Examination of VOC Concentration of Aroma Essential Oils and Their Major VOCs Diffused in Room Air (2024)

2.1. Essential Oils and Major Components

Essential oils: Four types of essential oils (scientific name and vender), lavender (Lavandula angustifolia, Ryohin Keikaku Co., Ltd., Tokyo, Japan), tea tree (Melaleuca alternifolia, Ryohin Keikaku Co., Ltd., Tokyo, Japan), eucalyptus (Eucalyptus, Ryohin Keikaku Co., Ltd., Tokyo, Japan), and melissa (Melissa officinalis, Insent Co., Nagano, Japan), were used in this study. Major components in the essential oils are researched from a text of phytotherapy [11] and reference papers and internet websites [12,13,14,15,16,17,18,19], as listed in Table 1.

The components and their ratios searched by two different resources (excluding melissa, who has no component ratio in internet surveys) are roughly consistent, and the small deviations are inevitable because the essential oils are naturally derived and can fluctuate depending on the production area and production time. The ratio for melissa and eucalyptus was adopted by phytotherapy, and lavender and Tea-tree by internet survey.

Effective components: 14 kinds of components were selected from the major components of four essential oils as effective components in this study. They are selected as representative aromatic constituents of four essential oils by excluding some major constituents which is considered closer to solvent rather than aromatic component, such as terpinolene, geraniol. Twelve effective components except for geranial and neral were used, and citral was used instead of geranial and neral because citral is a mixture of geranial and neral. Figure 1 shows the structural formulae of the 14 kinds of effective components. As most of both essential oils and chemicals are supplied as mixtures of enantiomers [20], also in this study, enantiomeric classification is simplified and omitted. The components are indicated by the numbers of shoulder brackets in Table 1 and Figure 1.

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Figure 1

Structural formulae of 14 effective components in this study. (1) terpinene-4-ol, (2) α-terpinene, (3) γ-terpinene, (4) α-terpineol, (5) 1,8-cineole, (6) (+)-limonene, (7) α-pinene, (8) β-pinene, (9) p-cymene, (10) linalool, (11) linalyl acetate, (12) citronellal, (13) geranial, (14) neral.

2.2. Diffusion of Essential Oil and VOC into Room Air

In the office room, the essential oils mentioned above were diffused using a commercially available aroma diffuser (Ryohin Keikaku Co., Tokyo, Japan). This aroma diffuser is selected from the viewpoint that it is desirable to use a diffuser that is easily available and has low peculiarities. It has a tank capacity of 350 mL for distilled water and possesses diffusing water on around 100 mL in 60 min. Essential oils of 0.5 mL are dropped with a microsyringe onto distilled water at the beginning, as shown in Figure 2. The aroma diffuser uses ultrasonic waves to diffuse water and the essential oils in the tank to produce a fragrant mist.

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Figure 2

Preparation of diffusing and collecting test: Dropwise 0.5 mL of essential oils and effective component to 350 mL water in the aroma diffuser.

The room air samples were collected with thermal desorption (TD) tubes (GERSTEL, GERSTEL K.K. Japan, Tokyo, Japan) as shown in Figure 3a, from the distance of 500 mm from the aroma diffuser while diffusing in the office room. The volume of collected air was 1 L which was pumped with a rate of 200 mL/min for 5 min. The interval of the collection is as shown in Figure 4. The 5-min-collection was carried out immediately on the start of aroma diffuser diffusing, hereinafter referred to as 0 min. In the same manner, the 5 min collection was also carried out from 30 to 35 min and from 60 to 65 min, hereinafter referred to as 30 and 60 min, respectively.

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Figure 3

(a) Diffusion of essential oils in the office room by aroma diffuser and (b) diffusion of 14 effective components in the draft chamber. The air in the office room and the draft chamber is collected by thermal desorption (TD) tubes using aspirator.

See Also

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Figure 4

Schedule of collecting air, including diffused essential oils and effective components by TD tubes.

