Popular Science │ Explanation of the Definition of Volatile Organic Compounds Worldwide

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Introduction: WeChat ID: VOCs99 discusses the definition of volatile organic compounds, analyzes the advantages and disadvantages of each definition, and points out that an ideal definition of VOCs should meet the following three conditions: first, it should cover all VOCs that are currently in use; second, it should make it easy to determine whether a compound is a VOC or not; third, it should take into account not only environmental effects but also health effects. Keywords: Volatile organic compounds (VOCs) ； photochemical reaction ； Solvent Directive ； Paint Directive Introduction The term VOCs is widely used in regulations related to the atmospheric environment and occupational health, yet its definition varies across different regulations, causing various difficulties in the identification, analysis, and testing of VOCs. Currently, at least four different definitions are in use worldwide, but none of them can accurately describe the same chemical in all situations. All definitions attempt to establish a distinction between volatile and non-volatile compounds. The four general definitions are summarized as follows: all organic compounds with a vapor pressure greater than 10 Pa at 20°C ； All organic compounds with a initial boiling point below 250°C at 1 atmosphere ； All organic compounds that participate in atmospheric photochemical reactions or have a Photochemical Ozone Creation Potential (POCP) value ； All organic compounds used as solvents or mixed solvents. Organic compounds are generally defined as all compounds that contain at least carbon, and at least one or more of hydrogen, halogens, oxygen, sulfur, phosphorus, silicon, or nitrogen, excluding carbon oxides and inorganic carbonates or bicarbonates. For anthropogenic sources, some definitions restrict the scope of VOCs by excluding methane, in order to distinguish them from VOCs released naturally by forests and thus better reflect the actual situation of human activities. Furthermore, these definitions show significant discrepancies regarding low-volatility organic compounds; for example, with the commonly used vapor pressure method (i.e., 10 Pa), certain low-volatility organic compounds are not considered VOCs, whereas they are classified as VOCs under the boiling point method (250°C). Nevertheless, the four definitions cover most of the solvents used in daily production and life. In comparison, using a stricter definition of VOCs in future legislation can at least restrict more than 90% of the organic compounds with evaporation properties in coatings. In any case, driven by legislation as well as the growing public demand for a better atmospheric environment and greater awareness of (occupational) health, the requirement to use low-volatility organic compounds will become increasingly stringent. Using four different definitions at the same time will confuse both solvent producers and users; therefore, it is necessary to choose clearer definitions. 1 All organic compounds with a vapor pressure greater than 10 Pa at 20°C; the vapor pressure of a particular compound is directly related to its maximum concentration in air. Since concentration is a quantity per unit volume, the maximum concentration of a given compound must increase as the volume increases. In fact, there is an “infinite” volume in the troposphere; the maximum concentration can never be reached. Thus, all solvents will evaporate from the product. For environmental protection, specifying the amount of solvent is more important than its volatility. However, this maximum concentration could be reached if we consider the working environment of painters who operate in very small work areas (tight workspaces). Under such circumstances, coating workers may be exposed to the maximum concentration of solvents, and this maximum concentration depends directly on the vapor pressure of the solvents. For health protection, it is important to limit the volatility of the solvents used. The lower the vapor pressure of the solvent used, the lower the risk of high exposure for coating workers. To define the scope of the Solvents Directive, the European Commission used the definition of \"all organic compounds with a vapor pressure greater than 10 Pa at 20°C\". This Council directive sets limits on the release of VOCs from industrial production processes and facilities. 10 Pa is a artificially selected limit value. The initially recommended limit was 1 Pa, but due to the requirements of the chemical industry, this limit was increased to 10 Pa. Subsequently, the European Commission and most EU Member States adopted this definition of VOCs. Since the first solvent directive was issued, all member states were forced to use this definition by 2002. 