Are Candles Toxic to Indoor Air Quality?

1       Introduction 

When it comes to air quality, most scientific studies and concerns focus on outdoor air quality. The Swiss environmental legislation on air quality is a perfect example. Regulations require companies to emit no more than a certain amount of a pollutant into the outdoor air, called an emission limit value. In addition, each pollutant is subject to an immission limit value, protecting the subject from being exposed to a higher value than the law requires [1]. However, these limits do not apply to indoor air. The only regulated source of indoor air pollution in Switzerland is radon. This is quite surprising knowing that a person spends almost 90% of his or her time indoors [2] and that indoor air is often more polluted than outdoor air. It is therefore important to study indoor air quality.

2       Results of the Current Studies 

This blog post draws on current studies to investigate the impact of candles on indoor air quality.

2.1       The Basic Principle : Combustion 

Above all, it is important to recall the very principle of a candle: combustion. Combustion is an exothermic redox reaction. This means that heat is released and electrons are transferred [3]. The wax rises by capillary action in a wick and vaporises. The gaseous paraffin reacts with the oxygen in the combustion zone and the flame is maintained.

For combustion to take place, 4 ingredients are needed:

  • A fuel, the wax in the case of a candle
  • An oxidizer allowing the combustion of the fuel, most of the time oxygen
  • An activation energy, that triggers the combustion, a match or a gas lighter.
  • Free radicals, that allow combustion to take place with flames. When there are no free radicals, combustion takes place but without flame. This is the case when there are only embers left in a fire.

There are two types of combustion:

Complete combustion: this is the case when the oxygen reacts completely with the oxidant. The end products can no longer be oxidised. This combustion occurs, for example, when methane reacts completely with oxygen to form water, CO2 and heat.

This type of combustion occurs, for example, when a burner emits a blue flame as in Figure 1 [4].

Incomplete combustion : This occurs when the quantity of oxidant is insufficient or when the contact time at a temperature that makes combustion possible is too short. It produces combustion residues that can pollute the air. This combustion occurs, for example, when methane reacts incompletely with oxygen to form water heat and undesirable products such as CO.

This is the case with a burning candle which emit a yellow flame as in Figure 1. Part of the combustion is incomplete because the wax flow is too high compare to the oxygen one. There is therefore not enough oxygen near the wick to allow a complete reaction. Unwanted products are released and pollute the indoor environment [4].

Figure 1 : On the left incomplete combustion, on the right complete combustion [5]

2.2       The Different Types of Candles 

There are many different types of candles, with different compositions. To simplify this study, three main families of candles are defined, as in the ADEME study [6] : candles, incense and catalytic lamps. In this study, candles are scented candles. Incenses are both in stick and cone form. Catalysis lamps are scented liquids.

This classification helps to simplify the analysis. However, it should be remembered that although emissions are probably related to the type of candle, there must necessarily be a large disparity within the same class. This will be discussed in the remainder of this study.

2.3       Combustion Rate 

The burning rate is expressed as mass per unit time. In the case of a candle, the unit is [g/h]. In the ADEME study [6] , the burning rates of different types of candles, described in section 2.2 , are analysed.

In Figure 2 it can be seen that the combustion rate of the catalytic lamp reaches 18 g/h, which is higher than both the candles and the incense. This is due to the combustion system that consists of evaporating a highly volatile liquid. Candles have a burning rate between 4 and 7 g/h whereas incense has the lowest burning rate, between 1 and 4 g/h. However, there is a wide variation in the burning rate within the same class.

Figure 2 : Combustion rate of the different type of candle : catalytic lamp (yellow), candles (blue) and incense (red) [6]

While the speed of combustion is one of the factors that will spread pollutants within a house, it is not the only factor that can tell whether a system is more harmful to the indoor air. Therefore, it is necessary to look at the emissions for these different types of candles.

2.4       The Emissions 

ADEME [6] has also analysed the average concentrations during the test [μg/m3] for the different pollutants emitted by the different types of candles.

The compounds researched are the following: VOCs (benzene, toluene, ethylbenzene, alpha-pinene, d-limonene, naphthalene), carbonyl compounds (formaldehyde, acetaldehyde, acrolein, acetone, propionaldehyde, benzaldehyde), polycyclic aromatic hydrocarbons (benzo(a)pyrene, benzo(a)anthracene, dibenzo(ah)anthracene, 9,10-anthraquinone), particulate matter, black carbon and NOx.

VOCs (Volatile Organic Compound) : for these types of pollutants, incense emits the most. For benzene, for example, emissions vary between 6 and 167 μg/m3. This shows the wide variation once again within the same class. It should be noted that the dominance of VOCs for incense and in particular for benzene is confirmed by the study of Werner Tirler and Gaetano Settimo [7].

Particulate matter : for incense, the majority of the particles emitted vary between 82 and 190 nm, as shown in Figure 3. This particle range is not in the zone of greatest deposition according to [8]. However, this type of particle is in the fine particle range and can penetrate quite far into the respiratory system.

For candles, the fine particle emission profile is not the same. Indeed, the particles emitted are below 100 nm, as shown in Figure 3. This means that some of these particles are named UFP (Ultra Fine Particles) with a diameter of less than 10 nm. This type of particle is particularly dangerous because it will penetrate the furthest into the respiratory system and UFP have a very high deposition fraction, i.e. above 80% [8].

Figure 3 : Size distribution of particles emitted by candles (right) and incense (left) [6]

Another important result shown in the ADEME study [6] is that the variation in particle size distribution over time. These results are shown in Figure 4. The incense and the candle are lit at t = 1h.

For incense, this variation is stable over time. The granulometry of the particles emitted does not vary between the time the incense is lit and well after its combustion.

