Volatile Organic Compounds

This compilation work briefly summarizes the definition, classification, and sources of volatile organic compounds found in indoor environments, the health impacts of volatile organic compounds, and general information about paints as a source of volatile organic compounds. Definition, Classification and Sources of Volatile Organic Compounds Volatile Organic Compounds, VOC (Volatile Organic Compounds, VOC) are the most common chemical pollutants found in the indoor air we breathe [1]. They are hydrocarbons with aliphatic or aromatic structure with boiling points up to 260ºC [2]. Volatile organic compounds, which have high vapor pressure and low solubility in water, are recognized as contributing significantly to air pollution through photochemical reactions [3, 4]. Today, particularly in cities, people spend much of their lives in environments such as homes, workplaces, schools and vehicles [5-6]. According to research conducted, 88% of a person's daily time is spent in these closed environments [7-8]. Since people spend much of their time in these closed environments, "indoor (indoor) air quality (indoor air quality)" has become at least as important as outdoor air quality [9]. Indoor air is defined as the air inside residences, non-industrial workplaces and official buildings (schools, hospitals, etc.) [5, 10]. In line with developments in the construction, paints and coatings, and furniture sectors, closed living spaces become more comfortable with increased use of synthetic materials, but at the same time, indoor air quality is degraded by these synthetic materials used [9]. As stated by the Environmental Protection Agency (Environmental Protection Agency, EPA), among the pollutants that degrade indoor air quality, volatile organic compounds and formaldehyde are the main ones [11- 12]. Volatile organic compounds are found in varying amounts in almost every environment, and their levels in indoor spaces are variable. Consumer products such as perfumes, deodorants, soaps, detergents, shampoos, air freshener sprays, coating materials such as paints and varnishes, building materials such as adhesives and flooring, and office machines such as photocopiers and fax machines are the main VOC sources in indoor environments [13-15] .

Figure 1. Indoor Air Pollutants

Figure 2. Sources of Volatile Organic Compounds

The image presented in Figure 1 shows indoor air pollutants, the graph presented in Figure 2 shows the sources of volatile organic compounds. In the report prepared by the European Commission Joint Research Centre - Environment Institute, organic compounds found in indoor air are generally classified as; (I) According to their chemical structure (alkanes, aromatic hydrocarbons, aldehydes, etc.), (II) According to their physical properties (boiling point, vapor pressure, carbon number, etc.) and (III) According to their potential health effects (irritants, neurotoxic, carcinogenic, etc.) [2]. According to the widely used classification made by the World Health Organization (World Health Organization, WHO) for organic-based indoor air pollutants, organic compounds are examined in four groups according to their boiling points: VVOC, VOC, SVOC and POM. The classification of indoor organic pollutants [2-16] is presented in Table 1.

Table 1. Classification of indoor organic pollutants [2-16]

As can be seen from the table, organic-based indoor air pollutants include compounds boiling in a very wide range from very volatile organic compounds (b.p. 0-50oC) to particulate organic compounds (b.p. >380oC). The group frequently observed in indoor air and referred to as VOC generally includes compounds from certain paraffins and benzene to naphthalene (b.p. 50-260oC). These compounds are present in vapor form in indoor air due to their relatively low boiling points [2, 11, 16-19]. Propane, butane and methyl chloride are examples of very volatile organic compounds (VVOC); pesticides (DDT), plasticizers (phthalates), flame retardants (polychlorinated biphenyls, PCBs and polybrominated biphenyl, PBB), semi-volatile organic compounds (SVOC) and limonene, toluene, acetone, ethyl alcohol, isopropyl alcohol, hexanal are examples of volatile organic compounds (VOC) [3].

BTEX

The most commonly observed volatile organic compounds are called BTEX, which includes "benzene, toluene, ethylbenzene and xylene", among which toluene is the compound generally found at the highest levels in indoor environments. Another important compound with carcinogenic effects on humans when its concentration in air is high is benzene [11, 16, 19]. The chemical structures of BTEX are presented in Figure 3.

Figure 3. Chemical structures of BTEX

Emissions from human activities and building materials are the most important sources of indoor air pollution, but the entry of pollutants present in outdoor air into indoor air through ventilation is also important [20]. From this perspective, it is necessary not to overlook the VOC content in outdoor air resulting from the exhaust of vehicles that are likely to affect indoor air quality through ventilation and mostly use fossil fuel (gasoline). Since one of the most important main sources of BTEX is gasoline, the percentage distribution of BTEX components in gasoline is presented in Figure 4.

Figure 4. Distribution of BTEX Components

Health Effects of Volatile Organic Compounds

The major potential health effects of volatile organic compounds are acute and chronic respiratory tract effects, neurological toxicity, lung cancer and eye and throat irritation [21]. Some of these compounds are associated with "sick building syndrome" (SBS), which causes mucosal irritation, headaches and fatigue [1]. The World Health Organization (WHO) has listed the symptoms seen in sick building syndrome (SBS) under five categories [9, 22-23]. (I) Irritation of the eyes, nose and throat, (II) Neurological or general health symptoms: headaches, dizziness, nausea, vomiting, physical and mental fatigue, memory loss, lack of concentration, (III) Irritation observed on the skin: skin redness, pain, itching and dryness, (IV) Unexplained hypersensitivity reactions: asthma-like symptoms in non-asthmatics, eye and nasal discharge, (V) Odor and taste findings: changes in sense of smell and taste. Many volatile organic compounds are considered toxic and potentially carcinogenic, mutagenic or teratogenic [1, 24, 25]. Furthermore, many indoor air pollutants such as volatile organic compounds can cause lung cancer. Lifetime cancer risk from volatile organic compounds is estimated to be quite similar to risk estimates from radon and secondhand smoke [21, 26].