The effective components are diffused in the air on the same method as above. However, in the office room, diffusing chemicals should be avoided from a safety point of view. The diffusion in the air was carried out in a draft chamber of 650 × 1300 × 1250 mm³, as shown in Figure 3b. During the diffusion, the height of a shutter was controlled for controlling flow rate of ventilation at 0.5 m/s.

2.3. GC/MS Analysis

Analysis of diffused essential oils and effective components in TD tube is carried out by GC/MS (Agilent GC 6890 and MS5973, Agilent Technologies Japan Ltd., Tokyo, Japan) with autosampler for TD tubes (GERSTEL TDS A2, GERSTEL), as shown in Figure 5. Collecting volatile compounds using TD tube is a highly sensitive GC/MS-analyzing method because of large volume collection thus it is more efficient and sensitive than other extraction methods such as solid-phase microextraction (SPMI) for trace-level analysis. Thermal desorption is the process of collection and desorption of analytes from solid sorbents using heat and a flow of inert gas, and then the analytes are trapped in a cold trap prior to entering the analytical column, resulting in narrow and symmetric peaks. The condition of GC/MS is listed in Table 2. The column used in this study, DB-WAX, is a high polarity type made of Polyethylene glycol (PEG) and recommended type for food, fragrance, and flavor applications.

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Figure 5

GC/MS system, TD tube, and PVDF bag.

The calibration method of GC/MS peak intensity and component concentration is as below. One microliter of essential oil was injected into 10 L of pure air filled in PDVF (polyvinyridene fluoride) bag, and the bag was kept overnight. The concentration of each component, linalool γ-terpinene, 1,8-cineole, and caryophyllene, for lavender, tea tree, eucalyptus, and melissa, respectively, in the bag can be calculated from the component ratio contained in the essential oil. Connecting the bags with TD tubes and collecting 100 mL, then the TD tubes were analyzed by the GC/MS system, and obtaining the peak area of each component was used as the reference. The relationship between concentration and peak area of each component is calculated.

3.1. GC/MS Results of Essential Oils Diffused into Office Room

Figure 6 shows the GC/MS chromatogram of lavender oil collected in the office room as an example. Descriptions of the peaks were from the analysis results using the MS database. The chromatogram of room air without any essential oils and effective components is the chromatogram [bg] in Figure 6, indicating that the main components of lavender oil were not included in the ingredients that were always present in the room. As for the chromatogram of the room air samples of the lapsed time of 0, 30, and 60 min from the oil diffusion test, [0 min] [30 min] [60 min], these peaks were compared to that of the sample gas in the PVDF bag as a reference, [bag], for obtaining the concentration of each component.

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Figure 6

GC/MS chromatogram of lavender. Descriptions of the peaks were from the analysis results using the MS database, and descriptions with structural formulae mean major components of lavender, as listed in Table 1. Samples of [bg] and [bag] chromatogram were the room air before lavender-diffusing as a background measurement and gas from PVDF bag as a reference, respectively.

The concentration of each component was calculated and listed in Table 3, with the molecular formula, molecular weight, and retention time. For the components which were unknown or chemicals that originated from the extraction process, such as ethanol and benzene, these concentrations were calibrated with the calculation based on the peak area of linalool.

For other essential oils, tea tree, eucalyptus, and melissa were also carried out on the same analysis and listed in Table 4, Table 5 and Table 6, respectively. GC/MS chromatogram of three essential oils are found in Supplementary Material, Figures S1–S3.

The concentration level of each main component of essential oils was very low and several to several tens of ppt (parts per trillion). The GC/MS peaks were calibrated with the calculation based on the peak area of linalool. For tea tree oil, Eucalyptus oil, and melissa oil, the peak area of γ-terpinene 1,8-cineole, and caryophyllene, was used for the calibration, respectively.

