LITERATURE REVIEW

2.1 CONCEPTUAL REVIEW

Major efforts have been made to address current indoor air problems since daily societal activities are mostly performed in confined environments. Consequently, people are exposed to a large variety of pollutants from chemical, biological, or physical origins (Weschler 2001). The scientific community has focused its attention on chemical pollutants, principally volatile organic compounds (VOCs). VOCs are defined as organic compounds with lower boiling points ranging from 50 to 100 °C and upper boiling points ranging from 240 to 260 °C (Brown et al. 1994; World Health Organization 2010).

Sources of indoor contaminants that might affect human health can be divided into three main categories: (i) infiltration from outdoor air, (ii) building materials and furnishings, and

(iii) indoor human activities (Missia et al. 2012). The concen- trations of these pollutants can vary widely with time as the values are driven by four different parameters, known as in- door air determinants. These determinants are (i) the emission rates of primary sources, (ii) the air exchange rate (AER) be- tween indoor and outdoor environments, (iii) interactions with surfaces, and (iv) homogeneous and heterogeneous reactivity (Weschler 2004). This fourth determinant should not be ruled out since pollutants can react with other compounds or oxi- dants, forming secondary species that are otherwise absent from indoor environments (Weschler and Shields 1997b).

Among indoor sources of pollutants, the main sources of chemical contaminants are linked to the activities of occu- pants, such as interior renovation or decoration, smoking, cooking, and cleaning activities (Kirchner et al. 2001). Cleaning is performed by the population to increase hygiene, improve esthetics, and preserve materials (Wolkoff et al. 1998). Furthermore, among the thousand available and marketed products, scented products are appealing because a pleasant odor provides the sensation of a cleaner environment. Despite all the benefits involved in cleaning activities, such products present many associated risks.

Essential oils are a group of odorous or fragrant chemicals extracted from plants that can contain hundreds of molecules, mainly belonging to the terpene chemical family (Zuzarte and Salgueiro 2015). Terpenes and terpenoids are large classes of chemicals, all derivatives of the isoprene (C5H8) and produced from biosynthesis by a large diversity of plants. Terpenes are strictly hydrocarbons of general molecular formula (C5H8)n, whereas terpenoids contain additional functional groups, mainly O-containing such as terpenes alcohols. Monoterpenes and sesquiterpenes are respectively constituted with 2 and 3 isoprenic units and are divided into acyclic, mono-, bi-, and tri-cyclic compounds. Essential oils promise a decrease in microorganism activity and an increase in indoor air quality due to their antibacterial properties (Koukos et al. 2000; Danh et al. 2012; Teixeira et al. 2013; Harkat-Madouri et al. 2015; Luís et al. 2016; Said et al. 2016). To these ends, essential oils are widely used in the formulation of household products to (i) naturally scent the products, (ii) enhance the purifying performance, and (iii) refresh and “purify” indoor environments with a “natural” claim.

Currently, as indoor air quality has become a major concern for human health, air purifiers, antibacterial air fresheners, and “naturally scented” cleaning products have become trend and market leaders among household products. However, based on the ambiguity of the words “purifying” and “natural,” consumer products formulated with essential oils have benefited from skillful marketing strategies. Indeed, products containing essential oils take advantage of “natural” or “green” formula- tions to promote an increase in indoor air quality by purifying and reducing synthetic chemical emissions.

2.2 CHEMICAL COMPOSITION OF ESSENTIAL OIL-BASED PRODUCTS

Cleaning agents are assembled in different product categories according to their technical function and purpose of use and are typically composed of one or more active components (Wolkoff et al. 1998). To address this point, six studies are summarized in Table 1 to provide information regarding house- hold product compositions and general formulations. These studies were performed between 1998 and 2013 in various countries. Table 1 presents this information by describing the chemical formulations of household products and classifying products according to their categories and diffusion modes.

A cleaning product is generally constituted by water, solvents, surfactants, preservatives, and fragrances. Depending on the usage purpose, other compounds can be included, such as disinfectants, acids, bases, bleaching agents, abrasives, or enzymes (Missia et al. 2012). According to Table 1, surfac- tants are the main active compounds in cleaning products. The function of surfactants is to enhance the effect of the cleaning agent by lowering the surface tension of water. In contrast, acids and bases can be used as active compounds as they enhance the dissolution of specific stains and improve the performance of surfactants by regulating the pH of the solu- tion (Wolkoff et al. 1998; Steinemann et al. 2011; Trantallidi et al. 2015).

