# Forest Volatile Organic Compounds and Their Effects on Human Health: A State-of-the-Art Review
## 3 Results
### A Qualitative and Quantitative Analysis of Forest VOC Emissions  
The composition of forest VOCs (probably the key VOC source [11]) is expected to consist mainly of constitutive compounds, but, since it cannot be excluded that, at any given moment, some individual plants in the forest may be under herbivore attack or abiotic stress, and provided that the rate of VOC emissions can be many times higher than baseline after a herbivore attack [29], a part of the total forest emissions can consist of biotic and abiotic-induced VOCs. This can be important for the evaluation of biological activities of forest VOCs because the range of biotic-induced compounds is often highly different from that of constitutive ones and can be also different from the abiotic-induced ones [29,30]. If seen from the plants' functional point of view, VOCs thus comprise all volatile constitutive defense compounds (volatile phytoanticipins, or phytoncides, as defined by [PERSON]), plus volatile inducible defense compounds (volatile phytoalexins or HIPVs) and non-defense compounds (attractants of pollinator and of seed dispersals). The biochemical composition of forest air tends to follow diurnal variations, with BVOCs concentration usually peaking early in the afternoon and, especially in summertime, even in the morning (around 2 h after dawn), and it can be markedly influenced by several factors, including meteorological conditions, altitude, seasons, sunlight exposure, and tree types [31,32]. For example, it has been demonstrated that the air of forests dominated by conifer trees has a higher concentration of VOCs if compared to that of forests with more deciduous trees [31]. However, beyond these differences, it is estimated that, from a biochemical point of view, the majority of forest VOCs are isoprene and terpen, especially monoterpenes, including limonene or pinene [33]. Specifically, according to [PERSON] and colleagues [3], isoprene dominates the emissions, with around 50% of the total BVOC emissions of 1 billion tons C/year. C1 and C2 oxygenated compounds such as methanol and acetaldehyde and C10 hydrocarbons such as \(\alpha\)-pinene, \(\beta\)-pinene, (E)-\(\beta\)-ocimene, and d-limonene contribute to another 30%, while the remaining 20% is dominated by 20 compounds (mostly terpenids), with the other 100 compounds accounting for a mere 3%. Evidence from research done on open-field emissions of both conifer and deciduous forests [34, 35, 36], and on un-detached leaves or branchlets [30,37], supports these estimates, indicating that, apart from isoprene, the most common and dominant forest VOCs in terms of percentage of the total emissions are monoterpenes such as \(\alpha\)-pinene, d-limonene, and \(\beta\)-pinene, followed by sabinene, myrcene,and camphene. Monoterpenes' global annual emissions amount to 330-480 million tons, with an average atmospheric concentration above forests from some pmol/mol to several nmol/mol. Other quantitatively important forests VOCs such as GLVs and acetaldehyde are emitted in quantities of around 260 million tons and show atmospheric concentrations of around 1-3 nmol/mol [11; 16]. From a quantitative point of view, studies which aimed to determine the atmospheric concentration of some categories of BVOCs under forest canopy reported heterogeneous findings, probably due to a significant influence of the abovementioned factors on the forest air composition: approximately, on the basis of available evidence, the total amount of terpen can vary from a minimum of 3.5 to a maximum of 56.0 \(\upmu\)g/m\({}^{3}\), isoprene can range from 0.1 to 28.0 \(\upmu\)g/m\({}^{3}\), and monoterpenes may have an average concentration of 4.5 \(\upmu\)g/m\({}^{3}\), whereas the amount of LOX pathway products is usually below the upper threshold of 3.4 \(\upmu\)g/m\({}^{3}\)[16; 31; 32; 34; 38; 39; 40; 41; 42; 43; 44] (Figure 3). In particular, if we only consider monoterpenes, the concentration range of seven of them under the canopy of a Scots pine forest was reported in a study [44] (Figure 4).