Alexis St-Gelais, M. Sc., chimiste
A while ago, I mentionned in an article about Boswellia serrata essential oil that it almost always contained toluene. This compound tends to trigger unrest for some essential oils reseller and consumers, as toluene is indeed a relatively common petroleum-derived industrial solvent. The assumption thus becomes that the oil has been contaminated in some way.
Let me sell the punch of this blogpost altogether: toluene is naturally found in some plants, and also apparently widespread (and thus expected) in many, many essential oils. Blaming producers on the basis that toluene has been reported in their oil is unfair, given the apparently very frequent occurence of toluene in essential oils that we are starting to unveil. The real question to ask should be if the level of toluene found is dangerous, which is not the case.
Let us have a closer look at this case. In a nutshell:
Why was toluene was called this way? A more formal name would be methylbenzene (which is by the way also accepted). As with most common names for volatile compounds, the most commonly used word “toluene” derives from the plant within which this molecule was first isolated.
You read right, toluene was discovered first in plants-derived material. But the distillations mentionned refer to dry distillations, or pyrolysis. This means that the plant material (often resin or wood) is heated enough to induce decomposition and rearrangement of the original molecules. So, toluene isolated from pine resins, Tolu balsam (Myroxylon balsamum) or wood distillates could arise from thermal cracking of other substances, and not be present “originally” in the fresh plant material. Still, toluene comes from “Tolu balsam”.
Actual occurence in plants and oils
Both spruce and pine needles litter emitted toluene upon standing . It was also shown that living sunflowers and pines spontaneously emit toluene as a reaction to stress from their environment .
The essential oil of Ferulago angulata was reported to contain 0.1% of toluene . Traces (0.01 ppm) of the compound were also reported in fruits of Genipa americana . As much as 0.7% of toluene is mention in Cynanchum paniculatum root oil although the fact that the authors also mention xylene may suggest external contamination no due to the plant . A credible study on mustard volatiles (Sinapis arvensis) also reports 0.1% of toluene . Several publications in Chinese journals also appear to mention toluene in essential oils, but we were not able to readily get access to those.
Recently, preliminary work on a more sensitive system lead us to detect toluene at levels between 0.01 and 0.11% in at least one oil from the following essential oils: frankincense serrata, frankincense carterii, frankincense frereana, lime, larch, Labrador tea, carrot seed, wormwood, agarwood, cedarwood, oregano, elemi, roman chamomile, lavender, cumin, basil (ct. methylchavicol), and amyris. Older analyses done in routine lead us to find toluene in e. g. pines (various species), most incenses, evergreen cypress, cistus/labdanum, hemlock, angelica root, curcuma, white cedar, ginger, juniper berry, firs, spruces, and cinnamons. These oils came from dozens of producers and suppliers, some of which we know, even a few cases in which we were the distillers. The list will likely keep expanding as we progressively become aware of the extent of this phenomenon.
When something is found in some many places at once, the most likely conclusion is that it is because it is to expected, rather than the result of isolated accidents.
We intend to keep systematically documenting the presence of toluene in commercial essential oils using our more sensitive system starting in a few months from now.
Why has this not really been reported before
There are overall relatively few publications that mention toluene in essential oils. There could be two main explanations to this:
A) Many laboratories dilute essential oils in a solvent prior to GC analysis. Usually, they then “skip” the first minutes of the chromatogram to omit the solvents that they have added. Toluene will be eluted in that portion of the chromatogram, and go unnoticed (or its presence could be attributed to an impurity from the main solvent, and thus the peak omitted).
B) Several laboratories do not integrate/consider peaks representing less than 0.05% of the total signal. Since toluene is, from our current experience, often found at concentrations that can be below this treshold, the peak might be simply ignored by analysts.
Toluene is emitted from both natural and anthropologic sources. It is found in fuels and many manufactured goods (such as, for example, nail polishes, glues, and paints), and is present in the air especially inside buildings. On the other hand, as we saw earlier, some plants were shown to spontaneously emit toluene, and toluene is emitted from plant litter in forests. As such, toluene in the environment has multiple origins.
At this point, it is very hard to tell how toluene ends up in plants. In some instances, it can passively be absorbed from the air into waxy or fat tissues of plants, such as citruses peels . Again, sunflowers were found to emit toluene when they are alive, and as a reaction to their environment. Toluene could be produced by decomposition of other molecules, either through the normal metabolism of plants, or after death during digestion by fungi and bacteria.
One could imagine that toluene would come from the emissions of fuel-ran engines on distillation facilities. Toluene is indeed found in exhaust from diesel- or gasoline-ran engines. Toluene could then be gradually absorbed by the essential oils upon standing nearby. However, this hypothesis is the least likely because emissions from fuel combustion also contain xylenes and, especially, benzene . So, if toluene arose from a contamination from fuel exhaust, other substances would accompany it: this is not in line with our observations.
Thus, at this point, little can be concluded as to toluene’s actual origin, but it most likely does not come from contamination at the production site. This should incline us to give the benefit of doubt to producers, as long as the concentration of toluene remains constitent between oil batches (since a given lot could still deviate strongly from the usual “background” level due to a specific contamination).
Putting numbers in perspective: how much toluene would you absorb from your oils?
So, is that toluene in essential oils a problem? Again, let us start with the punchline: even considering worse-case scenarios, natural levels of toluene in essential oils are not hazardous.
