## 1 Introduction
They found that (_R_)-(-)-carvone and a mixture of _cis_ and _trans_ isomers of limonene oxide are potent allergens. Their next study [31] showed that hydroperoxides of D-limonene are potent allergens. The _cis_- and _trans_-limonene-2-hydroperoxides (mw 168) are the most abundantly formed compounds in the autoxidation of D-limonene by air. Autoxidation of D-limonene resulted in other potent allergens, (+)-limonene-l,2-oxide and carvone (Figure 1). A later, more advanced study by [PERSON] et al. [32] revealed many other air-oxidized products of D-limonene. The products are _cis_- and _trans_-limonene-l,2-oxide, _cis_- and _trans_-carveol, _R_-(-)-carvone, _cis_- and _trans_-limonene-2-hydroperoxide, _cis_- and _trans_-\(p\)-mentha-2,8-dien-l-ol, _cis_- and _trans_-\(p\)-mentha-2,8-diene-l-hydroperoxide, _cis_- and _trans_-limonene-2  
Figure 1: Air oxidation of limonene. hydroperoxide, and \(p\)-menth-8-ene-l,2-diol. In 1999, data on oxidation kinetics and product yields were published for 23 terpen and 65 oxidation products [33]. Oxidized limonene and linalool caused contact allergy in dermatitis patients [34; 35]. Similarly, hydroperoxides of limonene were contact allergens [36]. The main oxidation product of linalool was isolated and identified as 7- hydroperoxy-3,7-dimethyl-octa-1,5-diene-3-ol [37]. Later, [PERSON] et al. [38] published contact allergens originating by air exposure of linalool **1** (mw 154). The primary allergens identified were 7- hydroperoxy-3,7-dimethyl-1,5-diene-3-ol (the main one in an oxidized sample) (mw 186) **2**, 6-hydroxy-2,6-dimethyl-2,7-dien (mw 168) **4**, 2,6-dimethyl-3,7-diene-2,6-diol (mw 170) **5**, 2,6-dimethyl-1,7-diene-3,6-diol (mw 170) **6**, 2-(5-methyl-5-vinyltetrahydrofuran-2-yl)propan-2-ol (mw 170) **8**, and 2,2,6-trimethyl-6-vinyltetrahydrofuran-2H-pyran-3-ol (mw 170) **9**. The presence of 6-h hydroperoxy-3,7-dimethyl-Meta-1,7-diene-3-ol (mw 186) **3** and 2,6-dimethyl-Meta-2,7-diene-1,6-diol (mw 170) **7** was also detected (Figure 2). Linalool is not an allergen, but air-oxidized linalool can trigger contact allergies [39; 40]. The atmospheric oxidation of three terpen was studied by [PERSON] et al. [41]. After sunlight irradiations of mixtures of terpene and NO in air, reaction products were positively identified. Limonene produced 4-acetyl-1-methylcyclohexene, formaldehyde, and glyoxal. \(\alpha\)-Pinene oxidation gave rise to formaldehyde, acetone, pinonaldehyde, and glyoxal. Oxidation products of \(\beta\)-pinene: 6,6-dimethylbicyclo[3.1.1]heptan-2-one, formaldehyde, and acetone. Caryophyllene oxidized to just one product with low-sensitizing potential (an allergen of moderate strength), carryophyllene oxide [42]. Later, these authors identified two hydroperoxides to be primary oxidation products, but due to instability, they are rapidly converted to a more stable and less reactive secondary oxidation product--caryophyllene oxide [43] (Figure 3). Thermal degradation of camphene, \(\Delta^{3}\)-carene, limonene, and \(\alpha\)-terpinene was published by [PERSON] et al. [44]. The major monoterpene alcohols of the essential oil undergo isomerization and oxidization reactions during steam distillation. Nerol and geraniol predominantly isomerize; their conversion gives rise to linalool and \(\alpha\)-terpineol. Linalool is converted into isomeric furan and pyran linalool oxides, 2,6-dimethyl-3,7-ocadiene-2,6-diol and 2,6-dimethylboct-7-en-2,6-diol (Figure 4). The chemical conversion of analytical targets  
Figure 2: Air oxidation of linalool. during sample clean-up by steam distillation is objectionable and interferes with precise hop oil analysis [45]. We must take into consideration that, upon aging, essential oils can undergo oxidation and polymerization, which may result in a loss of pharmacological properties. Heat, light, and air can lead to their oxidation, polymerization, isomerization, thermal rearrangement, or dehydrogenation [46].