# Forest Volatile Organic Compounds and Their Effects on Human Health: A State-of-the-Art Review
## 1 Introduction  
### Forest Volatile Organic Compounds (VOCs): Background and Natural Functions  
The first systematic analysis of the importance and role of volatile organic compounds (VOCs) emitted by plants in the atmosphere above forests was probably performed from the perspective of atmospheric chemists who were interested in the effects of VOCs on atmospheric composition and climate. In two seminal articles [1; 2], the Dutch scientist [PERSON] and his colleagues hypothesized that the blue haze observed above many forests was caused at least partially by volatile organic compounds emitted by plants (forest VOCs) and went on to estimate the impact that biogenic emissions have on the global balance of organic compounds released into the atmosphere, compared to anthropogenic production. The first estimate of global terpene emissions was 175 millions of tons C/year [1], but morerecent evaluations state that worldwide biogenic VOC (BVOC) emissions into the atmosphere could amount to around 1 billion of tons C/year [3]. Forests seem to be the greatest BVOC emitters. Specifically, tropical trees appear to be responsible for around 80% of terpenoid emissions and for around 50% of other BVOC emissions, while other tree species seem to contribute to around 10% of total BVOC emissions [3]. BVOCs form 80% to 90% of total VOC emissions in the atmosphere each year [4,5]. Some forest VOCs are also defined as \"phytoncides\" and this term, first introduced in the last century by the Russian biologist [PERSON], derives from the combination of two words: \"phyto\", which refers to plants and botany in ancient Greek, plus the suffix \"cide\", which indicates \"killing\" or \"extreminating\", thus underscoring the antimicrobial and insecticidal activity of these substances [6,7]. However, the term \"phytoncide\" may be misleading because, due its broad definition and etymology, it can indicate either plant-derived volatile substances with antiparasitic properties or any (volatile and nonvolatile) antimicrobial/insecticidal compound released by plants, any volatile VOC, or, in some cases, essential oils obtained from aromatic woods. In order to avoid potential misunderstandings, in this research work, which is focused on biogenic volatile substances found in forest air, the term \"forest VOCs\" will be preferred over the less specific \"phytoncide\", and BVOCs will be defined as any organic compound, except carbon dioxide and monoxide, emitted by plants and having vapor pressure high enough to be vaporized in relevant amounts. Usually, the definition excludes dimethyl sulfide and methane because it is still debated as to whether they are produced by terrestrial plants [8]. More than 1000 different BVOCs are released from plant flowers, vegetative parts, or roots [9]. These substances are largely lipophilic products with molecular masses under 300 Da, and the vast majority are isoprenoids, including hemieterpenes (C5H8) such as isoprene, monoterpenes (C10H16), irregular acyclic monoterpenes (C11H18 or C16H26), and sesquiterpenpenes (C15H24), both as hydrocarbons and as oxygenated compounds; there is also a number of low-molecular-weight molecules, especially C1 and C2 oxygenated compounds, such as methanol and acetaldehyde, and C6H10 fatty acid derivatives, including lipoxygenase pathway products such as green leaf volatiles (henceforth GLVs), plus benzenoids and phenylpropanoids [5,9,10,11,12,13]. Many BVOCs are common to phylogenetically distant plant species [14]. For instance, only some plant species, even belonging to taxonomically distant taxa (like _Bryophyta_ and _Quercus_), emit isoprene, while in close taxa, there can be both emitters (_Quercus_ spp.) and non-emitters (_Acer_ spp.) [11], thus suggesting that metabolic pathways related to their production have been highly preserved during evolution [14]. From the perspective of plant physiology, BVOCs can be divided into constitutive and inducible compounds.