**5.2 Skin sensitization**

In contrast to irritation, skin sensitization is a response of the adaptive immune system to certain chemical substances called sensitizers or haptens, which can modify skin proteins and induce a delayed T-cell-mediated allergic response [23]. Some of the ingredients which may trigger allergic reactions are listed in the seventh amendment of directive 76/768 CEE (directive 2003/15/CE) and include benzyl alcohol, cinnamyl alcohol, citral, eugenol, hydroxycitronellal, isoeugenol, benzyl salicylate, cinnamaldehyde, coumarin, geraniol, anisyl alcohol, benzyl cinnamate, farnesol, linalool, benzyl benzoate, citronellol, or limonene [24, 25]. Skin sensitization occurs on first exposure to a substance, with only a slight (or absent) effect on the skin. Subsequent exposure to the same compound/compounds will produce a severe inflammatory reaction caused by T-lymphocytes.

**Figure 2.** *Chemical structures of the main skin irritant compounds.*


**Table 2.**

*Essential oils potentially irritant to the skin (in alphabetical order of the botanical family) [22].*

Symptoms include a bright red rash, which may be painful to some individuals and sometimes a pigmentation of the skin, more frequently in Asians [21, 24]. In order to prevent sensitization, it is recommended to avoid known dermal sensitizers and avoid application of the same essential oil every day for a long period of time.

**Table 3** lists some of the oils considered to be dermal sensitizers, but the sensitization process can occur for any essential oil [22, 25].

Essential oils obtained from different species of *Pinus* and *Abies* should only be used when the level of peroxides is kept to the lowest practical level, preferably by adding anti-oxidants at the time of production [19].

Skin sensitization reactions are idiosyncratic, identification of the causative allergen(s) and their subsequent withdrawal generally leading to a resolution of the problem. Some standard mixtures (fragrance mixture) can be used in a patch test to screen for allergic reactions in susceptible individuals, but not all the allergies can be predicted by this method [26, 27].

#### **5.3 Photosensitization**

Photosensitization is a reaction between a phototoxin from an essential oil that is applied to the skin in the presence of sunlight or ultraviolet A (UVA) light. The interaction with the light may be either phototoxic or photoallergic.

Photoallergy is an immune-mediated skin reaction, while phototoxicity may lead to photocarcinogenesis. Furanocoumarins (psoralens) appear to be primarily responsible for phytophototoxic reactions in humans. Reactions can vary from pigmentation, blistering, to severe full-thickness burns. Furanocoumarins occur mainly in expressed citrus peel oils (*C. bergamia*, *C. aurantium, C. limon*, *C. aurantifolia*) although they are also found in angelica root (*Angelica archangelica*), rue (*Ruta graveolens*), cumin (*Cuminum cyminum*) parsley leaf (*Petroselinum crispum*), and marigold (*Tagetes minuta*) essential oils [19, 28].

The most common compounds are bergapten and psoralen (**Figure 3**). They are not found in distilled citrus peel oils [21].

**47**

reports [30, 31].

least 2 h [28].

**Table 3.**

**Figure 3.**

**6. Neurological toxicity of essential oils**

*Safety Profile of Essential Oils*

*DOI: http://dx.doi.org/10.5772/intechopen.91363*

**Latin name (botanical family)/common name Part used in EO extraction**

*Cananga odorata* (*Annonaceae*)/Ylang ylang Flowers *Inula helenium* (*Annonaceae*)/Elecampane Flowers, leaves

*Saussurea costus* (*Asteraceae*)/Costus Roots *Commiphora erythraea* (*Burseraceae*)/Opoponax Resin *Myroxylon pereirae* (*Fabaceae*)/Peru balsam Resin *Liquidambar styraciflua* (*Hamamelidaceae*)/Styrax Resin *Cinnamomum cassia* (*Lauraceae*)/Chinese cinnamon Barks *Cinnamomum zeylanicum* (*Lauraceae*)/True cinnamon Leaves, barks *Melissa officinalis* (*Lamiaceae*)/Lemon balm Leaves *Backhousia citriodora* (*Myrtaceae*)/Lemon myrtle Leaves *Melaleuca alternifolia* (*Myrtaceae*)/Tea tree Leaves *Pimenta racemosa* (*Myrtaceae*)/Bay rum tree Fruits, leaves *Syzygium aromaticum* (*Myrtaceae*)/Clove Buds, leaves, stems

*Pinus* spp. (*Pinaceae*)/Turpentine Leaves *Cymbopogon nardus* (*Poaceae*)/Lemongrass Aerial parts *Lippia citriodora* (*Verbenaceae*)/Lemon verbena Leaves

*Chemical structure of the most common furanocoumarins involved in photosensitization.*

