Candles are ubiquitous in homes for light, ambiance, scent, and relaxation. But because candles burn (i.e. undergo combustion), many people wonder: Can lighting a candle cause carbon monoxide (CO) poisoning? In short: it’s theoretically possible in extreme circumstances, but in ordinary use in a reasonably ventilated space, candle-related CO poisoning is extremely unlikely. Nonetheless, it’s worth understanding how and when risk might arise, what the underlying science is, and how to minimize even small risks.
This article will cover:
- Basics of combustion, incomplete combustion, and carbon monoxide
- How much CO candles typically produce
- Whether those amounts can reach dangerous levels
- Documented evidence or case reports
- Risk factors that increase danger
- Symptoms, diagnosis, and treatment of CO poisoning
- Practical safety advice
Combustion, Incomplete Combustion & Carbon Monoxide
To understand risk, we need to look at how candles burn.
Complete vs. incomplete combustion
- A carbon-based fuel (like wax, which is largely hydrocarbons) burns by reacting with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O).
- However, if oxygen is limited or the flame is starved, combustion becomes incomplete, and one byproduct is carbon monoxide (CO).
- Carbon monoxide is colorless, odorless, and binds with hemoglobin in the blood more strongly than oxygen, reducing the body’s capacity to carry oxygen.
Thus, the potential for CO arises any time combustion is imperfect (insufficient oxygen, soot formation, flame disturbances, drafts, etc.).
Candles are relatively low-energy combustion sources, so their potential for CO is much smaller than, say, a malfunctioning gas heater, vehicle exhaust, or other fuel appliances.
Emissions from candles: what studies show
Several laboratory and field studies have measured emissions from burning candles. A relevant recent study, supported by candle manufacturers, measured how much CO, CO₂, NOₓ, volatile organic compounds, and particulate matter are emitted from various kinds of candles (scented, unscented, different waxes) in a sealed chamber.
The results confirm:
- Candles do emit measurable CO (alongside CO₂, NOₓ, volatile organics, soot) under closed, controlled conditions.
- The emission rates depend on factors such as wax type, fragrance, wick, combustion efficiency, and ventilation.
- Under normal usage with ventilation, the CO concentrations tend to remain low compared to dangerous levels.
A case study by ADSL Laboratories highlighted that a candle left burning in a small, unventilated room could potentially allow CO to accumulate to levels that “pose a serious risk to health.”
In other sources, it is stated that candle sources “are blamed for causing elevated CO levels” but that in well-ventilated homes the risk is minimal.
Some consumer safety commentary (less scientific) suggests that, in practice, CO levels from candles “are generally low” but that in poor ventilation or with long burning times, CO might accumulate.
In sum: yes, candles can produce CO in small quantities — especially under less-than-ideal combustion, but whether that leads to poisoning depends heavily on environment and time.
Can the CO from a Candle Reach Poisonous Levels?
This is the crux of the question: under what conditions, if any, could candle CO become dangerous?
Typical CO concentrations and health thresholds
First, what levels of CO are considered harmful? While thresholds vary among guidelines, a few reference points:
- In general, CO exposures around 35 ppm for several hours may start to cause symptoms like headache and dizziness.
- At 100 ppm, mild headache may develop in 2–3 hours.
- Higher levels (400, 800 ppm, etc.) cause more severe symptoms, loss of consciousness, and death.
Comparing with candles: the emissions studies show that under test chamber conditions, CO levels from burning a candle are typically quite low relative to these benchmarks — especially once diluted by room air, ventilation, and diffusion.
Thus, for most real-world candle use, CO accumulation would be too slow or too diluted to reach poisoning thresholds — unless the room is very small, sealed, and the candle burns for a long time with very poor ventilation.
The ADSL case and commentary suggest that under very unfavorable conditions (small sealed space, no ventilation, prolonged burning) CO could build to harmful levels.
Practical examples & anecdotal reports
There are occasional media reports and anecdotes claiming individuals got CO poisoning from candles. For example:
- A tabloid article claimed someone was hospitalized with CO poisoning after burning multiple scented candles for hours.
- Another claim warns of “people quite often dying due to carbon monoxide poisoning after lighting candles in a room with air-conditioning on and all windows closed.”
However, these reports are not backed by rigorous scientific or medical documentation, and they may exaggerate or misunderstand the causation.
Importantly, medical advice in forums suggests that a single candle burning overnight in a closed room is “very unlikely” to produce CO at harmful levels.
Given the limited experimental data and rarity of documented cases, the consensus is that candle CO poisoning is extremely rare, but not impossible under extreme misuse.
Risk Factors That Increase the Danger
If conditions are unfavorable, the risk of CO accumulation from candles increases. Some key risk factors:
- Poor ventilation / sealed rooms: If a room is tightly closed with minimal air exchange, combustion products (including CO) can accumulate, and Lack of airflow prevents dilution.
- Small enclosed spaces: The same emission rate leads to higher concentrations in smaller volumes.
- Long burning times: The longer the candle burns, the more CO accumulates (if not ventilated).
- Multiple candles or large candles: More combustion means more emissions.
- Imperfect combustion / poorly trimmed wicks / flickering / soot formation: These conditions increase the fraction of incomplete combustion, producing more CO, soot, and other pollutants. and Flickering or soot suggests burned conditions are not ideal, and increased emissions may result.
- Using synthetic fragrances, dyes, or paraffin waxes: These may influence soot, combustion quality, and additional volatile emissions.
- Low oxygen levels or competing oxygen consumers: If there are other combustion sources or limited oxygen, incomplete combustion is more likely.