For lavender oil, see Figure 7, the concentration level of each main component, listed in Table 1, was several to several tens of ppt, and the total concentration of main components was up to 75 ppt. β-Phellandrene, α-terpineol, camphor, and lavendulol were not detected in this study. An interesting feature found in this study was that the concentration of each component varied with the elapsed diffusion time. That is, the ratio of components changed over diffusion time. However, the total concentration decreased slowly. This can be related to the molecular weight or retention time of GC/MS system. Four components of 1,8-cineole, trans-beta-ocimene, cis-beta-ocimene, and 3-octanone, whose GC/MS peaks appeared before the retention time of 14.2 min, show high concentration at 0 min then decreased fast by the elapsed time of 30 min. Other components, linalool, linalyl acetate, terpine-4-ol, and lavandulol acetate, whose GC/MS peaks appeared after the retention time of 18.5 min, showed high concentration at an elapsed time of 30 min.

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Figure 7

Temporal change of VOCs in the room air diffused by lavender oil.

For tea tree oil, see Figure 8, the concentration level of each component was several to several tens of ppt, similar to the case of lavender. However, the total concentration at elapsed time 0 was as high as 272 ppt and decreased to 30 ppt by 30 min. α-Terpinenol was detected from PVDF bag as a reference, however, not detected from room air. A total of 9 components with a retention time of up to 14.70 min showed high concentration at the beginning. Especially the concentration of γ-terpinene was high as 104 ppt at 0 min. After the retention time of 19.58 min, terpinen-4-ol delayed and showed peak concentration at 30 min.

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Figure 8

Temporal change of VOCs in the room air diffused by Tea tree oil.

For Eucalyptus oil, see Figure 9, the concentration level of each main component, listed in Table 1, was higher than that of other essential oils. Specifically, the concentration of 1,8-cineolewere 375 ppt at 0 min. (E)-pinocarveol and globulol were not detected in this study. Other ingredients started at tens of ppt. The total concentration at 555 ppt at the start at 1/6 in 30 min and 1/20 in 60 min. The concentration of any component decreases over time. All four components of the detection were detected before the retention time of 14.70 min, which was the boundary line where the peak of lavender and tea tree concentrations was either [0 min] or [30 min].

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Figure 9

Temporal change of VOCs in the room air diffused by Eucalyptus oil.

For Melissa oil, see Figure 10, the concentration of each component was several to tens of ppt, and the total concentration was about 17 to 55 ppt. Germacrene D, caryophyllene oxide, and geraniol were not detected in this study. In any component, the peak concentration appeared late [30 min]. All four components of detection were detected after the retention time 14.70 min, which is the boundary line where the peak of lavender and tea tree concentrations was either [0 min] or [30 min].

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Figure 10

Temporal change of VOCs in the room air diffused by Melissa oil.

3.2. GC/MS Results of 14 Effective Components Diffused into Draft Chamber

Effective components, which are major VOCs in the four essential oils, are diffused into the draft chamber, and the temporal changes of their concentration are measured by GC-MS. For safety reasons, the test of VOC diffusion was carried out in the draft chamber, but other experimental conditions were the same as that of essential oils. The details of GC/MS chromatogram of these components are found in Supplementary Material, in Figure S4.

Figure 11 shows the results of the concentration of the components in 0, 30, and 60 min. The component whose concentration decreased with the elapsed time shows a concentration of several parts per billion (ppb) immediately after the start of spraying but decreased to 0.2 ppb or less after 30 min. The tendency confirmed in this study that the higher the concentration at 0 min, the lower the concentration tended to be at 30 min was shown again clearly, and especially for α-terpinene, its GC/MS peak was not detected at 30 min. The components of the delayed or high molecular components, their concentration level was less than 1 ppb at 0 min, but the concentration was not so much changed. A specific change is shown in the cases of linalyl acetate and terpinen-4-ol. Their concentration in the room air gradually increased. The concentrations of linalool and α-terpineol reached their maximum at 30 min.