Among cleaning product constituents, water is the compound added in the highest proportion, followed by solvents. Both can be present in formulations at mass percentages of up to 95 and 50%, respectively. Water is added for various rea- sons: (i) to remove water-soluble deposits of minerals and inorganic salts, (ii) to dissolve metal ions that can decrease the effect of surfactants, and/or (iii) to be used as an emulsi- fying agent (Wolkoff et al. 1998; Missia et al. 2012).

Solvents are the most volatile constituents included in cleaning products. Solvents have boiling points ranging from 70 to 200 °C. Moreover, these chemical substances can be present at concentrations ranging from 2 to 50% w/w. Water- soluble solvents are used to dissolve oil and greases and to increase the “fast-dry effect” on cleaned surfaces (Missia et al. 2012).

When a cleaning product emphasizes a disinfectant property, complementary chemicals are added to the formulation to suppress or inhibit microbial activity. Disinfectant substances can be chlorine-releasing compounds, alcohols, formalde- hyde, or formaldehyde releasers (Wolkoff et al. 1998; Magnano et al. 2009). In the case of “green” cleaning product formulations, essential oil constituents, such as geraniol, can be considered active compounds due to their biocidal action. As reported, the mass concentrations of main constituents can vary within the same product category. The diffusion mode of the product determines the proportion of the main family of ingredient added, e.g., solvent, water, or surfactant. For example, for all-purpose cleaners in wipe format, the solvent content does not exceed 10% w/w, while for liquid and spray formats, concentrations can be 1.5 times higher (Missia et al. 2012).

2.3 FRAGRANCES CONTAINED IN HOUSEHOLD PRODUCTS

Fragrance mixtures and essential oils are used in cleaning product formulations either to provide a pleasant odor, to hide odors from other chemicals used, or to enhance the product’s antibacterial performance (Rastogi 2002; Steinemann 2009). Nevertheless, there is a clear difference between essential oils and fragrance mixtures. Essential oils are natural oils extracted from plants, while fragrance mixtures represent combinations of different compounds (synthetic, natural-like, or extracted) assembled to replicate a “natural-identical” scent (Teixeira et al. 2013; Zuzarte and Salgueiro 2015). For instance, house- hold products can be formulated with (i) synthetic fragrances,

(ii) pure essential oils, or (iii) mixtures of synthetic fragrances and essential oils to reduce production costs. Suppliers are required to label substances or mixtures contained in packag- ing in accordance with Classification, Labelling and Packaging (CLP) Regulation (EC) No. 1272/2008 (Classification, Labelling and Packaging of Substances and Mixtures) before placing a product on the market when the substance is classified as dangerous or the mixture contains one or more substances classified as dangerous above a certain threshold. However, fragrance mixtures can be confidential and thus not labeled on the ingredient list (Rastogi 2002; Lassen et al. 2008).

According to Table 1, cleaning products can contain TerVOCs at mass concentrations of up to 5% w/w. The case of air fresheners and purifiers is completely different. The main purpose of air fresheners and purifiers is to mask or “eliminate” unpleasant odors from confined environments. As a consequence, fragrances are their main compounds. Air fresheners and purifiers can contain from 50 to 100% fra- grance chemicals, depending on whether the product is diluted in an organic solvent. The concentration of the organic solvent added to the formulation is directly related to the diffusion mode of the product: aerosol, passive diffuser, etc. (Rastogi 2002; Ezendam et al. 2009; Steinemann 2009; Steinemann

et al. 2011).

Sarwar et al. (2004) quantified TerVOCs in four different household products: (i) a pine-scented solid air freshener, (ii) a lemon-scented general-purpose cleaner, (iii) a lime-scented liquid air freshener, and (iii) a wood floor cleaner. The main detected terpenes were α-pinene, β-pinene, 3-carene, limo- nene, and α-terpinene, with individual mass concentrations ranging from 0.2 to 2% w/w for individual compounds. The authors confirmed that the total TerVOC contents in the cleaning products reached a maximum of almost 5% w/w. In air fresheners, the limonene contribution was approximately 50% w/w, while the total terpene contribution was almost 100%.

Finally, cleaning products and air fresheners present a wide variety of formulations and chemical compositions, depend- ing mainly on their usage purpose (general cleaner, disinfec- tant, floor cleaner, etc.) and diffusion mode (cream, liquid, spray, etc.) (Rastogi 2002). The diversity of VOCs included in the formulations can result in different temporal emission profiles. Likewise, the chemical composition of household products can influence the kinetics of emissions according to

(i) the concentration of the cleaning constituents, (ii) the vol- atility of the constituents, and (iii) the chemical affinities be- tween product components. These parameters will affect the kinetics and dynamics of the concentrations of the chemicals transferred from the product to the gas phase.