*Essential oils considered to be dermal sensitizers (in alphabetical order of the botanical family) [21, 22].*

The factors influencing risk of photosensitization include the amount of essential oil applied topically and the area of exposure. It is considered there is no risk of photosensitization if the skin is covered to prevent exposure to UVA light for at

Essential oils could easily pass the blood-brain barrier, reaching the central nervous system after a systemic absorption. In an experimental setting, essential oils from *Salvia officinalis* and *Hyssopus officinalis* evoked convulsions after intraperitoneal administration in rats at doses of 0.5 g/kg and 0.13 g/kg, respectively [29]. In humans, essential oils from *Salvia officinalis*, *Thuja plicata*, *Cedrus* spp., *Hyssopus officinalis*, *Eucalyptus* spp., *Mentha pulegium*, *Cinnamonum camphora* and *Anethum graveolens* produced tonic-clonic convulsions, particularly in children and especially in those with a history of epileptic syndromes, according to several


#### **Table 3.**

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

*Cuminum cyminum* (*Apiaceae*)/Cumin Fruits *Tagetes minuta* (*Asteraceae*)/Marigold Leaves *Origanum vulgare* (*Lamiaceae*)/Oregano Aerial parts *Satureja hortensis* (*Lamiaceae*)/*S*ummer savory Leaves *Satureja montana* (*Lamiaceae*)/*W*inter savory Leaves *Thymus capitatus* ct. carvacrol or thymol (*Lamiaceae*)/Spanish oregano Aerial parts *Thymus serpyllum* (*Lamiaceae*)/Wild thyme Aerial parts *Thymus vulgaris* ct. phenol (*Lamiaceae*)/Red thyme Aerial parts *Cinnamomum cassia* (*Lauraceae*)/Chinese cinnamon Barks *Cinnamomum zeylanicum* (*Lauraceae*)/True cinnamon Leaves, barks *Pimenta racemosa* (*Myrtaceae*)/Bay rum tree Fruits, leaves *Syzygium aromaticum* (*Myrtaceae*)/*C*love Buds, leaves, stems *Cymbopogon citratus* (*Poaceae*)/Citronella Aerial parts *Cymbopogon nardus* (*Poaceae*)/Lemongrass Aerial parts *Lippia citriodora* (*Verbenaceae*)/Lemon verbena Leaves

Symptoms include a bright red rash, which may be painful to some individuals and sometimes a pigmentation of the skin, more frequently in Asians [21, 24]. In order to prevent sensitization, it is recommended to avoid known dermal sensitizers and avoid application of the same essential oil every day for a long period of time. **Table 3** lists some of the oils considered to be dermal sensitizers, but the sensiti-

*Essential oils potentially irritant to the skin (in alphabetical order of the botanical family) [22].*

**Latin name (botanical family)/common name Part used in EO extraction**

Essential oils obtained from different species of *Pinus* and *Abies* should only be used when the level of peroxides is kept to the lowest practical level, preferably by

Skin sensitization reactions are idiosyncratic, identification of the causative allergen(s) and their subsequent withdrawal generally leading to a resolution of the problem. Some standard mixtures (fragrance mixture) can be used in a patch test to screen for allergic reactions in susceptible individuals, but not all the allergies can

Photosensitization is a reaction between a phototoxin from an essential oil that is applied to the skin in the presence of sunlight or ultraviolet A (UVA) light. The

Photoallergy is an immune-mediated skin reaction, while phototoxicity may lead to photocarcinogenesis. Furanocoumarins (psoralens) appear to be primarily responsible for phytophototoxic reactions in humans. Reactions can vary from pigmentation, blistering, to severe full-thickness burns. Furanocoumarins occur mainly in expressed citrus peel oils (*C. bergamia*, *C. aurantium, C. limon*, *C. aurantifolia*) although they are also found in angelica root (*Angelica archangelica*), rue (*Ruta graveolens*), cumin (*Cuminum cyminum*) parsley leaf (*Petroselinum crispum*),

The most common compounds are bergapten and psoralen (**Figure 3**). They are

interaction with the light may be either phototoxic or photoallergic.

zation process can occur for any essential oil [22, 25].

adding anti-oxidants at the time of production [19].

and marigold (*Tagetes minuta*) essential oils [19, 28].

not found in distilled citrus peel oils [21].

be predicted by this method [26, 27].

**5.3 Photosensitization**

**Table 2.**

**46**

*Essential oils considered to be dermal sensitizers (in alphabetical order of the botanical family) [21, 22].*

#### **Figure 3.**

*Chemical structure of the most common furanocoumarins involved in photosensitization.*

The factors influencing risk of photosensitization include the amount of essential oil applied topically and the area of exposure. It is considered there is no risk of photosensitization if the skin is covered to prevent exposure to UVA light for at least 2 h [28].
