Volatile organic compounds (VOCs)

VOCs include aromatic hydrocarbons, aliphatics, aldehydes, ketones, ethers, acids and alcohols, with diverse functional groups (halogens, oxygen, sulfur, nitrogen or phosphorus, excluding carbon oxides and carbonates). They are rather inert lipophilic compounds capable of passing through biological membranes, with a toxicity that basically depends on their biotransformation within the body [7]. Although a large number of substances are considered VOCs, the most abundant in the environment are benzene and some of its organic derivatives, like toluene, ethylbenzene and xylene (o-, m- and p-), jointly named BTEX, which comprise over 60% of the VOCs found in urban areas [8]; hence, they are used as a reference to evaluate environmental levels and VOC exposure.

VOCs come mainly from natural sources, like forest fires and the transformation of biogenic precursors; nevertheless, anthropogenic activities have become important sources of toxic VOC emissions into the atmosphere, so much so that they account for 25% of VOCs in our global atmosphere [9]. Petroleum and natural gas extraction, petrochemical activities and the burning of fossil fuels in industries, homes and mobile sources, including automobiles, trucks, buses and motorcycles, ships and airplanes are the major contributors of VOC, followed by chemical and industrial processes (manufacturing of paints, lubricants, adhesives, oil derivatives), mining, commercial sources, gas leaks from stoves, residential water heaters and boilers and use of pesticides in agriculture [10, 11].

To determine the amount of pollutant emissions into the atmosphere, the US Environmental Protection Agency (EPA) developed the national emissions inventory (NEI), where a comprehensive and detailed estimate of air emissions of the most hazardous atmospheric pollutants and their precursors is made from different sources across its territory [12]. Pollutants included in the NEI are associated with the national ambient air quality standards (NAAQS) and with the EPAs air toxic program, which include hazardous air pollutants (HAPs). NEI estimates of compounds include carbon monoxide (CO), lead (Pb), particulate material (PM), sulfur dioxide (SO₂) and ozone precursors (O₃), which are nitrogen oxides (NO_x) and VOCs.

Regarding VOC emissions from industrial processes, the EPA reported in 2014 the activities with the highest VOC emissions into the atmosphere were oil and gas production, totaling more than 3 million tons that year, followed by storage and transportation, and new and existing chemicals (NEC) with nearly 200 thousand tons in both cases; whereas, oil refineries emitted just over 50 thousand tons [13]. It is worth mentioning that these figures account for only the United States, and so it is of concern that not all countries have reliable air pollution emission inventories, because despite the existence of the Global Atmosphere Watch (GAW), it monitors only VOCs present in large geographic areas, not at a regional municipal level, where health impacts occur.

Table 1 presents VOC pollution compositions and levels from a study on air quality in major cities in Mexico [14, 15, 16]. In MX, the composition is due to liquified gas, automobile and truck fuel combustion and the use of household products of industrial origin besides fuel combustion. In MY, a substantial contribution of compounds of industrial origin besides fuel combustion is observed; levels reached in the most polluted area in MY (Santa Catarina) are about twice the highest levels measured in MX (La Merced): 1051 ppbV versus 605.5 ppbV [14, 15]. GJ shows an intermediate combined composition of industry, gas and use of industrial products, with a total of 678.04 ppbV average [16].

The relative abundance of reactive VOCs that contribute to the formation of tropospheric ozone in MY and MX was studied [14, 15], and it was found that the potential for ozone formation in the most polluted industrial area in MY was four times higher (Table 2) than the potential in the most polluted area in MX. Nevertheless, measurements in 2015 indicated higher ozone levels in MX (50 to 80 ppb during the ozone generation period, from March to June, with a maximal value of 179 ppbV in June, at the south of the City), which is probably because the natural atmosphere in MX is very oxidative and UV radiation is high [17]. MY showed ozone levels between 20 and 36.6 ppbV on average throughout the year, with a maximum 46 ppbV in August in its NW area [18]. According to the EPAs air quality indices (AQI) for ozone, MY shows good indices; whereas, MX shifts between moderate to harmful for sensitive groups values [19].

