as frankincense or copal, which produce fewer synthetic chemical byproducts. Maximise ventilation regardless.
Plug-In Air Fresheners
Plug-in air fresheners heat a reservoir of scented oil to release fragrance continuously. The heating mechanism is the core problem — it accelerates off-gassing of VOCs and other compounds around the clock.
Research by Professor Anne Steinemann at the University of Melbourne found that common fragranced consumer products emitted between 1 and 8 toxic or hazardous chemicals each, with nearly half generating at least one carcinogenic air pollutant. None were listed on any product label. Products marketed as green, natural, or organic performed no better than conventional ones.
Phthalates are the most significant chemical concern. Research found phthalates in 86% of air freshener products tested, including those labelled natural. Phthalates are endocrine disruptors linked to:
- Hormone disruption and reproductive abnormalities
- Developmental problems in children
- Increased asthma and allergy risk
Because plug-in devices run continuously, exposure is constant rather than occasional. In bedrooms and children's rooms this means sustained overnight chemical exposure during the body's recovery period.
The straightforward recommendation: remove plug-in air fresheners and address underlying odour sources directly.
Room Sprays and Fragrance Sprays
Room sprays have a distinct exposure profile. Rather than releasing compounds gradually, a spray delivers a concentrated burst of chemicals simultaneously — aerosolised into fine particles that can be inhaled directly into the respiratory tract.
A nationally representative study found that 34.7% of adults reported health effects including migraines and respiratory difficulties when exposed to fragranced products. More than 15% reported losing workdays or employment as a result of fragranced product exposure in the workplace.
The particles from a room spray do not clear when the scent fades. They adsorb onto surfaces — soft furnishings, carpet, bedding, walls — and are re-released into the air over time. Cumulative use matters: research has found that simultaneous use of multiple fragranced products produces significantly higher formaldehyde concentrations than any single product alone.
Better choice: a few drops of pure essential oil in water in a small spray bottle, used in a ventilated room.
Aromatherapy Diffusers
Ultrasonic aromatherapy diffusers are meaningfully better than the other products on this list. No combustion, no synthetic carrier solvents, and when used with pure essential oils, no phthalates or undisclosed synthetic fragrance chemicals. For most people in most circumstances, they are a reasonable choice.
That said, they are not entirely without consideration. Essential oils are volatile organic compounds by definition — they evaporate at room temperature, which is what makes them smell. When dispersed into indoor air, terpene compounds in the oils react with ozone to form secondary pollutants including formaldehyde and ultrafine particles.
The health significance depends on concentration and ventilation. In well-ventilated spaces with intermittent use, the risk to healthy adults is low. The concern increases with:
- Continuous use in poorly ventilated rooms
- Use around infants and young children — eucalyptus oil in particular has been linked to respiratory distress in young children at low concentrations
- Use in homes with already elevated humidity, where added moisture from the diffuser can contribute to conditions that favour dust mite and mould growth
Better choice: pure essential oils rather than synthetic fragrance blends, used intermittently in a ventilated room.
So What Does a Healthy Home Smell Like?
Not much.
Clean indoor air — low in VOCs, particulates, and biological contaminants — is largely odourless. The association between pleasant smells and a healthy home is largely a product of marketing, not biology.
That doesn't mean home fragrance is categorically off limits. If you want fragrance, the hierarchy of better choices is roughly:
- Pure essential oil diffuser, used intermittently in a ventilated room
- Beeswax candle with pure essential oil fragrance, in a ventilated room
- Natural plant resins on charcoal, ventilated
But the most effective interventions for indoor air quality are also the simplest: clean your home with low-toxin products, ventilate well (windows, rangehoods, bathroom exhaust fans), and use a good air purifier with a multi-kilogram carbon filter to trap odours.
Book a free consultation to discuss the air quality in your home.