More information is known about pollutants in outdoor air than in indoor air, although we spend about 90% of our time in public or private indoor environments. There are various sources of indoor air pollution, to name a few, such as emissions from building materials and furniture, electric equipment, heating or cooling systems, combustion sources or from cleaning, cooking and other activities of occupants . This blog post focuses on candles as a source of indoor air pollutants by combustion. In the current studies, different candles have been compared with each other, based on different emission factors of pollutants.
1. FRAMEWORK OF THE STUDIES
1.1 Distinguishing factors of the examined candles
There are various candles available on the market. Candles distinguished by current studies have different colours, shapes, can have a container mainly out of glass, an anti-mosquito activity or the most popular type are the scented candles  & . The raw material is mostly out of paraffin wax and can have a different composition with different melting points or oil contents and different wicks can be used ,  & .
Another important factor is the burning condition. There are three modes of burning, the steady state condition with no visible smoke, the shooting burning with visible smoke and the smouldering mode with white smoke  & . In the following part only the steady state burning condition is considered.
1.2 Emitted Pollutants
Since investigations on candles that emit pollutants have been carried out, the first pollutants to be determined were lead, zinc and soot .
Other concerned emissions produced from burning candles are  & :
- Polycyclic aromatic hydrocarbons (PAHs)
- Volatile organic compounds (VOC’s)
- Aromatic hydrocarbons species (BTEX)
- Particulate matter (PMx)
PAHs result from the incomplete combustion of organic material and are known to be highly carcinogenic . The term volatile organic compounds (VOCs) covers a number of substances containing the element carbon C. VOCs can cause irritation to the eyes, nose and throat or headaches. In a higher concentration VOCs can cause irritation of the lungs, damage to the kidney, liver or central nervous system. Some VOCs can even cause cancer . BTEX are volatile organic compounds and the term stands for benzene, toluene, ethylbenzene and xylenes. They are listed separately because there is sufficient evidence that these compounds are carcinogenic .
Information on the health effects of inhalation of aldehydes is limited. However, it is known that inhalation of formaldehyde in high concentrations has caused nasal tumors in laboratory rats and in lower concentrations has caused irritation of the eyes and respiratory tract in humans .
In the case of PMx, size is the determining factor for health effects. Particles with a diameter of 10 μm are the most dangerous as they can enter the lungs or bloodstream. Health effects include premature death for people with heart or lung disease, worsened asthma and reduced lung function .
1.3 Test chamber
A test chamber should provide representative burning conditions for the candles and should not require much space. There are several test chambers, but one seemed to be particularly popular, see figure 1 .
The chamber consists of 3 parts. The space in which the candles are placed has a cylindrical shape with a diameter of 0.6m and a height of 0.4m. Then follows the conical shape with a height of 0.6m, in which the exhaust gases of the candle are mixed with the incoming air, thus achieving a uniform concentration. At the top is the stack with a height of 1.5m, from which the air samples are taken to determine the pollutants .
For each test 4 candles were burned in the test chamber to reduce heterogeneity between the candles of the same type. There is a cycle of 4h burning, which is repeated after 1h break. The emission factors are determined after 15 minutes of burning, because then the main combustion products and the oxygen reach a steady state. The emission factors are converted to 1g burned candle to compare the emission rates of pollutants of the different candles .
2 RESULTS OF THE CURRENT STUDIES
2.1 Candle Wick
Van Alphen (1999) had examined candles with a wick made from a lead core and found that these candles presented an unacceptable risk to human health . ONriagu & Kim (2000) considered lead and zinc emissions from candles with a metal-core wick. The release of lead from the metal-core wick ranges between 0.55 – 66 μg/h and of zinc 1.2-124 μg/h. No correlation between the two pollutants was found and the variations in emission rates indicate the different properties of the metal core element. The zinc in these quantities in indoor air is not considered to be a health risk. ONriagu & Kim (2000) calculated the lead level in a bedroom of 50 m3 in which they burned a candle for two hours. After the two hours 5 out of 14 different candles have exceeded the acceptable limit of 1.5 μg/m3 from the US Environmental Protection Agency guideline for ambient air. The conclusion is that lead-wick candles should be prohibited in every country .
2.2 Candle Composition
Derudi et al. (2013) examined 3 different container candles with different oil content to analyse the influence of the candle composition on the emission factors. The candle A consisted of fully refined paraffin wax, the other two candles B and C were slack waxes. To compare the emissions, they were calculated with an emission factor of 1 g of burnt candle.
A correlation between BTEX, PM, PAHs and CO emissions and wax composition was found, but not with wax additives. But there is no clear relationship between the concentration of PAHs in the wax and in the emitted gas. One reason for this could be that the PAHs in the wax are burned by the flame. it should be noted that BTEX does not occur in the composition of the wax, but is caused by the incomplete combustion of the wax. It was also found that the oil content in the raw material influences the released pollutants. A higher oil content leads to a worsening of the combustion process and thus to a higher content of BTEX and PAHs. These results are underlined by the values in tables 1 and 2 .
Table 2 shows that the highest emissions of BTEX and PAHs are from candle B, but the highest value of PMx is emitted by candle C. It should be noted that the oil content not only has an influence on the content of the emitted particles, but also on the particle size. Candle A has less than 50% of the particles that are smaller than 25 μm and of the other two candles the part of the particles that are smaller than 25 μm is larger than 90%, see figure 3 .
Only some exemplary pollutants were compared with the limit values and the concentration of sulphur and PM2.5 in the air exceeds these limit values. PAH and BTEX substances seem to have a negligible impact .
2.3 Scented Candles
Derudi et al. (2012) investigated scented candles and pure paraffin candles. They observed that aldehyde emissions are attributed to the properties of fragrance, see figure 4. This also resulted from the investigations of Gelosa et al. (2007) and Derudi et al. (2013).
Emissions of BTEX and PAHs were on average higher for pure paraffin waxes than for scented candles. This indicates that the emissions depend on the wax composition and not on the additives. From the scented candle the highest emission rates of a pollutant of the respective group of PAHs, BTEX or aldehydes were compared with its limit values .
Table 3 shows that the emissions are well below the limit value. However, Benzo(a)pyrene is about 40% of the limit value and the pure paraffin candle has a higher value. A more detailed study is needed to classify the toxicity for these candles. Orecchio (2011) has tested a total of 12 candles for PAH emissions. She differentiated the candles according to fragrance, colour, shape and container. The lowest emissions were found for candles with a cylindrical shape, which leads to the assumption that they provide a more complete combustion. The highest PAH emissions were from scented candles with anti-mosquito effects. Due to the different composition of the raw materials in the wax, large differences in emissions of PAHs were found. When calculating the concentration of the average emission of PAHS, the concentration is below the limits. However, it is important to note that if the interior is not well ventilated, a high concentration of PAHs can accumulate, which can be harmful to our health .
The emitted pollutants vary greatly depending on the candle. The studies mentioned above showed the dependence of the emitted pollutants on the raw material of the wax. A consistent result of all the studies is that aldehydes are produced by the scent properties of the candles, but these concentrations are well below the limit value. However, it was not possible to make a general statement about which type of candle is the least harmful to our health, as the emission concentrations of BTEX and PAH can be different for similar candles. This indicates the need for a simple and cheap method of measuring the emissions of pollutants from commercial candles to ensure that the limit values are not exceeded and that there is no health risk  & .
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