Benzene, toluene, ethylbenzene and xylenes (o-, m- and p-) (BTEX), as indicators of toxic VOC exposure

Benzene, toluene, ethylbenzene and xylenes (o-, m- and p-) are found in natural form in crude oil, diesel and gasoline, so they are released into the environment whether or not these fuels are burned. In addition, BTEX are highly used in the industry as additives and precursors of other substances: benzene is used in the manufacturing of synthetic materials and consumer products, including plastics, nylon, insecticides and paints; toluene is used as solvent for paints, coatings, rubbers, oils and resins; ethylbenzene may be found in paints, plastics and pesticides, and it is also used as additive for aviation fuel; xylenes are used as solvent in the printing, rubber and leather industries [20, 12].

Because BTEX are so widely used and are derived from gasoline and organic material combustion, they coexist as a mixture in the atmosphere in practically all ecosystems. The mixture composition varies and depends to a great extent on nearby sources, on the type of industry, on the activities developed in the region and on environmental conditions. These compounds may be found polluting not only the air but also the soil and the water, because, as already mentioned, their physical and chemical properties endow them with a great dispersion capacity, and once they are released into the environment, they can volatilize, dissolve in water, adhere to soil particles or be biologically degraded [21, 10]. High BTEX concentrations have been found in areas with intensive industrial activity, but they also reach high levels in large cities, mostly owing to vehicular traffic problems [22, 21]. Furthermore, there may be additional exposure owing to household products, like disinfectants, aerosols, varnishes, printing inks [23], as well as cigarette smoke, which turns them into ubiquitous pollutants. Higher BTEX concentrations have been reported in close spaces (i.e., indoors) than in open spaces [10], but the ban on indoor smoking and more careful measurements [24, 21] have demonstrated that personal exposure to VOCs may be due to outdoor pollutants, mainly in temperate and warm latitudes where doors and windows are opened more frequently, allowing for an air exchange. In MX, only toluene showed higher levels indoors than outdoors (26.11 ppbV vs 14.32 ppbV) [24].

For different polluting compounds in environmental matrices, the EPA has used information from the IRIS database to establish reference concentrations (RfCs) that cause critical effects [25]. Hence, the maximum environmental exposure concentration in air of benzene for an individual is 0.03 mg/m³; this concentration is due to the hematological effect of benzene in humans. For toluene, it is 5.0 mg/m³, based on neurological effects in humans and the increase in liver size. For ethyl-benzene, it is 1.0 mg/m³ based on effects in the respiratory system, hepatotoxicity and nephrotoxicity. For xylene mixture, a maximum concentration in air of 0.1 mg/m³ was estimated for effects on the respiratory and neurological systems and low birth weight (Table 3, bottom) [25]. All of the above refers to noncarcinogenic effects. Nevertheless, according to the monograph on specific reference values for benzene, an individual’s chronic exposure to a reference value of 0.03 mg/m³ accounts for an increase in the potential risk for developing leukemia to 1 out of 10, 000, compared with a baseline of 1 in one million for a population with exposures 2 order of magnitude lower [25].

Table 3 also shows environmental exposure levels measured in research studies of populations near BTEX sources, like industrial areas, refineries and a highly polluted Mexican river. In all these cases, the reported concentrations were lower than RfCs set forth by the EPA for noncarcinogenic effects. However, most data reported about benzene—the only carcinogenic compound—are near or above the inhalation unit risk (IUR) value of 1 µg/m³ in the air, increasing an individual’s cancer risk by 2.2 × 10^–6 up to 7.8 × 10^–6 for being exposed for a lifetime. For example, the 15.07 µg/m³ value reported by Lee [8] would increase the cancer risk for that population 100 times [25]. As can be observed, industrial areas in emerging economies, like Latin-American countries and Hong Kong, show the highest levels of benzene, demonstrating that economic growth and development are not related to environmental protection and health or well-being (Figure 1).

