Burning candles is one of the biggest sources of indoor air pollution, emitting ultrafine particles so small they can penetrate deep into your lungs and potentially enter your bloodstream. Recent research from Aarhus University found that candles produce significantly more harmful particles than cooking, with particles seven to eight nanometers in size, compared to 80 nanometers from cooking. These invisible particles can linger in the air for hours and pose particular risks to people with asthma, children, and older adults.

How Do Candle Emissions Compare to Other Indoor Pollution Sources?

When researchers at Aarhus University conducted controlled experiments comparing candle burning to cooking pork in an oven, the results were striking. While both activities emit high levels of particles due to combustion, candles released far more particles overall. The key difference was particle size: candles generated ultrafine particles around seven to eight nanometers wide, making them dramatically smaller and easier for lungs to inhale deeply. For context, these particles are more than a thousand times thinner than a human hair.

Beyond soot particles, burning candles also releases harmful chemical compounds into the air. The research detected nitrogen dioxide and polycyclic aromatic hydrocarbons, or PAHs, which are chemicals associated with inflammation and even cancer risk. The ultrafine particles from candles are remarkably similar in size and composition to those found in diesel exhaust, which have been linked to increased mortality rates from lung and cardiovascular diseases.

What Health Effects Can Candle Emissions Cause?

The health impacts of candle emissions extend beyond visible smoke. Because ultrafine particles are so small, they bypass the body's natural defenses and are easily inhaled into the deepest parts of the lungs, called the alveoli. Research has found that these particles are excreted from the body very slowly, meaning they can accumulate over time. Due to their size, they may even penetrate into the bloodstream and reach organs such as the heart and brain.

In studies of young adults with mild asthma, researchers observed subtle but measurable biological changes following exposure to candle emissions. Some markers of airway and blood inflammation were altered, and participants reported irritation and discomfort. Other studies among healthy adults have documented small decreases in lung function, changes in arterial stiffness and heart rate, and reduced cognitive function after exposure to burning candles.

People with chronic respiratory conditions face the greatest risk. Those with asthma or chronic obstructive pulmonary disease already have chronically inflamed airways, making them far more sensitive to particulate air pollution. Even relatively low levels of pollution from candles can be enough to trigger symptoms in these vulnerable populations. Children, older adults, and people with chronic illnesses are also particularly vulnerable due to immature or weakened immune systems.

How to Reduce Candle Emissions and Protect Your Indoor Air Quality

• Use LED candles: Switching to LED candles eliminates combustion entirely, preventing the release of ultrafine particles, soot, and harmful chemical compounds into your home.
• Limit the number of candles: Burning only a few candles at once significantly reduces the total particle concentration in your indoor air compared to lighting multiple candles simultaneously.
• Trim candle wicks: Keeping wicks trimmed to a shorter length reduces soot formation and the amount of particles released during burning.
• Position candles away from drafts: Placing candles away from air currents prevents flickering flames, which produce more soot and smoke than steady flames.
• Ventilate after burning: Opening windows after blowing out candles helps remove accumulated particles from the room and significantly reduces the concentration of harmful ultrafine particles in your indoor environment.
• Avoid candles near people with respiratory disease: Candles should never be burned in the presence of individuals with asthma, chronic obstructive pulmonary disease, or other respiratory conditions.

These practical steps can help maintain a healthier indoor climate without requiring you to eliminate candles entirely. The key is understanding that while candles create a pleasant atmosphere, their health impact depends on how often they're burned, how many are lit at once, and whether proper ventilation follows.

Why Should You Care About Candle Emissions in Winter?

The concern about candle emissions becomes especially relevant during winter months when people spend more time indoors with windows closed. In Denmark, where per-capita candle consumption is among the highest in Europe, around 75% of the population burns candles two or more times a week, and 34% use them daily during winter. Danes burn approximately 5.8 kilograms of candle wax per person per year, equivalent to about six bags of sugar.

This widespread use means that for many households, candles represent a significant and repeated source of indoor air pollution during the months when indoor air quality matters most. For people without respiratory disease, occasional candle use poses minimal risk. However, for vulnerable populations and those with chronic conditions, the cumulative effect of regular candle burning can meaningfully impact day-to-day symptom control and long-term respiratory health.

The research underscores an important principle: indoor air quality is not a minor detail but a key factor in respiratory health. By understanding how candles affect the air you breathe and taking simple precautions, you can enjoy the warmth and ambiance of candles while protecting your lungs and those of your family members.