During the winter months, when days are short and cold and nights are long and dark, creating a warm and cosy indoor atmosphere can feel especially important.

Where I live in Denmark, around 75% of the population burns candles two or more times a week, and 34% use them daily during the winter months.

In fact, Denmark is often said to be the country with the highest per-capita candle consumption in Europe. Danes burn around 5.8kg of candle wax per person per year – the equivalent of about six bags of sugar.

Yet, nice as they can look, studies show burning candles is one of the biggest sources of indoor air pollution. This is because when burning, candles emit a large number of ultrafine particles.

These particles are so small that they are invisible to the human eye, and more than a thousand times thinner than a human hair. They can remain airborne for long periods, and when candles are blown out, the concentration of these particles (including soot) increases even further.

Candles vs cooking

In our experiments conducted in exposure chambers (where the climate is controlled) at Aarhus University, we examined how indoor air is affected both by burning candles and cooking pork in an oven.

Cooking – and especially frying – is known to emit high concentrations of particles due to combustion (the process of burning something). Particles from combustion processes are among the most harmful to human health – but less is known about the role that particles emitted from candles plays when it comes to indoor air pollution.

Our research found that while both candles and cooking emit high levels of particles, the number of particles from candles was much higher. Even more significant was the difference in particle size: cooking produced particles about 80 nanometers wide, whereas candles generated particles around seven-to=eight nanometers in size – much smaller and much easier for our lungs to inhale.

We also measured chemical compounds in the air, and found that burning candles produced not only soot particles but gases such as nitrogen dioxide and polycyclic aromatic hydrocarbons (PAHs) – a group of chemicals associated with inflammation and even cancer.

The particles we breathe

Ultrafine particles are of particular concern from a health perspective. They are easily inhaled into our lungs, but research has found they are excreted from the body very slowly.

Due to their extremely small size, particles from candles can penetrate deep into our smallest airways, known as the alveoli, and may even enter the bloodstream. From there, they can reach organs such as the heart and brain.

Indeed, the particles formed when candles are burning are remarkably similar to those found in diesel exhausts in size and composition. These particles have been linked to increased mortality rates from lung and cardiovascular diseases.

In our study, we also investigated how candle burning affects young adults with mild asthma. We found 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 observed small decreases in lung function, changes in arterial stiffness and heart rate, and reduced cognitive function after exposure to burning candles.

Light and dark

You might be thinking: all this from just a candle? But it’s worth remembering that people with chronic conditions like asthma or chronic obstructive pulmonary disease are far more sensitive to particulate air pollution.

Because people with these conditions already have chronically inflamed airways, even relatively low levels of pollution — including particles released by burning candles — can be enough to trigger symptoms.

Children, older adults and people with chronic illnesses are also particularly vulnerable to air pollution, due to immaturity of their lungs or weakened immune systems.

Indeed, for anyone with asthma or other respiratory conditions, the quality of indoor air is not a minor detail, but a key factor in day-to-day symptom control and long-term respiratory health.

Blow out all the candles?

But that’s not to say those without lung disease or asthma aren’t affected. As research shows, burning candles affects not only the indoor environment but potentially everyone’s health. Fortunately, a few simple steps can help maintain a healthier indoor climate.

Try using LED candles, for example, or light only a few candles at once. You should also place candles away from drafts to avoid a flickering flame, which can produce more soot and smoke. Trim your candles’ wicks to reduce soot formation, too. And candles should never be burned near (or by) people with respiratory disease.

Most importantly, air out the room afterwards by opening your windows. Such measures can help reduce the number of particles significantly, and make all the difference when it comes to keeping your cosy or hygge-time healthy.