Biotransformation mechanisms and health effects of BTEX

BTEX compounds (Figure 2) are aromatic hydrocarbons derived from benzene; the toluene and ethylbenzene rings are monosubstituted with a methyl group and an ethyl group, respectively; the xylene ring is disubstituted with methyl groups, so that the xylene mixture comprises ortho-, meta- and para- (o-, m- and p-) compounds, according to the position of the ring substituents [26]. Because of these structures and their easy volatilization, they can enter organisms through various routes: the most common is inhalation, followed by skin and, at a lower percentage, orally through polluted water or food.

Toxicokinetic studies in humans and animals indicate that these lipophilic chemical products are distributed to highly vascular tissues rich in lipids, like the brain, the bone marrow and the body fat, and they are rapidly eliminated from the body. Nevertheless, such lipophilicity confers on them the capability to cross mucous epithelia of the respiratory tract and the cell membranes in various organs. The nose mucous epithelium and the tracheobronchial tree are metabolically active to BTEX exposure, although the liver is the organ with the highest activity, because most xenobiotic biotransformation processes take place in the liver [27].

BTEX are slightly reactive compounds, so their toxicity is determined by their biotransformation within the body [7]. Biotransformation comprises phase I and phase II processes.

Classical studies about BTEX biotransformation, like the toxicological profile for total petroleum hydrocarbons [28], considered that out of the four compounds, benzene has the highest toxicity potential because the formation of an epoxide was recognized from the oxidation of the benzene molecule by CYP2E1 action. However, more recent studies, such as those presented by the Agency for Toxic Substances and Disease Registry (ATSDR), reevaluated the joint toxic action of these chemicals, and now it is known that at low exposure concentrations BTEX are substrates of CYP2E1 and of other P450 isoenzymes, whose main function is to oxidize their substrates. Oxidized metabolites of this phase I reaction are highly reactive [20]. Furthermore, the action of compounds has a competitive metabolic inhibition mechanism, but only at high exposure levels; such mechanism has not been found at low levels. Hence, it was concluded that the biotransformation of the four substances is dose-dependent and generally extensive at dose levels that do not saturate the first metabolic step of each compound [29, 20].

All four chemicals can produce neurological impairment via parent compound-induced physical and chemical changes in nervous system membranes. Additionally, exposure to benzene can cause hematological effects, including aplastic anemia, with subsequent manifestation of acute myelogenous leukemia via the action of reactive metabolites [30, 20].

Benzene, mainly during its metabolism, may suffer a nonenzymatic rearrangement and form phenol, which by a second CYP2E1-mediated oxidation turns into hydroquinone. This may be accumulated in the bone marrow and act as a substrate of myeloperoxidase (MPO), forming benzoquinone that is both myelotoxic and clastogenic [31]. Its leukemogenic capacity has partly been attributed to this mechanism (Figure 3). It has been determined that benzene may suffer biotransformation in the bone marrow, but in a lower quantity compared to the liver, although the typical toxicity site of this compound is the bone marrow [32, 33]. At low doses, benzene turns into recognized toxic metabolites that include benzene epoxide or oxide, benzene dihydrodiol, hydroquinone, catechol, benzoquinones and muconaldehyde, while at a high dose, benzene inhibits hydroquinone formation from phenol, apparently through competition for the CYP2E1 active site, which also has an affinity for hydroquinone and catechol [31, 33]. This is important because environmental exposure to benzene typically occurs at very low doses.