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Figure 11

Summary of the temporal changes of 14 effective components diffused in the room air. (a) before, (b) after the retention time around 16 min. * Calculated by assuming the molar ratio of neral and geranial contained in citral is 1:1.

Before and after the retention time band from 14.6 to 17.7 min of the GC/MS, the components can be divided into two groups; with one group decreasing its concentration with time, including α-pinene, β-pinene, α-terpinene, (+)-limonene, 1,8-cineole, γ-terpinene, and p-cymene; and the other group of a delayed peak of concentration with the time, including citronellal, linalool, linalyl acetate, terpinen-4-ol, neral, α-terpineol, and geranial. This tendency was similar to the behavior of the concentration change of each component in the test in which the essential oil was diffused.

4.1. Temporal Change of the Concentration of VOCs in Essential Oils

The above results are summarized in Table 7. The component ratio fluctuates with elapsed time. For the reason why the scent changes over time, the peak concentration differs depending on the component. That is, eucalyptus and tea tree showed a dramatic reduction by elapsed time, but melissa and lavender lasted for an hour.

The aroma of fragrances is also classified into three basic categories [21], so-called notes, are similar to how musical notes make up a song, top notes, heart notes, and base notes. Top notes have higher volatility, and they appear faster within 10 or 15 min, while base notes are longer-lasting and appear for hours. This categorization or note is strongly related to how volatile the note is. Among four oils, lavender is only one top-note aroma, but the concentration was constantly lasted.

An almost linear relationship between the molecular weight and the retention time is observed as listed in Table 3, and several order mismatches are due to the polarity of the component, as the column material used in this study, DB-WAX, is the high polarity of PEG. Since components with a long retention time tend to have a large molecular weight, those with low volatility are expected to be applicable.

An interesting feature of the case of tea tree oil, the most lightweight component showed an explicate decrease with time, but terpinen-4-ol, which is considered the main component of the oil, kept a relatively high concentration for a long time. The content of is known as 38%, see Table 1, but was found to be small, around 5% in this study, and the main component was γ-terpinene, whose concentration decreased fast with diffusion time. In contrast, 1,8-cineole, which is the main component 80% in eucalyptus, showed high concentration and decreased gently to be 30 ppt after 60 min diffusion.

These reasons are that components having polar functional groups, such as hydroxyl groups and aldehyde groups, are less volatile and more soluble in water than components without these functional groups. In this study, 350 mL of water with 0.5 mL of essential oils was diffused by ultrasonication. Insoluble components in water are spread on the surface of the water and should be easily diffused immediately, whereas soluble components are dissolved in water, and the concentration is low, but long-term diffusion can be expected. That is, α-pinene, β-pinene, α-terpinene, (+)-limonene, 1,8-cineole, γ-terpinene, and p-cymene included in the one group of decreasing its concentration with the time in Figure 11 do not have the polar functional groups (excluding ether group), whereas citronellal, linalool, linalyl acetate, terpinen-4-ol, neral, α-terpineol, and geranial included in the other group of a delayed peak of concentration with the time have polar functional groups. The same tendency can be seen as the diffusion of 14 effective components in Figure 11 with respect to the analysis results of the essential oils in Figure 7, Figure 8, Figure 9 and Figure 10.

4.2. Safety Issue of VOCs

One important purpose of this study is to measure the concentration level of VOCs or the component of essential oils diffused into the room air by the common use of diffusion. When the aroma is used, the safety issue is a critical consideration, for example, to understand the concept of the Threshold of Toxicological Concern [22], and a question is whether the exposure of VOC is below the critical threshold for health or not. For the concentration of VOC is defined by ISO 16000-X series, and WHO and or Japanese government [23] declared the regulation on the maximum level of VOCs in the room air, for example, 260 μg/m³ or 6.9 ppb for toluene, 100 μg/m³ or 8.15 ppb for formaldehyde. There are several Japanese codes related to the use of fragrance or aroma, such as Food Sanitation Law, Chemical Substances Control Law, Pharmaceutical Affairs Law, but no clear threshold for aromatic VOC can be found. For fragrances, International Fragrance Association (IFRA) regulation or standard is only one reference regarding the acquisition, development, manufacture, and sale of raw materials for fragrances, which is checked by the expert agency of Research Institute for Fragrance Materials, Inc. (RIFM) for safety and health [24].