Figure 5. Diseases caused by indoor air pollutants

Figure 5 shows the diseases caused by indoor air pollutants.

Paints and Coatings as a Source of Volatile Organic Compounds

Synthetic and water-based paints have entered every area of our lives today. While the amount of VOC emitted and evaporated from industrial sources (chemical production facilities, oil refineries, etc.) and waste disposal areas (landfills, wastewater treatment plants, etc.) is not as high, paints are also among the important VOC sources. Among potential VOC emission sources in any indoor environment, commercial consumer products such as paints and related materials, adhesives, furniture and building materials are also present [21, 24]. Painting operations rank second among VOC sources at 38% [27]. Organic solvents and various additives used in paints fall into the VOC category and, due to their low boiling points, can easily be released into indoor air and present in enclosed spaces [21, 28]. For this reason, today, VOC emissions from paints have become a problem that needs to be solved [29]. Among the volatile organic compounds contained in paints are aliphatic hydrocarbons (n-hexane, n-heptane), aromatic hydrocarbons (toluene, xylene), halogenated hydrocarbons (methyl chloride, propylene dichloride), alcohols, ketones (methyl ethyl ketone), esters (ethyl acetate), ethers (methyl ether, ethyl ether, butyl ether) [28, 30]. As is known, apart from solvent-free paint and powder coating systems, the use of organic solvents in paint formulations, including water-based paints, is common. Paint systems can generally be examined in three groups: solvent-based paints, water-based paints and powder coatings. Solvent-based paints are classified according to their solid content ratio as "low-solid paints" containing 40% or less solids, "medium-solid paints" containing 40%-70% solids and "high-solid paints" containing 70% or more solids. High-solid paints, developed over the past forty years and still popular, reduce solvent content to below 30% [31]. With an increase in solid content in the paint formulation, reduced solvent use provides lower VOC content.

In water-based paints, depending on the type of resin used and the environment, "aqueous dispersions (latexes, emulsions)" and "water-thinnable (dilutable)" paints can be discussed [32].

In water-based latex paints produced using latex binders prepared by stably dispersing solid polymer particles in water, coalescing agents used for paint film drying are also among indoor VOC sources. Emulsion paints are obtained by stably dispersing liquid polymer spherules in water as a result of each monomer droplet being polymerized separately in water [31]. Water-based emulsion paints contain approximately 5%-15% organic solvent [30]. In water-thinnable paints, polymers containing hydrophilic groups such as free carboxyl or amine groups on the polymer chain are used as binders. The binder, which is acidic or basic in character depending on the functional group it contains, is neutralized in the presence of an appropriate amine or acid, and the resulting soluble salt is dissolved in a secondary solvent (alcohol, glycol ether, etc.) to impart water-thinning properties to the binder polymer [31, 33-34]. These types of paints contain approximately 10%-15% organic solvent by weight from binder synthesis, along with secondary solvents that are miscible with water [30] for a total of approximately 20%-30% organic solvent by weight, and the VOC content depending on the paint dilution ratio with water is lower compared to solvent-based paints.

UV and EB curing paints and powder coatings are solvent-free paints with no VOC emissions.

Organic solvents and other volatile additives contained in paints evaporate during the application and drying stages of the paint on the surface. Therefore, in solvent-based painting systems, solvent emissions vary depending on the method used during painting. Table 2 shows theoretical solvent emissions according to paint application method [30].

Table 2. Theoretical solvent emissions according to paint application method [30]

* Transfer efficiency in pneumatic spraying is 60%, ** in electrostatic spraying is approximately 75%

VOCs released into outdoor and indoor air from various sources and harmful to humans and plants even in very small concentrations react with CO and NOx present in the atmosphere in the presence of sunlight to form photochemical smog containing ozone [3]. Since VOC emissions affect climate change, plant growth, and the health of humans and all animals, these emissions must be limited and controlled [35]. For this reason, reducing the amount of organic solvents used in paint formulations, one of the main VOC sources, has become a necessity. To reduce solvent emissions, approaches include (I) the use of solvent-free or low-solvent products, (II) the use of low-solvent emission painting techniques, (III) the use of exhaust gas treatment systems in environments where painting operations are carried out [30]. Today, water-based coatings appear to be the most practical and effective solution for low VOC content. For this reason, all types of traditional coatings are being converted to water-based coating systems [36]. In addition, preferring high-solid systems in solvent-based coatings emerges as another alternative. Using different paint application methods (high-volume low-pressure spray gun (HVLP, High Volume Low Pressure spray gun) and electrostatic applications, etc.) to maximize paint transfer efficiency is also an important approach. In conclusion, in recent years, in the paints and coatings industry; due to environmental concerns, new regulations and directives related to this issue, and high costs, work continues on the production of various paint systems with low VOC content.

Assoc. Prof. Işıl Acar Department of Chemical Engineering Istanbul University

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