1.1 Advantages of using 10 Pa as the definition: The 10 Pa definition is the most effective physicochemical parameter. Although, in general, there are fixed formulas for the evaporation capacity of compounds, vapor pressure at least indicates the maximum concentration of the compound in the air, and it is more important for local (health) effects. Consistency in European law. Among all European laws, this definition of VOCs is considered the best. Using the 10 Pa definition in legislation on decorative paints and coatings is in line with the Solvents Directive. However, in the **maximum emission guidelines, the definition of VOCs is based on their contribution to tropospheric ozone. Reaching a consensus with the chemical industry. The European Solvent Industry Group (ESIG) favors the 10Pa definition, as it is the most effective parameter for indicating volatility. They agree with the empirical relationship between vapor pressure and boiling point, but this relationship is not absolute. According to ESIG, a vapor pressure of 10 Pa corresponds to a boiling point of around 216°C for hydrocarbon solvents. 1.2 Disadvantages of using 10 Pa for definition and inconsistency with the coating industry. The European Committee for the Paints, Inks and Pigments Industry (CEPE) approved the use of the 10 Pa definition, which was already established at the time of the initial proposal in the Paints Directive; however, the paint industry later changed its stance and strongly supported the 250°C definition. In terms of **countermeasures, some** use the 250°C definition (such as Germany). In legislation regarding decorative paints and coatings, they were forced to turn to the 250°C definition or use both. Since the Solvents Directive applies to industrial production processes and equipment, it still has an impact on the coatings industry. There is an urgent need to establish vapor pressure using standard methods. Several methods exist for determining the vapor pressure of compounds; however, all the extrapolation methods used yield different results. Although the definition of vapor pressure stipulated in the Solvent Directive has been in use for some time now, the test methods for determining vapor pressure are simple and conform to standards. Not all volatile organic compounds used in coating materials are covered. The definition of 10 Pa does not cover all VOCs used in coatings, such as organic solvents with high boiling points; this may lead to the use of more advanced organic solvents during the development of coatings. The coating industry addresses the challenges of the chemical industry. Coating manufacturers want to verify whether the solvents they use are VOCs, even though this information is provided by the suppliers. This is possible because paint manufacturers bear legal responsibility for the MSDS they provide. In fact, some small and medium-sized enterprises are unable to measure vapor pressure in most cases. Only laboratories equipped with special apparatus can determine vapor pressure, while distillation equipment is sufficient to determine the boiling point. However, according to the associations of paint manufacturers in Germany and France as well as CEPE, the paint industry has never tested the volatility of the solvents themselves. They merely checked with the dedicated laboratory symbolically as a formality. All organic compounds with a initial boiling point below 250°C at 2 atmospheres of pressure; although there is no direct relationship between a compound’s boiling point and its evaporation capacity or its maximum concentration in the air, there is still some empirical relationship between boiling point and vapor pressure. Since, in fact, the boiling point is a more easily understood physicochemical parameter, and it is often easier to determine the boiling point than the vapor pressure, the boiling point is also used in the definition of VOCs. The definition that “VOCs are all organic compounds with a initial boiling point of less than 250°C at 1 atmosphere” was developed by the CEPE working group for the ecological labeling of indoor coatings in Europe. The German representative particularly wishes to use this definition, as it is commonly used in Germany. The 250°C limit is artificially chosen. Since eco-labels are voluntary, there is no **mandate to use this definition. 2.1 The advantages of using 250°C have been agreed upon with the paint industry. Regarding the use of the 250°C definition, CEPE has reached a consensus among its members to incorporate this definition into legislation concerning decorative paints and coatings. CEPE prefers the 250°C definition because the boiling point is easier to determine, and this definition makes it simpler for paint manufacturers and applicators to understand the physical/chemical properties. The evaluation of general-purpose solvents for paints indicates that the 250°C boiling point definition is stricter than the 10 Pa definition. A relatively simple measurement. Establishing the initial boiling point is relatively simple and feasible, and detecting the boiling point is easy for anyone. 