Citronella candles can technically be burned indoors, but doing so combines two problems: they are surprisingly poor mosquito repellents in enclosed spaces, and burning any scented candle indoors introduces air pollutants you would not encounter if you chose a flameless alternative. Research consistently shows that citronella candles repel only about 14 to 29 percent of mosquitoes indoors, a performance so weak that the air-quality trade-off becomes hard to justify. Whether the health risks are large enough to worry about depends on how often you burn them, how well ventilated the room is, and who else is in it.
How Well Citronella Candles Actually Work Indoors
The appeal of a citronella candle is simple: light it, and mosquitoes leave you alone. Outdoors on a patio, that expectation is already inflated. Indoors, the reality is worse. A controlled experiment measuring mosquito landing rates found that citronella candles reduced bites by only about 29 percent compared to an unprotected control, while candles made with geraniol, a different plant-derived compound, reduced bites by roughly 85 percent.1Journal of the American Mosquito Control Association. Indoor Protection Against Mosquito and Sand Fly Bites: A Comparison Between Citronella, Linalool, and Geraniol Candles A separate study using traps rather than human subjects put the indoor repellency rate of citronella candles even lower, at just 14 percent.2PubMed. Efficacy of the botanical repellents geraniol, linalool, and citronella against mosquitoes
Those numbers mean that if ten mosquitoes would have bitten you in an unprotected room, a citronella candle might deter one or two of them. For a product people associate with mosquito control, that is a strikingly modest payoff. The problem is partly about how candles deliver the active compound. A flame produces heat and soot but disperses the volatile citronella oil unevenly and in small quantities. Much of the oil breaks down in the flame itself rather than evaporating into the air as a repellent vapor.
What Burning a Citronella Candle Releases Into Indoor Air
Every candle, scented or not, is a small combustion source. The flame converts wax into carbon dioxide, water vapor, soot particles, and a cocktail of trace chemicals. Scented candles add another layer because the fragrance compounds themselves vaporize and react with whatever is already in the room’s air. Research on commonly available scented candles has detected formaldehyde, toluene, and polycyclic aromatic hydrocarbons like naphthalene and pyrene in the fumes, with cheaper paraffin-wax candles tending to release higher quantities of these compounds.3PubMed Central. The unknown risks of scented candles! what science has to say: an editorial
Citronella candles carry an additional air-quality quirk. The essential oil they contain is rich in unsaturated compounds, meaning molecules with double bonds that react readily with ozone. On warm summer evenings, when people are most likely to light a citronella candle, indoor ozone levels tend to be higher because outdoor ozone drifts in through open windows and doors. Smog-chamber experiments showed that when ozone mixed with citronella candle emissions, it triggered rapid formation of secondary organic aerosol, tiny particles created not by the flame itself but by chemical reactions in the air after the fact.4PubMed Central. Organic aerosol formation in citronella candle plumes This means a citronella candle in a room with even modest ventilation from an open window can generate more airborne particles than you would expect from the candle alone.
To put the particle emissions in perspective, direct particulate matter from candle burning itself is relatively low. Measurements of various combustion sources found that candles produced well below 1 milligram of PM10 per hour, far less than incense or coil-type anti-mosquito products, which emitted upward of 48 milligrams per hour.5Building and Environment. Characteristics of particles and black carbon emitted by combustion of incenses, candles and anti-mosquito products So the candle flame itself is not the major concern. The secondary particles formed when citronella vapors react with ozone are the less obvious issue, and they are harder to control because they form invisibly in the air around you.
Are the Health Risks Serious for Occasional Use?
The short answer is that occasional, well-ventilated use is unlikely to cause measurable harm in a healthy person, but the evidence gets fuzzier with frequent or prolonged burning. A large Danish cohort study tracking thousands of adults over years found that people who used candles more than four times a week had hazard ratios for respiratory conditions like COPD, asthma, and pneumonia that were close to 1.0, meaning no clear increase in risk compared to people who rarely used candles.6PubMed Central. Use of candles and risk of cardiovascular and respiratory events in a Danish cohort study The associations were not statistically significant, which is reassuring for the average candle user.