An important fact we have confirmed for the first time is that the total concentration of VOCs in the room air diffused by the ultrasonic diffuser is as low as 0.6 ppb and decreased soon below 0.1 ppb, as shown in Figure 7, Figure 8, Figure 9 and Figure 10. Considering the threshold level and safety guidance discussed above, the concentration of VOCs in room air diffused by a common ultrasonic diffuser analyzed in this study is safe even for repeated use. For the VOC exposure, a more detailed study on inhalation is required to conduct a safety assessment. A 2-box air dispersion model [25] is a good example for indoor environmental air, which determines air exposure concentrations in two connected, enclosed zones. This information can be combined with our results in this study and be meaningful to design a clinical study with aroma compounds of four essential oil.

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1. Sasannejad P., Saeedi M., Shoeibi A., Gorji A., Abbasi M., Foroughipour M. Lavender essential oil in the treatment of migraine headache: A placebo-controlled clinical trial. Eur. Neurol. 2012;67:288–291. doi:10.1159/000335249. [PubMed] [CrossRef] [Google Scholar]

2. Yogi W., Tsukada M., Sato Y., Izuno T., Inoue T., Tsunokawa Y., Okumo T., Hisamitsu T., Sunagawa M. Influences of Lavender Essential Oil Inhalation on Stress Responses during Short-Duration Sleep Cycles: A Pilot Study. Healthcare. 2021;9:909. doi:10.3390/healthcare9070909. [PMC free article] [PubMed] [CrossRef] [Google Scholar]

3. Hart P.H., Brand C., Carson F., Riley T.V., Prager R.H., Finlay-Jones J.J. Terpinen-4-ol, the main component of the essential oil of Melaleuca alternifolia (tea tree oil), suppresses inflammatory mediator production by activated human monocytes. Inflamm. Res. 2000;49:619–626. doi:10.1007/s000110050639. [PubMed] [CrossRef] [Google Scholar]

4. Juergens U.R., Dethlefsen U., Steinkamp G., Gillissen A., Repges R., Vetter H. Anti-inflammatory activity of 1.8-cineol (eucalyptol) in bronchial asthma: A double-blind placebo-controlled trial. Respir. Med. 2003;97:250–256. doi:10.1053/rmed.2003.1432. [PubMed] [CrossRef] [Google Scholar]

5. Ballard C.G., O’Brien J.T., Reichelt K., Perry E.K. Aromatherapy as a safe and effective treatment for the management of agitation in severe dementia: The results of a double-blind, placebo-controlled trial with Melissa. J. Clin. Psychiatry. 2002;63:553–558. doi:10.4088/JCP.v63n0703. [PubMed] [CrossRef] [Google Scholar]

7. Peterfalvi A., Miko E., Nagy T., Reger B., Simon D., Miseta A., Czéh B., Szereday L. Much More Than a Pleasant Scent: A Review on Essential Oils Supporting the Immune System. Molecules. 2019;24:4530. doi:10.3390/molecules24244530. [PMC free article] [PubMed] [CrossRef] [Google Scholar]

8. Şentürk A., Tekinsoy Kartın P. The Effect of Lavender Oil Application via Inhalation Pathway on Hemodialysis Patients’ Anxiety Level and Sleep Quality. Holist. Nurs. Pract. 2018;32:324–335. doi:10.1097/HNP.0000000000000292. [PubMed] [CrossRef] [Google Scholar]

9. Sandner G., Heckmann M., Weghuber J. Immunomodulatory Activities of Selected Essential Oils. Biomolecules. 2020;10:1139. doi:10.3390/biom10081139. [PMC free article] [PubMed] [CrossRef] [Google Scholar]