2.4 INDUSTRIAL USE OF ESSENTIAL OILS

Essential oils are distilled volatile oils of plants materials that have strong aromatic components. These aromatic substances are made up of different chemical compounds that occur naturally in the plant. For example, alcohol, hydrocarbons, phenols, aldehydes, esters and ketones are some of the major components.

Among all types of plants in the world, only about 700 plants are considered aromatic, and therefore, they are all significant for the production of essential oils. Besides the limited source of supply, the small amount of essential oils that are contained in each aromatic plant makes it even more valuable.

Essential oils and plant extracts are the basis for the natural flavor and fragrance industry worldwide. They are extensively used globally for food flavoring, fragrances, aromatherapy and pharmaceuticals. The worldwide market for essential oils has been estimated at US$2.6 billion, with an annual growth rate of 7.5%.

The widespread use of aromatic plant products by consumers around the world and increasing international trade have been accompanied by attempts to identify new species of potential interest, to bring them into cultivation from the wild, and to introduce them to distant locations away from their places of origin. Among the aromatic substances of natural origin, which are currently used by the pharmaceutical, perfumery, cosmetic, and food industries, some are derived from plants, which have been cultivated for a long time whereas others are still obtained from species, which grow abundantly in the wild.

In order to get the best quality and quantity of essential oils, extraction procedure seems the key-controlling step. Factors such as types of plants, chemical make-up of oil, and location of oil within the plant (root, bark, wood, branch, leaf, flower, fruit and/or seed) are to be considered prior to the extraction. Choosing a proper extraction method is also important as well.

In general, there are four types of extraction methods:

Distillation

Carbon dioxide extraction

Cold pressing and

Solvent extraction

These extraction methods will be briefly discussed in the next chapter.

Applications of aromatic plants and volatile oils extracted from them include:

direct culinary use: as fresh or dried herbs, spices and condiments;

In perfumery, cosmetics, and household and personal hygiene products either directly as major or minor ingredients or as raw materials for the extraction and or synthesis of specific aroma chemicals;

In food, and drink: as flavoring agents;

In human and veterinary medicine: as components of pharmaceutical preparations;

In crop protection: as pesticides and insect repellents; and

As antibacterial and antifungal agents in a range of situations.

In addition to all application and use mentioned above essential oils are widely used in industries including the flavor and fragrance industries as a source of flavor and aroma, in the food and drink industries as processing and flavoring agents, in the paint, petroleum, and in many other industrial transformations. The industrial uses of essential oils are summarized in the following chart:

2.5 OVERVIEW OF APPLICATION METHODOLOGIES FOR AIR FRESHENERS AND PURIFIERS

The case of air fresheners differs from that of cleaning products: the use of air fresheners does not involve a specific process. Subsequently, the loading factor cannot be defined as it is for cleaning products since air fresheners are not directly applied onto surfaces. Generally, a loading factor of 1 diffusion unit per room (1 unit/room) is considered in such cases. Thus, primary emissions are mainly influenced by the emitted mass and the selected diffusion mode.

(i). Plug-in air freshener: The application mode only depends on the intensity selected on the electrical controller: low, medium, or high intensity. Therefore, the amount of dispersed species can vary. The products tested are weighed before and after use in all studies to quantify the mass of product vaporized.

(ii). Passive diffuser air freshener: This product does not involve an application process, but rather simply the positioning of the device in the test chamber. Generally, passive diffusers are used to continuously release odorous chemicals. The products tested are weighted before and after use in all studies to quantify the mass of product vaporized.

(iii). Spray aerosols: The application process depends on (i) the amount of product distributed by each spray (user dependent) and (ii) the number of sprays performed. The number of sprays might change depending on the volume of the experimental chamber. For spray air fresheners, all reviewed studies followed the indications provided by the manufacturer. The amount of product released can vary from 0.39 to 2.00 g/spray depending on the spray mechanism and the user (Missia et al. 2012).

Finally, depending on the diffusion mode of air fresheners, the dynamics of emissions might vary. Plug-in air fresheners and passive diffusers serve as continuous emissive sources, while spray aerosols are a one-time source. Therefore, the sampling strategy, i.e., sampling time and test duration, must be optimized depending on the tested product.