For candles, however, this is not the case at all. As can be seen in Figure 4, there is a drift effect in the particle size. When the candle is lit, the finest UFP are emitted, around 0.5 nm. It takes 25 minutes before the particles emitted are no longer UFP and go above 10 nm. Finally, when the flame is extinguished at t = 2 h, there is a “jump” in the size of the particles emitted, which increases to about 50 nm. The number of particles emitted increases also. The particle size will not exceed 100 nm which seems to represent the upper boundary for the size of the particles emitted for candles. These results are very important for two reasons:

Firstly, they show that the flame goes through several combustion states depending on the amount of oxygen and wax at time t. The related emissions will therefore depend on this.

Secondly, the size of the particles provides information on their suspension time in the air. Indeed, particles around 0.1 µm are the ones that settle the fastest on the ground [9]. Thus, the particles emitted by incense will remain in the air for a shorter period of time, as their diameter is around 0.1 µm. On the other hand, the particles emitted by candles will have a longer residence time in the air, as they are smaller. This means that in the case of natural ventilation, it will take longer to remove the same amount of incense particles as the candles.

In addition, the resuspension of particles in the air when a person walks through an indoor space for example increases with the size of the particles. The particles from incense will therefore settle faster and resuspend in the air more easily as shown in the study [10]. Thus, a particular attention should be paid to the time and frequency of ventilation if incense has been used in order to maintain good indoor air quality. These results are consistent with the study by Manoukian et al (2013) [11].

Figure 4 : Temporal profile of the particle size distribution of emitted particles by incense (left, EBENE-1) and candles (right, EBENE-1) [6]

Black carbon and NOx : Soot formation is by far the most varied emission within a single candle class according to [6]. This is mainly due to the quality of the wax for candles and the shape of the incense.

For candles, the quality of the wax seems to play an important role. A natural wax therefore emits less than a paraffin wax from petroleum products [12], [13] and [14]. The conclusion is the same for scented candles. They will start emitting pollutants even before they are lit [13]. Furthermore, once lit, scented candles will emit more soot and more NOx than unscented candles.

For incense, the stick shape seems to contain the soot better [6].

3       Conclusion 

As a conclusion, this blog post demonstrates through the various current studies that candles and incense contribute to degrading indoor air quality due to the incomplete combustion process. However, even if the combustion was complete, CO2 would be produced. CO2 is not an air pollutant, but it can cause headaches and above a certain concentration it can be fatal. It is therefore important to ensure that a building is well ventilated, so that the air is renewed.

Regarding the question “Are candles more or less harmful to health than incense”? It is difficult to answer. It would be necessary to see to what extent the pollutants emitted by one type of candle are more harmful to health than the other.

All in all, it is therefore important not to use too many candles at home and to ensure that the room is well ventilated during and after use.

4       Sources 

[1]  O. fédéral de l’environnement OFEV, ‘Valeurs limites pour la pollution de l’air’.–fachinformationen/luftqualitaet-in-der-schweiz/grenzwerte-fuer-die-luftbelastung.html (accessed Apr. 29, 2021).

[2]  N. E. Klepeis et al., ‘The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants’, J. Expo. Sci. Environ. Epidemiol., vol. 11, no. 3, Art. no. 3, Jul. 2001, doi: 10.1038/sj.jea.7500165.

[3]  ‘Combustion’, Wikipédia. Apr. 21, 2021, Accessed: Apr. 29, 2021. [Online]. Available:

[4]  ‘Combustions incomplètes’, MAXICOURS. (accessed Apr. 29, 2021).

[5]  ‘Bec Bunsen’, Wikipédia. Jan. 09, 2021, Accessed: Apr. 30, 2021. [Online]. Available:


[7]  W. Tirler and G. Settimo, ‘Incense, sparklers and cigarettes are significant contributors to indoor benzene and particle levels’, p. 6.

[8]  H. Majid and P. Madl, ‘Lung deposition predictions of airborne particles and the emergence of contemporary diseases Part-I’, theHealth, vol. 2, pp. 51–56, May 2011.

[9]  A. C. K. Lai and W. W. Nazaroff, ‘MODELING INDOOR PARTICLE DEPOSITION FROM TURBULENT FLOW ONTO SMOOTH SURFACES’, J. Aerosol Sci., vol. 31, no. 4, pp. 463–476, Apr. 2000, doi: 10.1016/S0021-8502(99)00536-4.

[10]     J. Qian and A. R. Ferro, ‘Resuspension of Dust Particles in a Chamber and Associated Environmental Factors’, Aerosol Sci. Technol., vol. 42, no. 7, pp. 566–578, May 2008, doi: 10.1080/02786820802220274.

[11]     A. Manoukian, E. Quivet, B. Temime-Roussel, M. Nicolas, F. Maupetit, and H. Wortham, ‘Emission characteristics of air pollutants from incense and candle burning in indoor atmospheres’, Environ. Sci. Pollut. Res., vol. 20, no. 7, pp. 4659–4670, Jul. 2013, doi: 10.1007/s11356-012-1394-y.

[12]     S. C. Lee and B. Wang, ‘Characteristics of emissions of air pollutants from mosquito coils and candles burning in a large environmental chamber’, Atmos. Environ., vol. 40, no. 12, pp. 2128–2138, Apr. 2006, doi: 10.1016/j.atmosenv.2005.11.047.

[13]     E. Uhde and N. Schulz, ‘Impact of room fragrance products on indoor air quality’, Atmos. Environ., vol. 106, pp. 492–502, Apr. 2015, doi: 10.1016/j.atmosenv.2014.11.020.

[14]     ‘L’impact environnemental de la bougie: plus important qu’imaginé!’, Fournisseur-Energie, Oct. 06, 2017. (accessed Apr. 29, 2021).

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