A study specifically looking at young adults who used scented candles found that burning them four or more times a month was associated with modestly higher odds of chest tightness and shortness of breath, but again, none of those associations reached statistical significance.7PubMed Central. Extent of exposure to scented candles and prevalence of respiratory and non-respiratory symptoms amongst young university students Burning for more than 60 minutes per session showed a non-significant uptick in headaches, sneezing, and wheezing. These findings do not prove candles are harmless, but they do suggest that low-to-moderate use in ventilated rooms is not a dramatic health threat for most people.
Where the calculus shifts is for people with existing respiratory conditions. A randomized controlled study exposing mild asthmatics to ultrafine particles from candle burning found weak or absent associations with systemic inflammation markers like CRP and cytokines, though there were some changes in lung-lining proteins measured in exhaled breath.8PubMed Central. Airway and systemic biomarkers of health effects after short-term exposure to indoor ultrafine particles from cooking and candles – A randomized controlled double-blind crossover study among mild asthmatic subjects For someone with well-controlled mild asthma, a single candle session is probably manageable. For someone with severe asthma, chronic bronchitis, or another condition that makes their airways twitchy, the sensible move is to skip the candle entirely and use a flameless repellent method.
Why Diffusers Outperform Candles Indoors
If you actually want citronella oil to repel mosquitoes in your living room, the delivery system matters more than the oil itself. The same study that measured a 14 percent repellency rate for citronella candles found that citronella diffusers, devices that evaporate the oil without combustion, achieved a 68 percent repellency rate in the same indoor conditions.2PubMed. Efficacy of the botanical repellents geraniol, linalool, and citronella against mosquitoes That is nearly five times more effective. Geraniol diffusers performed even better, reaching 97 percent repellency indoors.
The reason is straightforward. A diffuser releases the volatile oil directly into the air as a vapor. A candle flame destroys a large fraction of the oil through combustion before it ever reaches the surrounding air. You are essentially paying for the aroma of citronella, which may give you the psychological impression that mosquitoes are being repelled, while most of the active compound is going up in smoke. A diffuser preserves far more of the oil in its repellent-active form.
This also means that if your goal is mosquito control and you have been burning citronella candles indoors to achieve it, switching to a diffuser not only works better but eliminates the combustion byproducts entirely. No soot, no formaldehyde, no secondary aerosol from ozone reactions. You get more repellent effect with less air pollution.
Geraniol and Linalool as Stronger Alternatives
Citronella gets most of the brand recognition, but it is not the strongest botanical repellent. Research comparing three plant-based compounds head-to-head, citronella, linalool, and geraniol, consistently found that geraniol outperformed the other two by a wide margin. In indoor candle form, a five percent geraniol candle repelled about 85 percent of mosquitoes, compared to 71 percent for linalool and 29 percent for citronella.1Journal of the American Mosquito Control Association. Indoor Protection Against Mosquito and Sand Fly Bites: A Comparison Between Citronella, Linalool, and Geraniol Candles Geraniol candles were roughly five times more effective than citronella candles at preventing bites.
The same pattern held for sand flies, with geraniol candles repelling about 80 percent and citronella candles only about 25 percent. If you prefer the ritual of a candle over a plug-in diffuser, seeking out a geraniol-based candle would give you meaningfully better protection. Geraniol is found naturally in rose oil, palmarosa, and some varieties of citronella grass, so it has a pleasant floral scent rather than the strong lemongrass smell most people associate with insect repellent candles.
Linalool, the compound responsible for much of lavender’s scent, falls in the middle. It is a reasonable option if you find the smell appealing and want moderate repellency without the combustion-chemistry complications of citronella’s highly reactive unsaturated molecules. Both geraniol and linalool are less prone to the ozone-driven secondary aerosol formation that makes citronella a unique indoor air concern.
Pets in the Room
A common worry when burning scented candles around pets is whether the volatile compounds are toxic to dogs or cats. Citronella oil itself has relatively low acute toxicity in mammals. Research on essential-oil-based pesticides reports that most major essential oil constituents have oral toxicity thresholds in rodents ranging from 800 to over 3,000 milligrams per kilogram of body weight for pure compounds, and above 5,000 milligrams per kilogram for formulated products.9ScienceDirect. Plant Monoterpenoids (Prospective Pesticides) To put that in practical terms, a cat or dog would have to ingest an enormous amount of pure citronella oil for it to be acutely dangerous.