The FDA issues recommendations (which are nonbinding) for maximum residual solvents in pharmaceutical products. In the case of toluene, the limit is either 8.9 mg/day for the average person, or 890 ppm within a finished pharmaceutical product .
Usually, within essential oils, the peak for toluene represents less than 0.15% of total signal by the internal normalization method. Although this cannot be formally converted to a concentration in ppm without proper calibration, we can approximate that this would be equivalent to a maximum level between 100-300 ppm.
Essential oils should typically not be taken in massive doses. We will first consider the most direct intake method: oral consumption of an oil. For calculations below, 30 drops will be considered as equivalent to 1 mL. An oil containing 300 ppm of toluene taken at a rate of 30 drops a day (which is a quite important dose, several oils are dangerous regardless of toluene at much lower intakes) will be equivalent to an uptake 0.30 mg of toluene. This is 30 times below the recommended maximum from the FDA.
A more regular scenario would be consumption of 5 drops a day of an oil containing rather 150 ppm of toluene, bringing us at 360 times below the FDA’s maximum.
Toluene is absorbed through the skin, but less efficiently so than by oral intake. A study from the 60s estimated the absorption of toluene from water containing small concentrations of toluene. For each ppm of toluene, the rate of absorption was roughly of 1 µg per square centimeter of exposed skin every hour . Tisserand & Young  recommend that oils should be diluted in a carrier oil at a concentration of at most 5% for dermal application. For a full-body massage, this would be equivalent to the application of 1.5 mL total essential oil.
A rough estimation with the parameters above shows that skin absorption within the first hour is potentially faster than the total dose of toluene applied, meaning that all of the toluene would be absorbed, 0.45 mg in total (thus, as with oral consumption, being below the FDA’s guidelines – 20 times less). One should keep in mind that essential oils and their carriers may facilitate skin penetration compared to water. On the other hand, a fair share of toluene could simply dry out at the contact of warm skin, before being absorbed. It is likely that in reality, the toluene actually absorbed through dermal application of diluted essential oils would be smaller than 100% of the essential oils’ content, further lowering concerns for health.
In Quebec, toluene is considered safe at an average concentration of 50 ppm in the air in a workplace that you would go to 8 h a day, 40 h a week , which is in line with the regulations of several other countries including the USA . Irritation of the eyes and nose, as well as dizzyness, kick in at around 100 ppm during 6 h .
Let us hypothesize that one would vaporize 15 drops of our theoretical essential oil in a quite small (and a tad claustrophobic!) room of 13.8 m3 (which is 8′ x 8′ x 8′, or 2.4 m accross), with no ventilation at all. Our 0.15 mg of toluene distributed into the air of the room give a concentration 10.9 µg/m3, or 0.003 ppm (see  for conversion factor). This is not a concern at all.
There sure are a number of assumptions in the abovementionned scenarios, so they are by no mean precise. But they give an idea of the magnitude of the problem – or, more formally speaking, shows that there is not much of a problem, because even if the actual numbers were a bit higher, the level of hazard is negligible at most.
Note that in any scenario, you would be able to smell toluene before it would reach a dangerous level. As the odor of this compound is quite pungent and unpleasant, chances are you would be able to take proper action before the levels would become problematic.
Toluene in your everyday life
A Korean study found in 2002 median concentrations of 164 µg/m3 (range of 32 to 836) of toluene inside cars of suburban commuters, and 114 µg/m3 in public buses . This is due to exposure to exhaust from fuel engines. In comparison to our diffusion hypothesis above, these commuters are exposed to about 15 times more toluene in concentration.
A comparison of the indoor air quality in an urbanized environment in Hanover, versus presumably cleaner rural environment from Wedemark, was conducted in Germany in the early 2000s. Toluene was found in both cases at concentrations of around 15 µg/m3 in the summer. In the winter, the contents rose to 20-25 µg/m3, a bit higher in the city . Similarly, in La Plata in Argentina, concentrations of toluene were found to be roughly always in the 15-17 µg/m3 range indoors in urban, semi-rural and residential aeras, rising however to 36 µg/m3 in industrial zones . This is not very far from the theoretical level from our diffusion hypothesis, meaning that potentially the baseline level of toluene in your house is similar to what would be diffused.
Application of nail polish lead to exposure to 4000-14 000 µg/m3 of toluene in the ensuing 10-20 minutes, in a domestic environment with limited venting . This has been considered as a reasonably safe level in the EU, known to be relatively strict on exposure levels.
And as a reminder, the most commonly applied maximum tolerated exposure at a workplace for 40 h a week is 50 ppm, or 187 970 µg/m3 [12, 13]. This level is voluntarily set below the threshold at which health problems start to arise.
This is very far from being an exhaustive literature survey, but it shows that the levels of toluene that could arise from the diffusion of an essential oil are negligible, being very close to the expected baseline in the common household.
Bottom line: toluene is not a threat
Penalizing producers and distributors of essential oils on the basis of unreasonable absence criterion for toluene may lead to reduction of the supply and increase in cost, which may then lead to other problems such as adulteration. Breaking the equilibrium with overcaution is not desirable. We can thus only recommend that consumers take into account that toluene’s presence in essential oils below 300 ppm is presumably harmless, provided a safe usage as is recommended regardless of toluene.
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