2.2 The drawbacks of using 250°C are inconsistent with European legislation. In Europe, the Solvent Directive defines a value of 10 Pa, and the coating industry is reluctant to change its stance and revert to that definition, which prevents compliance with European laws. Inconsistency with the chemical industry. According to ESIG, the European Solvents Industry Group, boiling point and vapor pressure are only indirectly related, and the boiling point is an inappropriate parameter. The reason why ESIG favors the definition based on vapor pressure is likely that the definition at 250°C is stricter than that at 10 Pa, which results in limiting the use of most solvents. Member states are forced to use two different definitions. All **will be forced to use the 250°C definition (Paints Directive) and the 10 Pa requirement (Solvents Directive). Since neither definition covers all organic solvents, a state of confusion arises. It does not cover all VOCs used in coating materials. The 250°C definition also does not cover all volatile organic compounds used in coatings, such as high-boiling-point organic solvents. 3 The main reason for the reduction in VOC emissions by all organic compounds involved in atmospheric photochemical reactions or those with a POCP value is that, in the presence of NOx, VOCs contribute to the formation of tropospheric ozone. As VOC emissions are reduced, ozone formation is decreased. Different VOCs contribute to different amounts of ozone formation. Some VOCs have a high potential to contribute to ozone, while others have a low potential. This potential capacity is represented by POCP. Each VOC has its specific POCP value. Some people believe that reducing VOC emissions is necessary to decrease ozone formation, while others think that it is necessary to cut back on the emissions of those VOCs with a high potential impact. Thus, from an environmental perspective, it is very beneficial to use the POCP value for definition. However, certain low-reactivity solvents can cause additional problems, namely that they are highly harmful to human health, difficult to handle, persistently stable, or may lead to other negative environmental issues. More importantly, low-reactivity solvents (low POCP values) ultimately contribute to the formation of ozone. Ozone formed by high-POCP solvents accumulates around the source of release, whereas ozone generated by low-POCP solvents, due to their evaporation and low reactivity, moves away from the source of release. Due to the relatively high concentration of EU member states, the ozone formed by both is problematic. When it comes to decorative coatings used indoors, health considerations are of particular importance. Therefore, it is not feasible to use the POCP value to define VOC. In the 1979 draft on the control of VOC emissions and the long-distance transboundary air pollution caused by their cross-border movement, the following definition was used: VOCs are organic compounds of natural origin other than methane, which have the ability to react with nitrogen oxides under sunlight to produce photochemical oxidants. In this draft, POCP is defined as “the change in photochemical ozone levels caused by specific VOC emissions”. According to the draft UN POCP, POCP can be determined by photochemical model calculations or experiments. 3.1 Advantages of using POCP for definition: POCP is a very specific parameter. Although POCP merely indicates a compound’s ability to generate ozone, in fact it is equally important for its other environmental and health-related effects. For all effects, the volatility of the compound plays a decisive role; therefore, it is considered to be more important than vapor pressure or boiling point. In the **maximum release plate instruction, the above definition appears as the only one in the draft. 3.2 The disadvantage of using POCP definitions is its incompatibility with European law. In Europe, the definition defined by Directive 10/1999 on solvents is in use, so it will not be compatible with European law. It is inconsistent with the coating industry. CEPE has reached a consensus among its members on the use of the 250°C definition; decorative paints and coatings are legally not acceptable under the POCP definition. Furthermore, the coating industry also finds it difficult to accept the POCP definition for addressing health effects. Some agreements with the chemical industry. When choosing between vapor pressure and initial boiling point, ESIG prefers vapor pressure. However, when the main goal is to reduce ozone formation, using the POCP definition may be the most appropriate for European solvent industry groups. Obviously, for the solvent industry, a boundary value of 0 (POCP>0) may not be acceptable. Member states are forced to use different definitions. As with the 10 Pa definition (Solvent Directive), all **will be forced to use the POCP definition (Future Paints Directive, NEC Directive). Since neither definition covers those organic compounds mentioned above, this will be a complex situation. There is