That said, cats metabolize certain volatile compounds differently than dogs or humans, so concentrated essential oils can be more irritating to them. The concern with a burning candle is not really the citronella itself but the soot, particulate matter, and secondary aerosol that accumulate in the room. Birds are especially sensitive to airborne irritants because of their uniquely efficient respiratory systems. If you keep birds, burning any scented candle in the same room is a bad idea regardless of the specific fragrance.
For dogs and cats, occasional candle use in a ventilated room is unlikely to cause poisoning. The more practical risk is a pet knocking over a lit candle. Flameless diffusers sidestep that problem too.
Practical Tips If You Burn Them Anyway
Plenty of people burn citronella candles indoors during summer gatherings despite the modest repellent effect, either out of habit or because they like the smell. If that is you, a few adjustments can reduce the downsides:
- Ventilation first: Crack a window or run a fan to keep air moving. This dilutes the combustion byproducts and secondary aerosol. The paradox is that open windows also let in the mosquitoes you are trying to repel, so screens on any open windows are essential.
- Burn time limits: Keep sessions under an hour. The study linking longer exposures to higher symptom occurrence saw the uptick beyond 60 minutes of continuous burning.
- Trim the wick: A wick trimmed to about a quarter inch produces a smaller, steadier flame with less soot. Flickering or oversized flames generate more particulate matter.
- Choose soy or beeswax base: Paraffin candles tend to release higher quantities of formaldehyde and aromatic hydrocarbons compared to plant-based or beeswax candles.
- Skip them during high-ozone days: Air-quality forecasts report ozone levels. On days when ozone is elevated, citronella’s reactive oils will generate more secondary aerosol indoors.
These steps will not eliminate the air-quality cost of indoor candle burning, but they reduce it meaningfully. And if you swap from a citronella candle to a geraniol candle, you get a substantial improvement in repellency for the same effort.
When Screens and DEET Are Just Better
There is an honest case to be made that citronella candles, indoors or out, occupy a kind of comfort zone between doing nothing and using a method that actually works. If mosquitoes are getting into your home often enough that you are considering repellent candles, the most effective intervention is mechanical: intact window screens, door sweeps, and sealing gaps. These keep mosquitoes out without introducing any combustion products into your air.
For situations where you need personal repellency, such as sleeping in a room where mosquitoes are already present, topical repellents containing DEET, picaridin, or oil of lemon eucalyptus provide far higher protection than any area-effect candle or diffuser. These products are applied to skin or clothing and create a repellent zone right where mosquitoes are trying to bite, rather than relying on a plume of vapor to fill an entire room.
Citronella candles fit best as an ambient product for a screened porch or a brief outdoor dinner, where their modest repellency adds a small layer of deterrence and their combustion emissions dissipate into open air. Indoors, the combination of weak performance and measurable air-quality impact makes them one of the least efficient ways to deal with mosquitoes. A diffuser with geraniol oil, good screens, and a personal repellent for high-risk situations covers far more ground with fewer trade-offs.
Citronella-Treated Fabrics and Newer Delivery Methods
Researchers have been exploring ways to get citronella’s repellent compounds into the air without burning anything. One approach uses microencapsulated citronella oil applied to cotton fabric. The tiny capsules slowly release the oil over time, and treated fabrics maintained a repellent effect above 90 percent for about three weeks in testing, far outlasting a simple spray of citronella dissolved in alcohol.10PubMed. Microencapsulated citronella oil for mosquito repellent finishing of cotton textiles This kind of slow-release technology could eventually find its way into curtains, bed nets, or clothing designed for tropical climates, delivering area repellency without flame, smoke, or the secondary chemistry that makes indoor candle burning a concern.
These products are still more common in research settings than on store shelves, but they point toward a future where the useful properties of citronella and similar botanicals are separated from the least desirable delivery method, an open flame in your living room. For now, the simplest version of this concept is a plug-in or reed diffuser with a botanical oil, which achieves much the same goal with technology that already exists cheaply and widely.