From mechanism to measurement
The candle as a combustion and fragrance-release system
A sound interpretation follows the fuel, observes the state of the flame, and accounts for the conditions under which the measurements were collected.
From solid wax to flame
The flame creates the liquid fuel supply that allows it to continue burning.
A continuous cycle of melting and vaporisation
Once the wick is lit, heat melts the wax around its base. Pores in the wick lift the liquid by capillary action. Closer to the flame, the fuel is heated further, vaporises, and mixes with air. The visible flame is the region in which those vapours principally burn. [1][4]
The molten wax surface is therefore part of the fuel-delivery mechanism. Fragrance constituents can evaporate directly from the hot liquid wax, while those carried through the wick and into the flame are exposed to combustion. [1]
The balance is visible in the flame
During steady burning, much of the soot formed inside the flame can be oxidised before it escapes. Horizontal air movement can make the flame flicker and allow more carbonaceous particles to enter the surrounding air. [4]
Pagels et al. (2009) found that steady burning, sooting, and smouldering after extinction produced particles with different sizes and chemical compositions. Their experiment concerned two specific taper candles in a controlled chamber, but it demonstrates why flame state must be recorded in any comparison. [4]
Why the wax base is not a complete explanation
A raw material establishes some physical properties, but formulation and testing determine the finished behaviour.
Refinement, additives, and fragrance
Derudi et al. (2014) examined three container candles made from paraffin waxes with different degrees of refinement. Within those samples, the degree of refinement strongly influenced emission factors. In the group’s 2012 study, differences between scented candles were plausibly associated with both the paraffinic raw material and additives, with aldehyde emissions appearing more closely related to additives. [2][5]
The evidence does not support a general ranking of entire wax families. In the Salthammer et al. study, unscented palm, paraffin, soy, and stearin candles produced similar emission profiles, with no fuel showing a consistently better profile under the test conditions. A wax label cannot replace measurement of the finished candle. [1][2]
The wick contributes to the emission profile
During steady burning of the candles they tested, Pagels et al. detected ultrafine particles rich in phosphates or alkali nitrates. The authors identified wick flame-retardant additives as a likely source. Even a particle measured above a candle cannot automatically be attributed to the wax base alone. [4]
How fragrance travels from wax into the room
The measurements distinguish constituents evaporating from molten wax from compounds formed during combustion.
Evaporation from molten wax
In the Salthammer et al. study, the volatility of fragrance constituents affected the rate at which they passed from the hot liquid wax into the air. Constituents with higher vapour pressures dominated the air samples, while different fragrance families produced different volatile-compound profiles. [1]
Salthammer et al. (2021) recorded significant differences between the profiles of scented and unscented candles. Their measurements included both typical combustion products and evaporated constituents of the fragrance mixtures. That distinction matters: detecting a volatile compound does not by itself show that the flame created it. [1]
What the study did not measure
Every scented candle in the study used the same fragrance load, 5% by weight. Perceived scent intensity, duration, and balance were not assessed. The results therefore describe chemical emission profiles and do not establish an ideal fragrance percentage or the sensory performance of a candle. [1]
What emission studies can tell us
Measurements are most useful when read alongside the tested sample, chamber conditions, and assumptions of the exposure scenario.
From analytical detection to assessment
Orecchio (2011), Derudi et al. (2014), and Salthammer et al. (2021) detected gaseous and particulate emissions from burning candles. Concentrations and emission factors varied substantially between samples. That variation supports assessment of specific formulations, not a single conclusion for every candle. [1][2][3]
Salthammer et al. tested 24 purpose-made candle types in an 8 m³ chamber and converted the emission rates into scenarios for a 30 m³ reference room with 0.5 air changes per hour and defined use assumptions. Most calculated concentrations were below the comparison values they used. Exceptions were reported for NO₂ in some short-term comparisons and for acrolein and benzo[a]pyrene in some long-term comparisons. [1]