Classic Candles Under the Chemical Microscope: What We Breathe During Christmas and How to Ensure a Safe Home Environment

For many of us, the Christmas season is associated with a cozy atmosphere, the warmth of home, and the scent of candles. However, the flicker of a classic paraffin candle is far more than just an aesthetic element from a chemical perspective, it is a source of numerous substances that can affect indoor air quality, human health, and the environment. 

In this article, we will examine the composition of common candles, the chemical reactions that occur during burning, and what we are actually inhaling. We will also provide an overview of safer, natural alternatives.

Watrex: Classic Candles Under the Chemical Microscope: What We Breathe During Christmas and How to Ensure a Safe Home Environment

What is a common candle made of? 

Most commercial candles are made from paraffin, a mixture of saturated hydrocarbons (mostly alkanes C20–C40) produced during petroleum distillation. These candles often contain synthetic fragrances, dyes, and metal-core wicks. When a candle is lit, the hydrocarbons undergo pyrolysis, breaking the long paraffin chains into shorter hydrocarbons, which then oxidize. This process is never perfectly efficient and depends on the flame temperature, wax composition, wick type, and the presence of aromatic additives. Synthetic fragrances, often containing terpenes, aldehydes, and esters, decompose during burning into reactive products that can further interact with the surrounding air. 

What enters the air when a candle burns? 

Candle burning produces a wide range of chemical substances. The most significant from a health perspective include benzene, toluene, formaldehyde, and acrolein. Benzene and formaldehyde are classified by the IARC as carcinogenic to humans (Group 1). Long-term exposure to benzene is associated with an increased risk of leukemia, while formaldehyde irritates the eyes, nose, and upper respiratory tract. Acrolein, which is especially produced when burning oil-based waxes, is not classified as a carcinogen but is highly irritating to the respiratory system and can worsen asthma. 

Additionally, polycyclic aromatic hydrocarbons (PAHs) such as naphthalene and benzo[a]pyrene are formed. These are carcinogenic and mutagenic and attach to fine soot particles. These ultrafine particles (PM2.5 and PM1) penetrate deep into the lungs, impair cardiovascular health, and trigger inflammation. Aromatic components of scented candles, such as limonene or pinene, oxidize during burning and can react with indoor ozone, forming secondary organic aerosols that also irritate the respiratory tract. These substances can be analytically monitored using GC/MS for VOCs, HPLC for aldehydes, and optical methods for ultrafine particles. 

Why is this particularly risky at home? 

During winter, indoor spaces are often less ventilated, allowing concentrations of released chemicals to rise and accumulate quickly. Candles burn for long periods, especially during the Christmas season, increasing the total exposure to carcinogenic and irritating substances. The accumulation of VOCs, PAHs, and ultrafine particles, combined with other emission sources such as baking or cooking, creates a chemically complex environment that can be particularly burdensome for sensitive individuals — children, seniors, or asthma sufferers. 

How to minimize risks while keeping the Christmas atmosphere 

Choosing natural waxes, such as beeswax, soy, coconut, or rapeseed, is chemically advantageous. These waxes produce fewer VOCs and soot when burned, resulting in lower amounts of carcinogenic substances. For a fragrant effect, it is better to use natural essential oils, which are less reactive than synthetic fragrances, though thermal degradation should still be considered. Wicks should be made of cotton or wood, without metal cores, to minimize the release of metal traces. Short, intensive ventilation after burning candles significantly reduces VOCs, PAHs, and secondary aerosols. 

Watrex Fig.2: Beeswax candles with cotton wick

Conclusion 

The Christmas season should be a time of calm, family, and comfort. Yet even a small oversight using candles meant to create a warm atmosphere can contribute to indoor pollution. Understanding the chemical processes during burning and choosing safer, natural alternatives allows you to enjoy candlelight without unnecessary health risks. 

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