10. Api A., Belsito D., Bruze M., Cadby P., Calow P., Dagli M., Dekant W., Ellis G., Fryer A., f*ckayama M., et al. Criteria for the Research Institute for Fragrance Materials, Inc. (RIFM) safety evaluation process for fragrance ingredients. Food Chem. Toxicol. 2015;82:S1–S19. doi:10.1016/j.fct.2014.11.014. [PubMed] [CrossRef] [Google Scholar]

11. Koike K. Phytotherapy. Kyoto Hirokawa, Co.; Kyoto, Japan: 2016. (In Japanese) [Google Scholar]

13. Nurzyńska-Wierdak R., Bogucka-Kocka A., Szymczak G. Volatile constituents of Melissa officinalis leaves determined by plant age. Nat. Prod. Commun. 2014;9:703–706. doi:10.1177/1934578X1400900531. [PubMed] [CrossRef] [Google Scholar]

15. Pokajewicz K., Biało’n M., Svydenko L., Fedin R., Hudz N. Chemical Composition of the Essential Oil of the New Cultivars of Lavandula angustifolia Mill. Bred in Ukraine. Molecules. 2021;26:5681. doi:10.3390/molecules26185681. [PMC free article] [PubMed] [CrossRef] [Google Scholar]

17. Sebei K., Sakouhi F., Herchi W., Khouja M.L., Boukhchina S. Chemical composition and antibacterial activities of seven Eucalyptus species essential oils leaves. Biol. Res. 2015;48:7. doi:10.1186/0717-6287-48-7. [PMC free article] [PubMed] [CrossRef] [Google Scholar]

19. Carson C.F., Hammer K.A., Riley T.V. Melaleuca alternifolia (Tea Tree) oil: A review of antimicrobial and other medicinal properties. Clin. Microbiol. Rev. 2006;19:50–62. doi:10.1128/CMR.19.1.50-62.2006. [PMC free article] [PubMed] [CrossRef] [Google Scholar]

20. Davies N.W., Larkman T., Marriott P.J., Khan A.I. Determination of Enantiomeric Distribution of Terpenes for Quality Assessment of Australian Tea Tree Oil. J. Agric. Food Chem. 2016;64:4817–4819. doi:10.1021/acs.jafc.6b01803. [PubMed] [CrossRef] [Google Scholar]

21. Armanino N., Charpentier J., Flachsmann F., Goeke A., Liniger M., Kraft P. What’s Hot, What’s Not: The Trends of the Past 20 Years in the Chemistry of Odourants. Angew. Chem. Int. Ed. 2020;59:16310. doi:10.1002/anie.202005719. [PubMed] [CrossRef] [Google Scholar]

22. Kroes R., Renwick A.G., Feron V., Galli C.L., Gibney M., Greim H., Guy R.H., Lhuguenot J.C., van de Sandt J.J.M. Application of the threshold of toxicological concern (TTC) to the safety evaluation of cosmetic ingredients. Food Chem. Toxicol. 2007;45:2533–2562. doi:10.1016/j.fct.2007.06.021. [PubMed] [CrossRef] [Google Scholar]

24. Silva G.V., Martins A.O., Martins S.D.S. Indoor Air Quality: Assessment of Dangerous Substances in Incense Products. Int. J. Environ. Res. Public Health. 2021;18:8086. doi:10.3390/ijerph18158086. [PMC free article] [PubMed] [CrossRef] [Google Scholar]

25. Petry T., Vitale D., Joachim F.J., Smith B., Cruse L., Mascarenhas R., Schneider S., Singal M. Human health risk evaluation of selected VOC, SVOC and particulate emissions from scented candles. Regul. Toxicol. Pharmacol. 2014;69:55–70. doi:10.1016/j.yrtph.2014.02.010. [PubMed] [CrossRef] [Google Scholar]