So, while under typical use the risks are very low, if someone burns multiple candles in a sealed, small room for many hours, with poor ventilation, the risk may increase.
Symptoms, Diagnosis, and Treatment of CO Poisoning
Because CO is odorless and colorless, poisoning is often insidious and misdiagnosed. Below is a summary relevant to any CO exposure, not specific to candles.
How CO affects the body
- CO binds to hemoglobin in red blood cells (~200–250 times more strongly than oxygen), forming carboxyhemoglobin (COHb).
- This reduces oxygen delivery to tissues, leading to hypoxia (oxygen starvation) at the cellular level.
- Organs with high oxygen demand (brain, heart) are especially vulnerable.
Symptoms
Symptoms often reflect gradual oxygen deprivation and progress with exposure and concentration:
- Early symptoms: headache, dizziness, nausea, fatigue, shortness of breath, confusion, visual disturbances
- Moderate exposure: chest pain (esp. in those with heart disease), weakness, syncope (fainting)
- Severe exposure: seizures, arrhythmias, unconsciousness, brain damage, death
Because these symptoms are nonspecific, CO poisoning is sometimes mistaken for flu, food poisoning, or general malaise.
Diagnosis
- The key diagnostic test is measuring carboxyhemoglobin (COHb) levels in blood (from a blood sample).
- Pulse-oximeters and standard oxygen saturation measurements are unreliable, because they cannot distinguish COHb from oxyhemoglobin.
- Clinical history — exposure in a confined space, multiple people presenting with similar symptoms, correlation with combustion sources — is crucial.
Treatment
- Immediate removal from exposure and getting fresh air (100% oxygen)
- Administer high flow 100% oxygen (via mask) to displace CO from hemoglobin
- In serious cases, hyperbaric oxygen therapy (in pressurized chamber) may be used
- Supportive care for complications (cardiac monitoring, neurologic evaluation)
CO poisoning is a medical emergency; if suspected, seek immediate medical attention.
Can Candle Use Cause Long-Term Health Effects?
While the primary concern for “poisoning” is acute CO exposure, candles also produce other pollutants (soot, volatile organic compounds, polycyclic aromatic hydrocarbons, formaldehyde, particulate matter) which may have chronic effects. The medical literature is mixed:
- A systematic assessment of candle use and the risk of cardiovascular or respiratory hospital events in a Danish cohort (6,757 participants) found no statistically significant associations between frequent candle use (>4 times/week) and cardiovascular or respiratory events compared to infrequent use.
- However, experimental (short-term) studies and indoor air chemistry studies show that candle emissions can transiently elevate fine particles, volatile organic compounds (VOCs), and irritant gases.
- The “unknown risks of scented candles” article notes complaints of headaches, dizziness, throat irritation, respiratory symptoms associated with candle use, likely tied to VOCs, soot, and other combustion byproducts.
So, while chronic CO poisoning from candles is not a major documented concern, indirect impacts from air quality and inhaled pollutants are plausible over long durations, especially in vulnerable individuals (asthma, COPD, cardiovascular disease).
Practical Safety Advice
Because the risk of significant CO poisoning from candles is low under normal usage, safety measures are mostly commonsense. But here are best practices to minimize even small risks and improve indoor air quality:
1. Ensure ventilation
- Burn candles in rooms with fresh air exchange (open window, slight airflow)
- Avoid burning for long periods with no ventilation
2. Use high-quality candles
- Prefer natural waxes (soy, beeswax)
- Avoid cheap paraffin with heavy dyes and synthetic fragrance additives
- Use well-manufactured wicks (lead-free, properly centered)
3. Trim the wick
- Keep wick length around ¼ inch (≈ 6 mm)
- A shorter wick tends to burn more cleanly, reducing soot and incomplete combustion
4. Avoid drafts and flickering
- Place candles away from drafts, fans, vents, or open windows that cause flame instability
- A stable flame burns more efficiently
5. Limit number and size
- Don’t burn too many candles simultaneously in small spaces
- Use smaller quantities or space them out
6. Don’t leave candles unattended or burn overnight
- Extinguish candles before sleeping
- Never leave a burning candle when leaving the room
7. Consider alternatives
- Use LED (battery or electric) “candles” for ambiance
- Use essential oil diffusers rather than combustion
- Use good indoor air filtration or purification in rooms with frequent candle use
8. Be alert for symptoms
- If headaches, dizziness, confusion, or nausea develop in a room with burning candles, extinguish them immediately and ventilate
- Don’t ignore symptoms, especially if others in the space also feel unwell
9. CO detectors
- Although candles alone are unlikely to trigger alarms, having a carbon monoxide detector in your home (especially near fuel-burning appliances) is always prudent.
If you are especially cautious (children, people with heart or lung disease, small enclosed rooms), err on the side of better ventilation or reducing candle use.
Summary & Take-Home Message
- Yes, candles can produce carbon monoxide under conditions of incomplete combustion, especially in poorly ventilated or small spaces.
- However, the amount of CO from a normal candle in a typical ventilated room is extremely unlikely to reach levels that cause poisoning.
- Documented cases of CO poisoning solely from candles are exceedingly rare, mostly anecdotal, and not well supported by rigorous medical evidence.
- Risk is higher when many candles burn in a sealed room for long hours, with poor ventilation and unstable flame conditions.
- More relevant from a health standpoint may be the other pollutants (soot, VOCs, particulates) produced by candles, especially scented ones—these may irritate airways or influence indoor air quality over time.
- Use safe practices: proper ventilation, good quality candles, wick trimming, avoiding excessive numbers of candles, never leaving candles unattended, installing CO detectors, and substituting with safer alternatives when possible.