an urgent need to establish a standard method for POCP values, as the main problem with using POCP values is the lack of a consistent standard method for calculating them. The POCP values can vary significantly between different estimates, and in some cases the difference can be as much as 4 times. The POCP value is estimated based on individual cases and specific calculations (maximum ozone concentration, total ozone concentration, or average ozone concentration). Vague nature. The POCP value varies over time and space; therefore, it is quite ambiguous to use POCP to define VOCs. 4 All organic compounds used as solvents or mixed solvents – this definition is often used in everyday speech. In the eyes of many, it is a perfectly clear definition. Although it is useful in daily life, this definition of practicality cannot be used in law because it is vague and not employed. 5 Discuss the definitions of vapor pressure and boiling point; both have a common drawback, namely that they cannot cover all volatile organic compounds in coatings. This may result in the high-boiling-point solvents in the coating not being covered. To solve this problem, one option is to accept the limit values of the two definitions. In fact, 1 Pa is not exactly the dividing line, as vapor pressures below 1 Pa cannot be detected. However, if the boiling point is increased, surfactants or plasticizers that do not have an evaporating effect may also be included. An ideal definition of VOCs should meet the following conditions: it should cover all VOCs that are currently in use. Because it is hoped that all the VOCs used will evaporate from the dry film and eventually enter the troposphere. Therefore, all VOCs, namely solvents, mixed solvents, film-forming aids, monomers, and other potentially volatile additives, must be included in the definition. Only in this way can VOCs in decorative paints and coatings be restricted. Such a result will include all VOCs and all organic compounds containing POCPs. It is easy to determine whether a compound is a VOC. The initial boiling point can be determined immediately using a standardized method. ESIG has been committed to developing standard methods for the determination of vapor pressures of aliphatic compounds. In addition, the Department of Environmental Chemistry at the University of Amsterdam has also developed a simple method for measuring low-volatility esters. It is impossible to establish a standard method for measuring POCP values in a short period of time. The definition should include not only environmental effects but also health effects. Compounds also produce health effects after evaporation; therefore, it may be a better approach to express volatility in terms of parameters, as this makes it easier to reach agreement with most tissues. Obviously, it would be best if every organization adopted the same definition. Since it is impossible for each organization to use its own definition, the definitions can be established by the \"important\" and \"less important\" organizations. Using the POCP value to define VOCs meets only the first condition, although it receives attention in the coating industry. Studies indicate that all solvents used in decorative coatings have a POCP value. It may be appropriate to define it based on vapor pressure or initial boiling point. However, in order to include all VOCs, the limits currently in use should be changed; the downside is that changing all defined limits will result in a loss of consistency. On the other hand, the health effects are already included in both definitions; since they cover all VOCs, those that are most volatile and neurotoxic are also included. The initial boiling point is simpler than vapor pressure, but once vapor pressure is measured using standard methods, it too becomes the simplest detection method. Even if the initial boiling point limit is not 250°C, people still hope that CEPE will adopt a definition based on the initial boiling point in a relatively simple manner. When a better definition is available, the European Commission is also prepared to abandon the current definition based on the initial boiling point. ESIG does not agree with the definition of the initial boiling point, as volatility is not clearly addressed. However, the definition based on the initial boiling point is stricter than one based on vapor pressure, as the former essentially includes all VOCs. 6 Conclusion: By using any one of the four commonly used definitions, approximately 90% of the solvents used in coatings can be covered in future legislation. Under these circumstances, it seems somewhat redundant to discuss which definition is more appropriate. However, the use of low-volatility organic compounds is increasing. If one definition classifies a certain compound as a VOC, while another definition does not, there is a discrepancy in the definitions. The practical definition that “VOCs are all organic compounds used as solvents and mixed solvents” is not used due to its ambiguity.