December 1, 2023

Where does a candle go, if it burns at all? Is it healthy to inhale melted candle particles? How worried should I be?

– Abigail B., Washington, DC

When candles burn, most of their matter goes up in the air.

The light and warmth of a candle come from the burning wax. When you light the wick, the flame will melt some of the wax, flow up the wick and evaporate, and then the wax vapor will burn. The wick, which is made of cotton, also burns, although the wax contributes most of the heat. The puddles you sometimes see around the base are made of wax that has been spilled and dripped down without burning.

Wax is made up of hydrogen and carbon. When a candle burns, hydrogen and carbon from the wax combine with the oxygen in the air to form carbon dioxide and water vapor. Most of the matter in the candle ends up being these two gases.

Carbon dioxide and water aren’t exactly safe – too much of either can be extremely dangerous, as anyone who’s on a submarine can tell you – but in small amounts they are normal pieces of air. The amount of any gas produced by a candle is small – comparable to the amount that could be exhaled by another person in the room.

If candles burned completely, every molecule in the wax would combine with oxygen to form CO₂ or water vapor, but candles don’t burn perfectly. At the edges of the flame, clumps of carbon molecules – perhaps 0.1 percent of the mass of the candle – are thrown away before they burn to the end, much like scraps of food that are sprayed out by a kitchen mixer. These particles contribute to smoke and soot.

Aneta Wierzbicka, a scientist at Lund University in Sweden who studies indoor air pollution, conducted a series of experiments to measure particle emissions from candle flames. “Candles in non-smoking households are one of the strongest sources of particles indoors, followed by cooking,” she told me.

She said constant exposure to these tiny particles can lead to cardiovascular and respiratory diseases. For people who occasionally burn candles, fire hazards are likely a bigger problem than air pollution. But she said if people burn lots of candles every day, it pays to take steps to minimize exposure to airborne particles. She suggests making sure the room is well ventilated and using clean, white candles without too many additives or ingredients, as everything in the candle ends up in the air. (She also mentions that electronic candles have gotten pretty good lately – at first glance, some of them might even fool a candle expert!)

When a candle burns, the CO₂ and the water vapor cool down and mix with the air in the room. They can no longer be distinguished from other CO₂ or water molecules. As the air in your room exchanges with the air outside over the next few hours, the molecules in your candle will escape from the room and begin to diffuse into the atmosphere. After about a year, your candle’s atoms will have completely spread around the world.

For the next several years, every time someone breathes in air, they will inhale a few carbon atoms from the wax and a few oxygen atoms from the air in your room.