The American Conference of Governmental Industrial Hygienists (ACGIH) [34] made recommendations for the occupational biological exposure index (BEI®) in urine specimens for workers at industries producing environmental pollutants. In 2005, they recommended the use of certain metabolites to determine exposure to benzene and toluene, establishing that trans, trans-muconic acid (t,t-MA) was a good biomarker of benzene exposure with a BEI® of 500 µg/g of creatinine; for toluene, they established hippuric acid (HA) as an exposure biomarker with a BEI® of 1.6 g/g of creatinine. Table 4 shows levels found in children in various studies outside the working and industrial environments in Latin-American countries and in areas with similar pollution problems. As pointed out by Bolden et al. [10], levels found are generally lower than those set forth in international reference values, like the US-EPA or ACGIH, for occupational exposure, but effects on health or on the measured biomarker are reported.

Tuakuila et al. [35] used S-phenylmercapturic acid (S-PMA) to measure benzene exposure. This metabolite was proposed by the ACGIH in its 2015 publication as the best benzene exposure biomarker [36], which had rendered previous levels measured by t,t-MA questionable. However, Fang et al. [37] demonstrated that there is a good correlation between the two metabolites in experimental animals and in children exposed to benzene in an industrial area in Korea.

BTEX and their relationship to diseases in children

Due to the interaction of metabolites resulting from biotransformation of compounds with macromolecules, one can assume that systems will be affected in different ways, contributing to the development of a diversity of diseases, of which a vast collection of research has been carefully reviewed by Bolden et al. [10] They reached the important conclusion that even though environmental levels of these compounds are below reference levels, they cause various diseases because of the known characteristics of each BTEX component. BTEX are distributed to the central nervous system, the bone marrow and highly vascular tissues rich in lipids and are biotransformed both in the respiratory system and in the liver [38, 39, 40, 41], where biotransformation can generate different endpoints of toxicity at the cellular level (Figure 3).

Studies on environmental exposure largely found a relationship with diseases such as asthma and respiratory complications (Table 4) [40, 42, 43, 44]. Studied populations are located close to a source of BTEX, including industrial complexes, petrochemical facilities or refineries or large cities.

A study was conducted on children in elementary schools, one near a refinery and the other in an area far from the petrochemical complex. A relationship was observed between respiratory syndromes and resulting absenteeism and BTEX levels, and a significant difference was found in p-, m-xylenes concentration, which was greater in the area close to the refinery and correlated with sore throat, cough and cold in the children within the community [44].

Another study estimated concentrations of VOCs, including benzene, toluene and ethyl-benzene, with daily activity patterns in a sample of school age children in a nonindustrial area. This study was correlated with measurements of the forced expiratory volume (FEV) and the forced vital capacity (FVC), with the use of spirometric tests to identify respiratory capacity problems, and it found that the increase in total exposure was significantly associated with a decrease in FEV. Consequently, they suggest that this type of exposure may be related to the occurrence of asthma [40].

Lopes de Moraes et al. [43] found an association between wheezing and the levels of VOC at home; whereas, Wichman et al. [42] found more respiratory problems during the winter season. Similar results were seen in elderly people when looking for associations between the loss of pulmonary function measured by FEV and FVC tests and BTEX exposure measured through BTEX metabolites in urine, and the results were correlated with oxidative stress biomarkers (MDA, TBA, 8-OHdG), demonstrating that exposure to toluene and xylene has harmful effects on pulmonary function and that oxidative stress could be involved in the pathogenesis of this population [39].

This result matches the result seen in a model of young rats that were simultaneously administered a dose of toluene, chloroform and methylene chloride for three days, which produced an increase in lipid peroxidation measured by TBARS, a loss in antioxidant capacity determined by SOD, glutathione peroxidase and glutathione reductase diminished activities, as well as an increase in CYP2E1 hepatic activity. Individual administration of each compound did not induce a significant response in the animals [45].

In this model, the liver and bone marrow were affected. All this is consistent with the reconstruction of events from different studies shown in Figure 4, which represents BTEX-induced effects of metabolism and antioxidant response, as well as possible cell-wide toxic effects. In their review of studies conducted on children and teenagers and at birth, Bolden et al. [10] presented the following as the main targets affected by these mechanisms: the respiratory system, the immune system, the reticuloendothelial tissue, the skin and the developing embryo.